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Interpreting Arterial Blood GasesSuccessfully 2.6 www.aorn.org/CE

BRENDA G. LARKIN, MS, RN, ACNS-BC, CNS-CP, CNOR;ROBERT J. ZIMMANCK, MD

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available for this activity. Earn the CE contact hours byreading this article, reviewing the purpose/goal and objectives,and completing the online Examination and Learner Evalua-tion at http://www.aorn.org/CE. A score of 70% correct on theexamination is required for credit. Participants receive feed-back on incorrect answers. Each applicant who successfullycompletes this program can immediately print a certificate ofcompletion.

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Purpose/GoalTo provide the learner with knowledge specific to interpretingarterial blood gases (ABGs).

Objectives1. Explain what ABGs are.2. Discuss what ABGs measure.3. Discuss how acidosis and alkalosis may be identified using

ABG results.4. Explain how the nurse can determine whether respiratory

or metabolic factors are causing an imbalance.

AccreditationAORN is accredited as a provider of continuing nursingeducation by the American Nurses Credentialing Center’sCommission on Accreditation.

ApprovalsThis program meets criteria for CNOR and CRNFA recerti-fication, as well as other CE requirements.

AORN is provider-approved by the California Board ofRegistered Nursing, Provider Number CEP 13019. Checkwith your state board of nursing for acceptance of this activityfor relicensure.

Conflict-of-Interest DisclosuresBrenda G. Larkin, MS, RN, ACNS-BC, CNS-CP, CNOR,and Robert J. Zimmanck, MD, have no declared affiliationsthat could be perceived as posing potential conflicts of interestin the publication of this article.

The behavioral objectives for this program were created byHelen Starbuck Pashley, MA, BSN, CNOR, clinical editor,with consultation from Susan Bakewell, MS, RN-BC, direc-tor, Perioperative Education. Ms Starbuck Pashley andMs Bakewell have no declared affiliations that could beperceived as posing potential conflicts of interest in the pub-lication of this article.

Sponsorship or Commercial SupportNo sponsorship or commercial support was received for thisarticle.

DisclaimerAORN recognizes these activities as CE for RNs. Thisrecognition does not imply that AORN or the AmericanNurses Credentialing Center approves or endorses productsmentioned in the activity.

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Interpreting Arterial BloodGases Successfully 2.6 www.aorn.org/CE

BRENDA G. LARKIN, MS, RN, ACNS-BC, CNS-CP, CNOR;ROBERT J. ZIMMANCK, MD

ABSTRACTArterial blood gas (ABG) analysis is a crucial skill for perioperative nurses, in particular the RN circu-lator. This article provides the physiological basis for assessing ABGs perioperatively and presents asystematic approach to blood gas analysis using the Romanski method. Blood gas sample data allowthe reader to practice ABG interpretation. In addition, four case studies are presented that give thereader the opportunity to analyze ABGs within the context of surgical patient scenarios. The ability toaccurately assess ABGs allows the perioperative nurse to assist surgical team members in restoring apatient’s acid-base balance. AORN J 102 (October 2015) 344-354. ª AORN, Inc, 2015. http://dx.doi.org/10.1016/j.aorn.2015.08.002

Key words: arterial blood gases, perioperative, Romanski method, blood gas interpretation.

Leviticus 17:11 states that “the life of a creature is inthe blood.”1(p118) The ability of health careproviders to appropriately interpret the clinical

relevance of elements carried in the blood is essential tomaintaining homeostasis and patients’ lives. Nowhere is thismore important than in the OR, where patients entrust theperioperative team with their lives, relying on theirknowledge and expertise. The RN circulator is charged withthe oversight of the OR and with assisting the team insuccessfully intervening when difficulties are encountered.When there is the potential for large blood loss or majorfluid shifts during surgery, the interpretation of arterialblood gases (ABGs) and correct intervention by the RNcirculator and the anesthesia professional can mean thedifference between life and death for the patient.

To accurately interpret ABG samples, perioperative nursesmust understand all the components that are measured andhow they contribute to maintaining the individual’s normalphysiological function. Arterial blood gas test results show thepatient’s acid-base balance, which is measured by the hydrogenion (Hþ) concentration present in the blood (pH), its oxygensaturation (SaO2), partial pressure of oxygen (PaO2), partial

pressure of carbon dioxide (PaCO2), concentration of bicar-bonate (HCO3

�), and base excess and base deficit (Table 1).2

It is critical that perioperative nurses know how to interpretABGs and what interventions may contribute to the fullrestoration of homeostasis. The results of an ABG test canprovide a plethora of information about a surgical patient’sphysiological state. In addition to pH, blood gases providedata about the adequacy of a patient’s oxygenation andventilation and indicate the primary source of a disturbance(ie, respiratory or metabolic) in homeostasis. Additionally,ABG results can indicate how effectively the patient’s bodyis compensating for the acid-base disturbance and whetherthe patient’s total blood volume is adequate for transportingall the nutrients that the body’s tissues require.

Several shortcuts have been suggested to determine themeaning of acid-base disorders in an effort to make the processless daunting, such as assigning colors to values or using only afew values to make a determination.3-5 This article presentsthe Romanski method6 of blood gas analysis because it isstraightforward and is easily mastered (Figure 1). Fourexamples are presented to illustrate how to apply theRomanski method. The discussion of each example helps

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the nurse determine suitable interventions and treatments tohelp restore any acid-base imbalance in patients. Finally,four case studies and the correct answers are presented toallow nurses to practice what they have learned.

THE NEED FOR ABG MEASUREMENTSBlood gas interpretation can be useful in a wide variety ofsurgical procedures. The results of an ABG test can beparticularly helpful in any surgery in which large fluid shifts orblood loss are expected (eg, bowel resection, Whipple proce-dure, liver resection), thus providing information regarding thepatient’s fluid status and metabolic state. A patient requiringfluid resuscitation can have a large base deficit and metabolicacidosis.7 In any thoracic procedure, especially those in whichone-lung ventilation is used, blood gas analysis can provideinformation on adequate gas exchange. In addition to theseprocedures, the anesthesia professional and surgeon rely onABGs during cardiac, neurological, or long oncologicalprocedures to direct care.

INTERPRETING ABG MEASUREMENTSSuccessful interpretation of ABG results begins with an un-derstanding of pH and the effect of acidosis and alkalosis ontissue oxygenation. The body’s regulation of Hþ is influencedby both the respiratory and metabolic systems. Nurses shouldknow the relationships between all the ions that contribute to

blood pH because changes in the concentration of any ion willresult in a change in the overall blood pH.

The normal range for pH in human blood is 7.35 to 7.45.Neutral blood pH is considered to be 7.4. A pHapproaching 7.35 is considered acidic. Conversely, as thepH approaches 7.4, it is considered alkalotic.8 Whencarbon dioxide (CO2) concentration is increased in theblood, via the respiratory system, water present in theblood plasma (H2O) dissociates into Hþ and hydroxideions (OH�). Hydrogen and OH� also react with sodiumions (Naþ) circulating in the blood, which creates sodiumbicarbonate (NaHCO3). This leads to the followingchemical reaction:

CO2þH2O4H2CO34HþþHCO �3 4Naþ4NaHCO3

The amounts of the ions present can then shift back andforth depending on either the metabolic or respiratory stateof the individual. According to the Henderson-Hasselbachequation, pH is calculated based on the relationship be-tween the concentrations of CO2 and HCO3

�9(A/B Reg)and is used to accurately determine pH in a solution such asblood. This equation is expressed as follows:

pH ¼ 6:1þ log10

� �HCO3

��0:03� pCO2

�

Table 1. Components of Arterial Blood Gas Test Results1,2

Measurement Meaning Normal Range Critical Values

pH Concentration of hydrogen ions (Hþ) in blood 7.35 to 7.45 <7.25 or >7.60

SaO2 Percent saturation of oxygen (O2) in hemoglobin 80% to 100% <80%

PaO2 Partial pressure of O2 in arterial blood 80 mm Hg to 100 mm Hg <50 mm Hg

PaCO2 Partial pressure of carbon dioxide (CO2) inarterial blood

35 mm Hg to 45 mm Hg Acidosis<20 mm Hg or>60 mmHg>45 mm HgAlkalosis<35 mm Hg alkalosis

HCO3� Concentration of bicarbonate in blood 22 mEq/L to 26 mEq/L Alkalosis

<10 mEq/L or>40 mEq/L>26 mEq/LAcidosis<22 mEq/L

Base excess/basedeficit

Excess or deficit of bicarbonate in blood �2 mEq/L to þ2 mEq/L <�2 mEq/L or>2 mEq/L

1.Common Laboratory (Lab) ValuesdABGs. Globalrph. http://www.globalrph.com/abg_analysis.htm. Accessed June 30, 2015.2.Blood Gas Critical Values. Dartmouth-Hitchcock Medical Center. http://labhandbook.hitchcock.org/criticalTestLimits.html#BloodGas.

Accessed June 30, 2015.

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http://labhandbook.hitchcock.org/criticalTestLimits.html#BloodGas


These two equations show why it is important to know therelationships of all the ions that contribute to blood pH.Changes in the concentration of any ion will result in achange in overall pH. The following overview discusses acid-base physiology and the effects of the respiratory and meta-bolic systems on blood pH.

Respiratory System EffectsThe lungs regulate the amount of CO2 in the blood, sochanges in CO2 concentration can be referred to as either

respiratory acidosis or respiratory alkalosis. Carbon dioxide isexpressed in ABG results as PaCO2 and represents the totalamount of dissolved CO2 in arterial blood. Clinically, forexample, a patient with a disease such as chronic obstructivepulmonary disease (COPD) or emphysema will have addi-tional CO2 in the blood because of respiratory compromise.Other conditions that can result in acidosis are shown inTable 2.8 As CO2 increases, it forces the Hþ concentration toincrease, and the patient’s blood becomes more acidic and pHvalues decrease from 7.4 toward 7.35 or lower.

A patient who is hyperventilating or who has a medical con-dition that affects respiratory rates experiences a decrease inCO2, and his or her ABGs will show a pH of 7.4 or higher,which is known as alkalosis.8 As CO2 decreases, it forces Hþions to decrease, resulting in alkalosis. Respiratory alkalosis is arare physiological phenomenon. It is associated withconditions in which CO2 is “blown off” or exhaled at arapid rate (Table 3). It can also be seen in patients withsigns and symptoms of vasoconstriction and perhapshypocalcemia, as well as when there is artificialoverventilation. Flash pulmonary edema would result inCO2 retention and acidosis, as would asthma and hypoxia.

Metabolic System EffectsWhen changes in the acid-base balance result from primarychanges in extracellular HCO3

�, they are referred to asmetabolic acid-base disorders.9 If the bicarbonateconcentration increases, the blood’s Hþ concentrationdecreases. This causes the pH value to increase from 7.4toward 7.45 or higher, which is called metabolic alkalosis.Metabolic alkalosis is caused by a primary decrease ofHCO3

� from conditions such as kidney disease, electrolyteimbalances, prolonged vomiting, hypovolemia, diuretic use,and hypokalemia.10

Metabolic acidosis occurs when the bicarbonate concentrationdecreases, resulting in the Hþ concentration increasing, which

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Figure 1. The Romanski method of arterial blood gasevaluation.

Table 2. Conditions Associated With Respiratory Acidosis by Physiological Mechanism1

Central Nervous System Ventilation Control Peripheral Nervous System Ventilation Control Ventilation-Perfusion Mismatch

Anesthetic medication toxicity Myasthenia gravis Pneumothorax

Benzodiazepine overdose Poliomyelitis Pleural effusion

Opioid overdose Polymyopathy Atelectasis

Stroke Neuromuscular blockade Pneumonia

Spinal cord injury Pulmonary edema

1.Nelligan PJ, Deutschman CS. Perioperative acid-base balance. In: Miller RD. ed. Miller’s Anesthesia. 8th ed. Philadelphia, PA: ElsevierSaunders; 2015:1811-1829.

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in turn lowers the pH from 7.4 to 7.35 or lower. Metabolicacidosis is seen in patients with an accumulation of ketonesand lactic acid, as in shock-induced hypoxemia (which resultsin anaerobic metabolism), renal failure, excessive physical ex-ercise without adequate caloric intake, drug or toxin ingestion,and gastrointestinal or renal HCO3

� loss.10

Tissue Oxygenation StatusArterial blood samples are used for diagnostic testing. Atpressures less than 60 mm Hg, O2 dissociates from

hemoglobin (Hgb) rapidly. This is the normal physiologicalpicture in the capillary system in which O2 is released to thetissues and CO2 binds to Hgb and is carried to the lungsfor exchange.

Physiological Shift of the OxyhemoglobinCurve as it Relates to pHThe oxyhemoglobin dissociation curve, also known as theoxyhemoglobin dissociation curve, is shown in Figure 2. Theoxyhemoglobin dissociation curve depicts the relationshipbetween PaO2 in the blood to the percent saturation ofHgb with O2. The middle solid line shows thisrelationship when the pH is normal. A patient’s pH levelhas a major influence on the degree of saturation of Hgbin red blood cells because it affects the ability of red bloodcells to transport O2 to all the tissues of the body andremove CO2 from the tissues. The 30-60-90 rule showsthe typical relationship of PaO2 and SaO2. A PaO2 lowerthan 60% is considered a critical level because this mayindicate hypoxia.11,12 For example, in a patient with acutealkalosis, in which pH levels are higher, the oxyhemoglobincurve shifts toward the left and produces an increase in the

Table 3. Conditions Associated With RespiratoryAlkalosis by Physiological Mechanism1

Central Nervous System Pulmonary Function

Head injury Asthma

Stroke Pulmonary edema

Hypoxia Embolism

Hyperventilation

1.Neligan PJ, Deutschman CS. Perioperative acid-base balance.In: Miller, Ronald D, ed. Miller’s Anesthesia. 8th ed.Philadelphia, PA: Elsevier Saunders; 2015:1811-1829.

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Figure 2. The oxyhemoglobin dissociation curve (also known as the oxygen-hemoglobin dissociation curve). O2 ¼oxygen, PaCO2 ¼ partial pressure of carbon dioxide, PaO2 ¼ partial pressure of oxygen, SaO2 ¼ oxygen saturation.




affinity of Hgb for O2. This results in a higher saturation ofHgb with O2 than at neutral pH at the same PaO2. Theconverse is true for acute acidosis, in which pH levels aredecreased. When this occurs, there is a decrease in theaffinity of Hgb for O2, and this results in less O2

saturation of Hgb than occurs at neutral pH for the samePaO2. To see this more clearly, locate the * on the curvein Figure 1 at PaO2 30 mm Hg and you will see that apatient with acidosis will only have 42% saturation, whileat the same PaO2, a patient with normal pH will have anSaO2 of 60%.

Respiratory and Metabolic CompensationTwo main organs regulate acid-base balance: the lungs and thekidneys. The respiratory system regulates the amount of car-bonic acid in the blood by controlling PaCO2, and a patient’srespiratory rate will change to attempt to correct PaCO2 levelsthat are out of the normal range. Respiratory regulation andcompensation can occur within minutes. If patients are unableto compensate via their own respiratory system, mechanicalinterventions such as a ventilator, continuous positive airwaypressure (CPAP), or bilevel positive airway pressure (BiPAP)may be needed.

The patient’s kidneys control the NaHCO3 buffering sys-tem and are able to excrete both Hþ and HCO3

� into theurine. Metabolic compensation occurs primarily in thekidneys and can take from three to five days to occur. Whenacidosis persists, the kidneys are also able to produce newHCO3

�, which further contributes to the restoration ofnormal pH.9 When there is a large of amount of Hþpresent in the renal tubules, more Hþ is excreted in theurine than HCO3

�, which results in the urine becomingmore acidic and the blood becoming more alkalotic.Because it takes time to filter blood in the kidneys, thisprocess is much slower than the regulation of CO2 bythe lungs.9

Actions of pHA patient’s pH level indicates the status of acid-base balance.Values less than 7.35 indicate that the patient is experiencingacidosis, while values above 7.45 indicate alkalosis. However,information about pH values are insufficient to diagnose thecause of an imbalance or to determine whether the body isattempting to compensate in response to the imbalance. OtherABG measurements that are crucial for determining the pa-tient’s full physiological status include PaO2, PaCO2, SaO2,bicarbonate, and the base excess or base deficit. These mea-surements guide the perioperative team toward instituting

interventions that may be needed to assist the patient inreturning to a normal pH.

PaO2 and PaCO2The PaO2 and PaCO2 values represent the status of a patient’srespiratory function. If the PaO2 values fall below 80 mm Hg,this may indicate that the patient does not have sufficientoxygen to adequately fuel cellular aerobic respiration. Cellsthat are not adequately oxygenated are unable to fuel the tri-cyclic acid cycle production of adenosine triphosphate (ATP),the primary source of cellular energy. In this situation, the cellsresort to using anaerobic respiration. Anaerobic respiration isless efficient than aerobic and contributes protein acid waste inthe form of lactic acid. As lactic acid builds up in the patient’scirculation, it increases the severity of acidosis.8

Patients with hypoxemia have inadequate PaO2 levels. Hyp-oxemia may exist in diseases that inhibit O2 from reaching thebloodstream in the lungs, such as COPD and emphysema.Other conditions, such as anemia or acute blood loss fromtrauma, decrease the oxygen-carrying capacity of blood bydecreasing the amount of available Hgb.8

Respiratory rate influences PaCO2 levels in arterial blood.When there are increases in a patient’s ventilatory rate, such asoccur in anxiety states or as a result of sudden anemia from anycause, PaCO2 levels will decrease. Levels of PaCO2 below35 mm Hg produce primary respiratory alkalosis. Increases inrespiratory rates that result in alkalosis are rare eventscompared with respiratory acidosis. Respiratory acidosis existswhen PaCO2 rises above 45 mm Hg. Levels of PaCO2 rise inmedical conditions that also cause decreases in PaO2.

SaO2Hemoglobin saturation with oxygen is expressed as a per-centage. As discussed earlier, the saturation of Hgb is directlyrelated to PaO2. The normal range for SaO2 is 80% to 100%.The SaO2 value represents the amount of oxygen available tothe patient’s tissues from the arterial blood. Values less than80% may not be sufficient to meet the oxygen needs of tissues,especially tissues that have normally high oxygen consumptionrequirements (eg, brain, heart, kidneys). Low SaO2 levelsalong with low PaO2 values indicate that the patient is in ahypoxemic state, which requires supplemental oxygenadministration. Hypoxemia is classified as mild, moderate,or severe:

� mild hypoxemia is defined as a PaO2 of 60 mm Hg to70 mm Hg,

� moderate hypoxemia is defined as a PaO2 of 40 mm Hg to59 mm Hg, and




� severe hypoxemia is defined as a PaO2 of less than 40 mmHg.7

The resulting SaO2 values can be determined from theoxyhemoglobin curve after taking into account any shifts inthe curve from acid-base disturbances and other factors thatmay shift the curve.

BicarbonateBicarbonate values range from 22 mEq/L to 26 mEq/L andrepresent the metabolic component of the ABG result. Bi-carbonate levels exceeding 26 mEq/L indicate metabolicalkalosis, while levels below 22 mEq/L indicate metabolicacidosis. Medical conditions that are associated with metabolicacidosis include starvation, diabetic ketoacidosis, and severediarrhea. Renal failure is the most common cause of metabolicacidosis. Metabolic alkalosis caused by high levels of HCO3

�

(as opposed to low levels of Hþ) are associated with vomitingor the removal of gastric secretions via nasogastric suction.8

Base Excess/Base DeficitThe value of the base excess or base deficit of bicarbonate isuseful in evaluating metabolic compensatory efforts and canalso estimate the blood volume deficits associated with trau-matic blood loss and direct the titration of fluid and bloodproducts needed for resuscitating trauma patients in hypo-volemic shock.7 This measurement has direct implications forlife-saving interventions by perioperative team membersbecause many trauma patients require surgical procedures.

Bicarbonate values provide information on the metaboliccomponent of acid-base disorders, while base deficit valuesprovide information on the level of excess or deficit of bicar-bonate available in the patient’s system. Some laboratoriesreport only base excess; therefore, a negative base excess isactually a base deficit. Values outside the normal rangeof �2 mEq/L to þ2 mEq/L suggest a metabolic cause for theacid-base imbalance. Base excess is defined as the amount ofHþ that would be required to return the pH of blood to 7.35if the PaCO2 were adjusted to normal.13 Severe volumedepletion is related to the amount of base deficit present asshown in the ABG result. Base deficit in relation to meanarterial pressure ranges is shown in Table 4.7

Compensation for Acidosis and AlkalosisAfter the nurse identifies the cause of the pH imbalance(whether the respiratory component [PaCO2] or the metaboliccomponent [HCO3

�]), the remaining value is used to deter-mine whether the patient’s body is making an effort to correctthe condition. If the anesthesia professional identifies an

acid-base disorder as primarily respiratory, the metaboliccomponent indicates whether compensation is occurring andto what degree. In the same way, if the disorder is primarilyfrom a metabolic cause, the respiratory component must alsobe evaluated for the existence of compensation and its extent.Compensation may be complete if pH levels are nearly neutral(ie, 7.40). Partial compensation is indicated if the pH valuesare moving toward normal. The best way to understand thisprocess is to practice ABG analysis with several examples.

ROMANSKI METHOD OF ABG ANALYSISAnalyzing ABGs is the first step in managing the patient’sacid-base status. Results of blood gas analysis must be usedalong with an assessment of the patient’s history, comorbid-ities, and other diagnostic blood tests to determine whichinterventions, if any, will be needed. The RN circulator mustwork in concert with the anesthesia professional to develop thebest plan of action after results are obtained.

Some clinicians master the skill of evaluating ABG resultsquickly by just looking at them, while others need to work itout step by step.14 The Romanski method of blood gasanalysis5 uses each value of the blood gas result to determinewhich acid-base disorder is present and what is the primarycause of the disorder. Further analysis using this methodreveals whether the results show respiratory or metaboliccompensation. The Romanski method breaks the process ofABG evaluation into four steps.6

While most blood gas results provide all the measurementsdiscussed previously, it is possible to analyze an ABG withoutSaO2 and base deficit/base excess values. The hypotheticalABG scenarios to follow discuss how the RN can analyze testresults. These four examples lead the reader through ABGanalysis using a step-by-step process, and sidebars 1 through 4

Table 4. Severity of Circulating Blood Volume Lossin Relation to Base Deficit Values1

Blood VolumeLoss Base Deficit Range

Mean ArterialPressure

Mild 2 mEq/L to �5mEq/L

92 � 2.3 mm Hg

Moderate �6 mEq/L to �14mEq/L

84 � 2.4 mm Hg

Severe ��15 mEq/L 68 � 4.2 mm Hg

1.Davis JW, Shackford SR, Mackersie RC, Hoyt DB. Base deficit asa guide to volume resuscitation. J Trauma. 1988;28(10):1464-1467.




provide the RN with additional case studies of the ABG resultsthat demonstrate how to determine the patient’s conditionsand potential interventions.

Scenario 1On room air, the patient has a pH of 7.24, PaCO2 of 38 mmHg, PaO2 of 80 mm Hg, and HCO3

� of 15.5 mEq/L.

� In step 1, the RN should look at the pH. In this case, thepH value is below the low end of normal (ie, 7.35), whichindicates acidosis.

� In step 2, the RN should evaluate the results for respiratoryor metabolic components. In this scenario, the PaCO2 isnormal and the HCO3

� indicates acidosis.� In step 3, the RN should look for the value consistent withthe pH results. The value that is consistent with the pH inthis scenario is the HCO3

�, indicating a metabolic cause.� In step 4, the RN should evaluate for evidence ofcompensation. To do this, he or she would search for thevalue that is not consistent with the pH. In this scenario,that is the PaCO2, which is within normal range, indicatingthat there is no compensation occurring.

After this analysis, the nurse determines that this patient is inmetabolic acidosis with no compensation. These data areinsufficient to determine an appropriate intervention. Thenurse needs to perform a physical assessment and review thepatient’s history to guide the next steps in treatment.

Scenario 2The patient has a pH of 7.39, PaCO2 of 51 mm Hg, PaO2 of59 mm Hg, and HCO3

� of 30 mEq/L.

� Starting with step 1, the nurse notes that the pH is in in thenormal range, using 7.4 as an absolute value to determinethe presence of acidosis or alkalosis; 7.39 is lower than 7.4,so the patient is experiencing acidosis.

� In step 2, the nurse notes that the patient’s PaCO2 is higherthan normal, indicating acidosis, and the HCO3

� is in thealkalotic range.

� Finding a consistent value (step 3), the nurse sees that thevalue that matches the acidotic state is PaCO2, indicatingthat the cause of the acidosis is respiratory.

� To determine if compensation is occurring (step 4), thenurse sees that the patient’s HCO3

� is greatly elevated,indicating a metabolic effort to compensate.

Based on the assessment, the nurse determines that thispatient is in respiratory acidosis with complete metaboliccompensation because the pH has returned to the normalrange (ie, just 0.1 below absolute normal of 7.40). Thispatient may need supplemental oxygen because PaO2 levels

are critically low. However, a complete patient history andphysical examination also are needed before any treatment iscarried out. If the patient has COPD, attempting to increaseoxygenation could disable the patient’s respiratory drivecenters in the brain.

Scenario 3While undergoing surgery via general anesthesia, the pa-tient’s pH is noted to be 7.45, PaCO2 is 32 mm Hg, PaO2 is138 mm Hg, HCO3

� is 23 mEq/L, base deficit is 1 mEq/L,and SaO2 is 92%.

� In step 1, the nurse determines that the pH indicates alka-losis. The patient’s PaCO2 is in the alkalotic range, while theHCO3

� is at the low end of the normal range.� In step 2, the nurse evaluates for respiratory or metaboliccomponents and determines that the patient is in respiratoryalkalosis.

� The consistent value (step 3) is the PaCO2, which is belownormal range and is consistent with the alkalotic pH value.

� Because the HCO3� is in the normal range, there is no

compensation (step 4), and the base deficit and SaO2 areboth in normal range.

Based on these results, the nurse determines that this patientis in respiratory alkalosis with no compensation. To correctthis, the anesthesia professional may attempt to overcomethe alkalosis by hyperventilating the patient (as indicated bythe high PaO2) or may gather more data to determinewhether the hyperventilation of the patient may be causingthe respiratory alkalosis. More data are needed to guidefurther intervention.

Scenario 4The ABG results indicate that the patient’s pH is 7.27, PaCO2

is 55 mm Hg, PaO2 is 93 mm Hg, HCO3� is 41 mEq/L, base

excess/base deficit is 10 mEq/L, and SaO2 is 82%.

� Based on these results, the nurse determines that the pa-tient’s pH is acidotic (step 1).

� The elevated PaCO2 indicates acidosis, and the elevatedHCO3

� indicates alkalosis (step 2).� The value that is consistent (step 3) with the pH is thePaCO2; this indicates that acidosis is from a respiratorycause.

� The HCO3� value is not consistent with the pH and is in

the alkalotic range, so there is a metabolic attempt tocompensate (step 4).

However, compensation is only partial because the pH has notreturned to normal. The nurse determines that this patient isin respiratory acidosis with partial metabolic compensation;




therefore, the nurse knows that the patient has a pre-existingrespiratory condition because it takes more than three daysfor metabolic compensation efforts to be effective. Althoughthe patient’s PaO2 is in the normal range (because of sup-plemental O2), the patient’s saturation is below normal. Thisindicates that the cause of the respiratory acidosis is ventilationmismatch, such as occurs in obstructive lung disease.

Case StudiesUsing the Romanski method of analysis, the following ex-amples walk the nurse through the process of identifying apatient’s condition.

Case Study OneMr F is an 83-year-old man with the following comor-bidities: aortic valve disease with systemic hypertension,dyslipidemia, and mild to moderate mitral regurgitation.He has good functional capacity and is able to walk withoutdyspnea, angina, or dizziness, but gets tired in the eveningand is taking more naps. According to his wife, he is moretired and has episodes of confusion with slight memoryimpairment. He is well nourished and his baseline vitalsigns are within normal limits. His physicians have opti-mized his medical status to prepare him for an aortic valvereplacement with myocardial revascularization. After theprocedure, the surgical team transfers him to the intensivecare unit with an endotracheal tube in place to maintain hisairway, and they place him on a ventilator. The surgeonorders an arterial blood gas (ABG) to be drawn, whichshows the following results.

Blood GasMeasure Values Acidotic? Alkalotic? Normal?

pH 7.44 Yes

Partial Pressureof CarbonDioxide(PaCO2)

36 mm Hg Yes

Partial Pressureof Oxygen(PaO2)

349 mm Hg No, onventilator

Concentrationof Bicarbonate(HCO3

�)

24 mEq/L Yes

Base Deficit/Base Excess

0 mEq/L Yes

OxygenSaturation(SaO2)

99% Yes

Using the Romanski method,1 the RN interprets theABG analysis.

� Step 1 (evaluate pH): pH is in the normal range; using7.40 as the cutoff point, the pH value indicates alkalosis.

� Step 2 (evaluate respiratory and metabolic compo-nents): The PaCO2 is low, indicating alkalosis; theHCO3

� is normal.� Step 3 (determine consistent value): The value that isconsistent with the pH is the PaCO2; this indicates thatacidosis is from a respiratory cause.

� Step 4 (determine compensation): The HCO3� value is

not consistent with the pH and is normal, so there is nometabolic attempt to compensate. The base deficit/baseexcess and SaO2 are both normal.

Results: This patient is in respiratory alkalosis with nocompensation.

Interpretation: The PaO2 setting and/or the rate ofventilation must be corrected by the nurse and physicianmanaging the patient to correct the acid-base imbalancewithout affecting SaO2 by managing the settings on theventilator to achieve desired outcomes. Additionally, thenurse should consider that the patient’s hyperventilationcould be the result of anxiety from being intubated post-operatively, causing ventilator hyperventilation, or it couldbe the result of pain from the surgical incision.

1. Romanski SO. Interpreting ABGs in four easy steps. Nursing.1986;16(9):58-64.

Case Study TwoMs A is a 78-year-old woman suspected by her gynecol-ogist of having a rectovaginal fistula. The gynecologistrefers her to a colorectal surgeon for fistula repair. Inaddition to the suspected fistula, she has the followingsignificant comorbidities: well-controlled diabetes, hy-pertension, hyperlipidemia, emphysema, protein-caloriemalnutrition, history of malignant neoplasm of the thy-roid for which she takes daily levothyroxine, and historyof neoplasm of the uterus and cervix. The colorectalsurgeon performs an exploratory laparotomy with lysis ofadhesions and small-bowel resection. Before surgery, thepatient exhibits extreme anxiety with rapid respiratory rate(ie, hyperventilation). The anesthesia professional ordersmidazolam 2 mg IV for anxiety reduction and orders anarterial blood gas (ABG) analysis to be drawn, whichshows the following results.



http://refhub.elsevier.com/S0001-2092(15)00700-0/sref1




� Step 1 (evaluate pH): This patient’s pH is in the normalrange; using 7.40 as the cutoff, the patient is acidotic.

� Step 2 (evaluate respiratory and metabolic compo-nents): PaCO2 indicates acidosis; HCO3

� is in thealkalotic range.

� Step 3 (determine consistent value): PaCO2 is consis-tent with the pH; therefore, there is respiratory acidosis.

� Step 4 (determine compensation): HCO3� is in the

alkalotic range and the pH is in the normal range,resulting in full compensation. The base deficit/baseexcess is normal and SaO2 is low.

Results: This patient is in respiratory acidosis with fullmetabolic compensation.

Interpretation: Because the patient’s SaO2 is low and herPaO2 value is nearing the critical point, she may needsupplemental O2 to correct the SaO2 toward normal, butnot enough O2 that it deters her from taking spontaneousrespirations postoperatively. The determination to givesupplemental O2 should be made in light of her preoper-ative need for O2 because of her emphysema and baselinehypoxic respiratory drive. Additionally, the 2 mg of mid-azolam given for anxiety reduction preoperatively couldhave resulted in a decreased respiratory drive. The nurse willneed to monitor this patient’s respiratory status carefullyboth preoperatively and postoperatively.


Case Study ThreeMr W is a 78-year-old man with a history of recent pul-monary embolism and cardiovascular stent placement forcoronary artery disease. Other significant comorbiditiesinclude bronchoalveolar carcinoma, surgically treated bypartial lung removal of the left upper lobe, and seizuredisorder. He presents at an emergency department withsudden vision changes in his left eye. Computed tomog-raphy reveals an occipital mass, and the emergency physi-cian refers him to a neurosurgeon. Mr W undergoes surgeryfor right occipital craniotomy to remove the brain lesion, hehas an episode of respiratory distress and complains of chestpain. The nurse acts on an existing order to have a respi-ratory therapist draw an arterial blood gas (ABG), whichshows the following results.


� Step 1 (evaluate pH): The pH value indicates alkalosis.� Step 2 (evaluate respiratory and metabolic compo-nents): PaCO2 indicates alkalosis; HCO3

� is in thenormal range.

� Step 3 (determine consistent value): PaCO2 is consis-tent with the alkalotic pH.

� Step 4 (determine compensation): The value that is notconsistent with the pH is HCO3

�, and it is within thenormal range, so there is no compensation occurring. Thebase deficit/base excess and SaO2 are normal.

Blood GasMeasure Values Acidotic? Alkalotic? Normal?

pH 7.46 Yes


34 mm Hg Yes


86 mm Hg Yes


�)

24 mEq/L Yes


1 mEq/L Yes


97% Yes

Blood Gas Measure Values Acidotic? Alkalotic? Normal?

pH 7.37 Yes


58 mm Hg Yes


65 mm Hg No

Concentration ofBicarbonate(HCO3

�)

29 mEq/L Yes


0 mEq/L


87% No






Analysis of ABGs and Interventions forPerioperative RNsMajor procedures are not the only instances during whichblood gas analyses are important. In any patient with pul-monary disease (eg, COPD, severe asthma, interstitial lungdisease), a blood gas analysis also may be critical. Forexample, patients with severe COPD may have an elevatedbaseline PaCO2. Decreasing the patient’s PaCO2 to normalperioperatively may suppress respiratory drive. This affectsthe anesthesia professional’s ability to extubate the patientat the end of the procedure because the patient would lackthe “normal” hypercapneic drive to breathe, which is higherthan for healthy adults. This may necessitate the need forventilator support in the postanesthesia care unit or theintensive care unit. In the postanesthesia care unit or theintensive care unit, blood gas measurement can provideimportant insight into the etiology of a patient’s alteredmental status or varied respiratory pattern. The case studiesgive the reader an opportunity to practice ABG analysisusing the Romanski method and increase confidence inclinical ABG analysis.

Results: This patient is in respiratory alkalosis with nocompensation.

Interpretation: This scenario can be caused by hyperven-tilation associated with pain and/or anxiety. The respiratorydistress the patient is experiencing also may be related to thecomplaint of chest pain and not an oxygenation/ventilationmismatch from the previous lung surgery or a pulmonaryembolism. If the patient is hyperventilating, the nurseshould take measures to calm and reassure the patient tohelp him slow his breathing. Additionally, assessing for painand obtaining analgesic medication orders is also an option.


Case Study FourMs P is a 67-year-old woman with recent episodes ofpostmenopausal vaginal bleeding and the followingcomorbidities: cerebral palsy with chronic pain and spas-ticity, neurogenic bladder, hypothyroidism, hyperlipidemia,hypercholesterolemia, peripheral artery disease, systolicheart failure, chronic hypokalemia, hypomagnesemia,controlled type 2 diabetes, mitral valve disorder, and acuteanemia. Her physician has cleared her to undergo a totalabdominal hysterectomy with bilateral salpingo-oophorectomy and possible pelvic and periaortic lympha-denectomy. The anesthesia professional draws an arterialblood gas (ABG) during the first 30 minutes of the pro-cedure, which shows the following results.


� Step 1 (evaluate pH): The pH value clearly indicatesalkalosis.

Blood Gas Measure Values Acidotic? Alkalotic? Normal?

pH 7.49 Yes


39 mm Hg Yes


249 mm Hg Elevated


�)

30 mEq/L Yes


6 mEq/L Elevated

Oxygen Saturation(SaO2)

99% Yes

� Step 2 (evaluate respiratory and metabolic compo-nents): PaCO2 is in the normal range; HCO3

� is alkalotic.� Step 3 (determine consistent value): The HCO3

� valueis consistent with the pH, so the cause of imbalance ismetabolic.

� Step 4 (determine compensation): PaCO2 is in thenormal range, so there is no compensation occurring.Base excess is elevated and SaO2 is normal.

Results: This patient is in metabolic alkalosis with nocompensation.

Intervention: There is presence of a base excess, which in-dicates that the patient may need fluid resuscitation and/orblood products to correct the acute anemia associated withthe vaginal blood losses. The RN circulator should be readyto assist the anesthesia professional by helping to establish asecondary IV access or an arterial line. Circulating volumemay need to be replaced with either IV lactated Ringer’ssolution or blood products. Other interventions to considerinclude reviewing the patient’s history for recent episodes ofvomiting, nasogastric tube placement with suction, anddiuretic use leading to volume and electrolyte depletion.Other diagnostic tests may include checking the patient’shematocrit, basic chemistry panel, and blood glucose.









CONCLUSIONAlthough the need for accurate interpretation of ABGs may berare in some OR settings, it is essential for the perioperativeRN circulator to be able to quickly assess and interpret ABGresults when blood gas reports arrive in the OR. These skillshelp the perioperative RN assist the entire surgical team,especially the anesthesia professional, in helping the patientmaintain acid-base balance. The perioperative nurse shouldanticipate the need for ABGs in patients with pre-existingrespiratory compromise, such as COPD, or metaboliccompromise, such as hypermetabolic states (eg, malignanthyperthermia), as well as in patients undergoing trauma,oncological, cardiothoracic, and neurological or other lengthyprocedures. Understanding the interpretation of ABGs helpsensure that the perioperative RN is prepared to respond tocritical acid-base imbalances and provide appropriateinterventions. �Acknowledgment: The authors acknowledge Holly Schmidtke,MBA, BSN, RN, CNML, chief nurse executive, Aurora HealthCare, Milwaukee, WI, and Steven Mayo, MD, chief of medicalstaff and anesthesia for the Burlington-Walworth Patients ServiceMarket of Aurora Health Care, Milwaukee, WI, for support forthis article.

References1. Leviticus 17:11. In: Thompson FC, ed. Thompson’s Chain-

Referenced Bible. New International Version. Grand Rapids, MI:Zondervan Bible Publishers; 1982:118.

2. Common Laboratory (Lab) ValuesdABGs. Globalrph. http://www.globalrph.com/abg_analysis.htm. Accessed June 30, 2015.

3. Shoulders-Odom B. Using an algorithm to interpret arterial bloodgases. Dimens Crit Care Nurs. 2000;19(1):36-41.

4. Wallace LS. Using color to simplify ABG interpretation. MedsurgNurs. 2000;9(4):205-207.

5. Wong FW. A new approach to ABG interpretation. Am J Nurs.1999;99(8):34-36.


7. Davis JW, Shackford SR, Mackersie RC, Hoyt DB. Base deficit as aguide to volume resuscitation. J Trauma. 1988;28(10):1464-1467.

8. Neligan PJ, Deutschman CS. Perioperative acid-base balance. In:Miller RD, ed. Miller’s Anesthesia. 8th ed. Philadelphia, PA:Elsevier Saunders; 2015:1811-1829.

9. Acid-base regulation. In: Hall JE, ed. Pocket Companion to Guytonand Hall Textbook of Medical Physiology. 12th ed. Philadelphia,PA: Elsevier Saunders; 2012:230-237.

10. Metabolic acidosis. Medscape. http://emedicine.medscape.com/article/243160-overview. Accessed June 30, 2015.

11. The pulmonary system. In: McCance KL, Huether SE, eds. Path-ophysiology: The Biologic Basis for Diseases in Adults andChildren. 7th ed. Philadelphia, PA: Elsevier Mosby; 2014:1229-1232.

12. Horne C, Derrico D. Mastering ABGs. The art of arterial blood gasmeasurement. Am J Nurs. 1999;99(8):26-32.

13. Pathophysiology of shock. In: Vincent JL, Abraham E, Moore FA,Kohanek PM, Fink MP, eds. Textbook of Critical Care. Philadelphia,PA: Elsevier Saunders; 2011:677-683.

14. Pruitt WC, Jacobs M. Interpreting arterial blood gases: easy asABC. Nursing. 2004;34(8):50-53.

Brenda G. Larkin, MS, RN, ACNS-BC, CNS-CP,CNOR, is a perioperative and gastrointestinal clinicalnurse specialist for Aurora Health Care, Burlington-Walworth Patient Service Market, Milwaukee, WI. MsLarkin has no declared affiliation that could be perceivedas posing a potential conflict of interest in the publicationof this article.

Robert J. Zimmanck, MD, is a staff anesthesiologistat Aurora Health Care, Milwaukee, WI. Dr Zimmanck hasno declared affiliation that could be perceived as posinga potential conflict of interest in the publication of thisarticle.
























http://emedicine.medscape.com/article/243160-overview

http://emedicine.medscape.com/article/243160-overview













EXAMINATION

Continuing Education:Interpreting Arterial Blood GasesSuccessfully 2.6 www.aorn.org/CE

PURPOSE/GOALTo provide the learner with knowledge specific to interpreting arterial blood gases (ABGs).

OBJECTIVES1. Explain what ABGs are.2. Discuss what ABGs measure.3. Discuss how acidosis and alkalosis may be identified using ABG results.4. Explain how the nurse can determine whether respiratory or metabolic factors are causing an imbalance.

The Examination and Learner Evaluation are printed here for your convenience. To receivecontinuing education credit, you must complete the online Examination and Learner Evaluationat http://www.aorn.org/CE.

QUESTIONS1. Clinical interpretation of elements carried in the blood is

essential to health care providers’ ability to maintain pa-tients’ lives and homeostasis when providing care.a. true b. false

2. Two events that trigger the need for ABG results are1. shifts in calcium levels.2. shifts in fluid levels.3. major blood loss.

a. 1 and 2 b. 1 and 3c. 2 and 3 d. 1, 2, and 3

3. Arterial blood gas test results include measurements of1. the concentration of hydrogen ions present in the

blood (pH).2. oxygen saturation (SaO2).3. partial pressure of oxygen (PaO2).4. partial pressure of carbon dioxide (PaCO2).5. the concentration of bicarbonate (HCO3

�).6. base excess and base deficit.

a. 1, 3, and 5 b. 2, 4, and 6c. 2, 3, 5, and 6 d. 1, 2, 3, 4, 5, and 6

4. In addition to pH, blood gases provide data about1. the adequacy of a patient’s oxygenation.2. disturbances in homeostasis.3. levels of carbon monoxide in the blood.4. whether the disturbances are respiratory or metabolic.

a. 1 and 3 b. 2 and 4c. 1, 2, and 4 d. 1, 2, 3, and 4

5. Arterial blood gas results can indicate how effectively thepatient’s body is compensating for the acid-base distur-bance and whether the patient’s total blood volumeis adequate for transporting the substances measured bythe test.a. true b. false

6. The results of an ABG test can be particularly helpful in1. procedures in which large fluid shifts or blood loss

are expected.2. procedures that require fluid resuscitation.3. procedures of the oropharynx.4. procedures in which one-lung ventilation is used.






7. As CO2 increases, it forces the hydrogen ion concentrationto increase and the patient’s blood becomes more alkalotic.a. true b. false

8. What might be seen in the ABG results of a patient withalkalosis?1. A decrease in CO2.2. An increase in CO2.3. A pH of 7.5 or higher.4. A pH of 7.4 or lower.


9. Which element represents metabolic changes in acid-basestatus?a. Intracellular HCO3

�.b. Partial pressure of PaCO2.c. Base excess.d. Extracellular HCO3

L.

10. Which element represents respiratory changes in acid-base status?a. pH. b. PaO2.c. Base excess. d. PaCO2.




LEARNER EVALUATION

Continuing Education:Interpreting Arterial Blood GasesSuccessfully 2.6 www.aorn.org/CE

This evaluation is used to determine the extent towhich this continuing education program met yourlearning needs. The evaluation is printed here for

your convenience. To receive continuing education credit, youmust complete the online Examination and Learner Evaluationat http://www.aorn.org/CE. Rate the items as described below.

OBJECTIVESTo what extent were the following objectives of thiscontinuing education program achieved?1. Explain what arterial blood gases (ABGs) are.

Low 1. 2. 3. 4. 5. High

2. Discuss what ABGs measure.Low 1. 2. 3. 4. 5. High

3. Discuss how acidosis and alkalosis may be identifiedusing ABG results.Low 1. 2. 3. 4. 5. High

4. Explain how the nurse can determine whether respiratoryor metabolic factors are causing an imbalance.Low 1. 2. 3. 4. 5. High

CONTENT5. To what extent did this article increase your knowledge of

the subject matter?Low 1. 2. 3. 4. 5. High

6. To what extent were your individual objectives met?Low 1. 2. 3. 4. 5. High

7. Will you be able to use the information from this articlein your work setting?1. Yes 2. No

8. Will you change your practice as a result of reading thisarticle? (If yes, answer question #8A. If no, answerquestion #8B.)

8A. How will you change your practice? (Select all thatapply)1. I will provide education to my team regarding why

change is needed.2. I will work with management to change/implement

a policy and procedure.3. I will plan an informational meeting with physicians

to seek their input and acceptance of the need forchange.

4. I will implement change and evaluate the effect ofthe change at regular intervals until the change isincorporated as best practice.

5. Other: __________________________________

8B. If you will not change your practice as a result ofreading this article, why? (Select all that apply)1. The content of the article is not relevant to my

practice.2. I do not have enough time to teach others about the

purpose of the needed change.3. I do not have management support to make a

change.4. Other: __________________________________

9. Our accrediting body requires that we verify the timeyou needed to complete the 2.6 continuing educationcontact hour (156-minute) program: _____________





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