Arterial blood gas

ByMrs. Amala Rajan

Reader

Medical Nursing Dept

“Life is a struggle, not against sin, not against the Money Power, not against malicious animal magnetism, but against hydrogen ions." H.L. MENCKEN

What is an ABGArterial Blood Gas

Drawn from artery- radial, brachial, femoral

It is an invasive procedure.

Caution must be taken with patient on anticoagulants.

Arterial blood gas analysis is an essential part of diagnosing and managing the patient’s oxygenation status, ventilation failure and acid base balance.

What Is An ABG?pH [H+]

PCO2 Partial pressure CO2

PO2 Partial pressure O2

HCO3 Bicarbonate

BE Base excess

SaO2 Oxygen Saturation

Acid/Base Balance The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to the no. of (H+) in the blood. The normal pH range is 7.35-7.45.Changes in body system functions that occur in an acidic state decreases the force of cardiac contractions, decreases the vascular response to catecholamines, and a diminished response to the effects and actions of certain medications.An alkalotic state interferes with tissue oxygenation and normal neurological and muscular functioning.Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.

H2O + CO2 H2CO3 HCO3

+ H+

Acid/Base Relationship

There are two buffers that work in pairs

H2CO3 NaHCO3

Carbonic acid base bicarbonate These buffers are linked to the respiratory and renal compensatory system

Buffers

The Respiratory buffer response

• The blood pH will change acc.to the level of H2CO3 present.

• This triggers the lungs to either increase or decrease the rate and depth of ventilation

• Activation of the lungs to compensate for an imbalance starts to occur within 1-3 minutes

The Renal Buffer Response

•The kidneys excrete or retain bicarbonate(HCO3-).

• If blood pH decreases, the kidneys will compensate by retaining HCO3

•Renal system may take from hours to days to correct the imbalance.

ACID BASE DISORDERRes. Acidosis• is defined as a pH less than 7.35

with a pCO2 greater than 45 mmHg.• Acidosis –accumulation of co2,

combines with water in the body to produce carbonic acid, thus lowering the pH of the blood.

• Any condition that results in hypoventilation can cause respiratory acidosis.

Causes 1. Central nervous system depression r/t

medications such as narcotics, sedatives, or anesthesia.

2. Impaired muscle function r/t spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs.

3. Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema or bronchial obstruction

4. Massive pulmonary embolus5. Hypoventilation due to pain chest wall

injury, or abdominal pain.

Signs & symptoms of Respiratory Acidosis

• Respiratory : Dyspnoea, respiratory distress and/or shallow respiration.

• Nervous: Headache, restlessness and confusion. If co2 level extremely high drowsiness and unresponsiveness may be noted.

• CVS: Tacycardia and dysrhythmias

Management• Increase the ventilation. • Causes can be treated rapidly

include pneumothorax, pain and CNS depression r/t medication.

• If the cause can not be readily resolved, mechanical ventilation.

Respiratory alkalosis

• Psychological responses, anxiety or fear.• Pain• Increased metabolic demands such as

fever, sepsis, pregnancy or thyrotoxicosis.• Medications such as respiratory

stimulants.• Central nervous system lesions

Signs & symptoms• CNS: Light Headedness, numbness,

tingling, confusion, inability to concentrate and blurred vision.

• Dysrhythmias and palpitations• Dry mouth, diaphoresis and tetanic

spasms of the arms and legs.

Management

• Resolve the underlying problem• Monitor for respiratory muscle

fatigue• When the respiratory muscle

become exhausted, acute respiratory failure may ensue

Metabolic Acidosis

• Bicarbonate less than 22mEq/L with a pH of less than 7.35.

• Renal failure• Diabetic ketoacidosis• Anaerobic metabolism• Starvation• Salicylate intoxication

Sign & symptoms

• CNS: Headache, confusion and restlessness progressing to lethargy, then stupor or coma.

• CVS: Dysrhythmias• Kussmaul’s respirations• Warm, flushed skin as well as nausea

and vomiting

Management

• Treat the cause• Hypoxia of any tissue bed will produce

metabolic acids as a result of anaerobic metabolism even if the pao2 is normal

• Restore tissue perfusion to the hypoxic tissues

• The use of bicarbonate is indicated for known bicarbonate - responsive acidosis such as seen with renal failure

Metabolic alkalosis

• Bicarbonate more than 26m Eq /L with a pH more than 7.45

• Excess of base /loss of acid can cause• Ingestion of excess antacids, excess use of

bicarbonate, or use of lactate in dialysis.• Protracted vomiting, gastric

suction,hypchoremia,excess use of diuretics, or high levels of aldesterone.

Signs/symptoms

• CNS: Dizziness, lethargy disorientation, siezures & coma.

• M/S: weakness, muscle twitching, muscle cramps and tetany.

• Nausea, vomiting and respiratory depression.

• It is difficult to treat.

COMPONENTS OF THE ABG

pH: Measurement of acidity or alkalinity, based on the hydrogen (H+)

7.35 – 7.45Pao2 The partial pressure oxygen that is dissolved in arterial blood.

80-100 mm Hg. PCO2: The amount of carbon dioxide dissolved in arterial blood.

35– 45 mmHgHCO3

: The calculated value of the amount of bicarbonate in the blood 22 – 26 mmol/L

B.E: The base excess indicates the amount of excess or insufficient level of bicarbonate. -2 to +2mEq/L(A negative base excess indicates a base deficit in blood)

SaO2:The arterial oxygen saturation. >95%

Stepwise approach to ABG

• Step 1: Acidemic or Alkalemic? • Step 2: Is the primary disturbance

respiratory or metabolic? • Step 3. Asses to Pa O2. A value below 80mm

Hg indicates Hypoxemia. For a respiratory disturbance, determine whether it is acute or chronic.

• Step 4. For a metabolic acidosis, determine whether an anion gap is present.

• Step 5. Assess the normal compensation by the respiratory system for a metabolic disturbance

STEPS TO AN ABG INTERPRETATION

•Step:1•Assess the pH

–acidotic/alkalotic•If above 7.5 – alkalotic•If below 7.35 – acidotic

Contd…..• Step 2:• Assess the paCO2 level.• pH decreases below 7.35, the paCO2

should rise.• If pH rises above 7.45 paCO2 should

fall.• If pH and paCO2 moves in opposite

direction – primary respiratory problem.

contd

• Step:2

• Assess HCO3 value

• If pH increases the HCO3 should also increase

• If pH decreases HCO3 should also decrease

• They are moving in the same direction• primary problem is metabolic

• Step 3Assess pao2 < 80 mm Hg - HypoxemiaFor a resp. disturbance : acute, chronic

The differentiation between A/C & CHR.respiratory disorders is based on whether there is associated acidemia / alkalemia.

If the change in paco2 is associated with the change in pH, the disorder is acute.

In chronic process the compensatory process brings the pH to within the clinically acceptable range ( 7.30 – 7.50)

• J is a 45 years old female admitted with the severe attack of asthma. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows:

• pH : 7.22• paCO2 : 55• HCO3 : 25• Follow the steps• pH is low – acidosis• paCO2 is high – in the opposite direction of the pH. • Hco3 is Normal.• Respiratory Acidosis• Need to improve ventilation by oxygen

therapy, mechanical ventilation, pulmonary toilet or by administering bronchodilators.

• EXAMPLE 2: Mr. D is a 55 years old admitted with recurring bowel obstruction has been experiencing intractable vomiting for the last several hours. His ABG is:

• pH : 7.5• paCO2 :42• HCO3 : 33•Metabolic alkalosis•Management: IV fluids,

measures to reduce the excess base

Respiratory acidosis

pH PaCo2 HC03

normal

Respiratory

Alkalosis

normal

Metabolic Acidosis

normal

Metabolic Alkalosis

normal

BASE EXCESS

• Is a calculated value estimates the metabolic component of an acid based abnormality.

• It is an estimate of the amount of strong acid or base needed to correct the met. component of an acid base disorder (restore plasma pH to 7.40at a Paco2 40 mmHg)

Formula• With the base excess is -10 in a 50kg

person with metabolic acidosis mM of Hco3 needed for correction is:

= 0.3 X body weight X BE = 0.3 X 50 X10 = 150 mM

Anion GAP

Step 4• Calculation of AG is useful approach to

analyse metabolic acidosis AG = (Na+ + K+) – (cl- + Hco3-)• * A change in the pH of 0.08 for each 10

mm Hg indicates an ACUTE condition.* A change in the pH of 0.03 for each 10 mm Hg indicates a CHRONIC condition.

• K etoacidosis• U remia• S epsis• S alicylate & other drugs• M ethanol• A lcohol (Ethanol)• L actic acidosis• E thylene glycol

REMEMBER

COMPENSATION• Step 5• A patient can be uncompensated or

partially compensated or fully compensated

• pH remains outside the normal range• pH has returned within normal range-

fully compensated though other values may be still abnormal

• Be aware that neither the system has the ability to overcompensate

Step 5 cont…Determine if there is a compensatory mechanism working to try to correct the pH.

ie: if have primary respiratory acidosis will have increased PaCO2 and decreased pH. Compensation occurs when the kidneys retain HCO3.

ABG Interpretation

Assess the PaCO2

• In an uncompensated state – when the pH and paCO2 moves in the same direction: the primary problem is metabolic.

• The decreasing paco2 indicates that the lungs acting as a buffer response (blowing of the excess CO2)

• If evidence of compensation is present but the pH has not been corrected to within the normal range, this would be described as metabolic disorder with the partial respiratory compensation.

Assess the HCO3

• The pH and the HCO3 moving in the opposite directions, we would conclude that the primary disorder is respiratory and the kidneys acting as a buffer response: are compensating by retaining HCO3 to return the pH to normal range.

Example 3• Mrs. H is admitted, he is kidney

dialysis patient who has missed his last 2 appointments at the dialysis centre his ABG results:

• pH : 7.32• paCo2 : 32• HCO3 : 18• Pao2 : 88• Partially compensated metabolic

Acidosis

Example 4• Mr. K with COPD.His ABG is:• pH : 7.35

• PaCO2 : 48

• HCO3 : 28

• PaO2 : 90

• Fully compensated Respiratory Acidosis

Example 5• Mr. S is a 53 year old man presented

to ED with the following ABG.• pH : 7.51• PaCO2 : 50• HCO3 : 40• Pao2 : 40 (21%O2)• He has metabolic alkalosis• Acute respiratory alkalosis

(acute hyperventilation).

FULLY COMPENSATEDpH paco2 Hco3

Resp.Acidosis Normal

but<7.40

Resp.Alkalosis Normal

but>7.40

Met. Acidosis Normal

but<7.40

Met. Alkalosis Normal

but>7.40

Partially compensated

pH paco2 Hco3

Res.Acidosis

Res.Alkalosis

Met. Acidosis

Met.Alkalosis

~ PaCO2 – pH Relationship

80 7.20

60 7.30

40 7.40

30 7.50

20 7.60

Precautions Excessive Heparin Decreases bicarbonate

and PaCO2

Large Air bubbles not expelled from sample PaO2 rises, PaCO2 may fall slightly.

Fever or Hypothermia, Hyperventilation or breath holding (Due to anxiety) may lead to erroneous lab results

Care must be taken to prevent bleeding

2SD NORMAL CL.ACCEPTABLE

• PH 7.35 – 7.45 7.30 – 7.50

• PCO2 35 – 45 30 – 50

• PO2 97 >80 (ON 21% O2)

(ON VENTILATOR) 60 – 90

• HCO3 24 - 28

Take Home Message: Valuable information can be gained from an ABG as to the patients physiologic condition

Remember that ABG analysis if only part of the patient assessment. Be systematic with your analysis, start with ABC’s as always and look for hypoxia (which you can usually treat quickly), then follow the four steps. A quick assessment of patient oxygenation can be achieved with a pulse oximeter which measures SaO2.

It’s not magic understanding ABG’s, it just

takes a little practice!

Practice ABG’s

1. PaO2 90 SaO2 95 pH 7.48 PaCO2 32 HCO3 242. PaO2 60 SaO2 90 pH 7.32 PaCO2 48 HCO3 253. PaO2 95 SaO2 100 pH 7.30 PaCO2 40 HCO3 184. PaO2 87 SaO2 94 pH 7.38 PaCO2 48 HCO3 285. PaO2 94 SaO2 99 pH 7.49 PaCO2 40 HCO3 306. PaO2 62 SaO2 91 pH 7.35 PaCO2 48 HCO3 277. PaO2 93 SaO2 97 pH 7.45 PaCO2 47 HCO3 298. PaO2 95 SaO2 99 pH 7.31 PaCO2 38 HCO3 159. PaO2 65 SaO2 89 pH 7.30 PaCO2 50 HCO3 2410. PaO2 110 SaO2 100 pH 7.48 PaCO2 40 HCO3 30

What is going on?

Answers to Practice ABG’s

1. Respiratory alkalosis2. Respiratory acidosis3. Metabolic acidosis4. Compensated Respiratory acidosis5. Metabolic alkalosis6. Compensated Respiratory acidosis7. Compensated Metabolic alkalosis8. Metabolic acidosis9. Respiratory acidosis10.Metabolic alkalosis

Any Questions?