the basics of blood gas and acid-base

The Basics of Blood The Basics of Blood Gas and Acid-baseGas and Acid-base

Kristen Hibbetts, DVM, DACVIM, Kristen Hibbetts, DVM, DACVIM, DACVECCDACVECC

VetStatVetStat

Measures 3 categories of resultsMeasures 3 categories of results

– (Chemistry) Electrolytes(Chemistry) Electrolytes– Blood gasesBlood gases– Acid-base statusAcid-base status

ElectrolytesElectrolytes

Electrolytes keep the cells functioningElectrolytes keep the cells functioning We pay the most attention toWe pay the most attention to- NaNa++, K, K++, Cl, Cl-- and HCO and HCO33

--

Na+

KK++

Cl-

HCO3-


Sodium (NaSodium (Na++): maintains plasma volume ): maintains plasma volume (osmolality) and blood pressure(osmolality) and blood pressure

Potassium (KPotassium (K++): important for cell membrane ): important for cell membrane excitabilityexcitability

Chloride (ClChloride (Cl--): moves with sodium to maintain ): moves with sodium to maintain plasma volume, and important in acid-base plasma volume, and important in acid-base regulationregulation

Bicarbonate (HCOBicarbonate (HCO33--) helps “buffer” changes in pH) helps “buffer” changes in pH

Total CO2 (TCOTotal CO2 (TCO22): 97% HCO): 97% HCO33--, 3% dissolved gases; , 3% dissolved gases;

reflects HCOreflects HCO33-- when respiratory function is normal when respiratory function is normal


We must maintain normal levels of We must maintain normal levels of electrolytes in our blood to maintain normal electrolytes in our blood to maintain normal cell functioncell function

Clinicians can alter their fluid administration Clinicians can alter their fluid administration to either add or dilute certain electrolytesto either add or dilute certain electrolytes

Blood GasesBlood Gases

Blood gases are literally gases (OBlood gases are literally gases (O2 2 and and

COCO22) that circulate around in our blood) that circulate around in our blood

We measure oxygen (pOWe measure oxygen (pO22) and carbon ) and carbon

dioxide (pCOdioxide (pCO22))

The “p” stands for partial pressure, and we The “p” stands for partial pressure, and we measure it in mmHgmeasure it in mmHg

Blood GasesBlood Gases

Oxygen (OOxygen (O22))

– Oxygen is what our cells use to live off of Oxygen is what our cells use to live off of (no oxygen = cell death)(no oxygen = cell death)

– Measuring p0Measuring p022 tells us if there is enough tells us if there is enough

oxygen circulating around for cells to oxygen circulating around for cells to survivesurvive

Blood GasesBlood Gases Carbon dioxide (COCarbon dioxide (CO22))

– Carbon dioxide is what is left over when Carbon dioxide is what is left over when the cell uses the oxygenthe cell uses the oxygen

– The job of the lungs is to breath in the The job of the lungs is to breath in the oxygen and breath out the carbon dioxideoxygen and breath out the carbon dioxide

CO2

O2

Blood Gas ParametersBlood Gas Parameters

Oxygen (OOxygen (O22))

– normal pnormal paaOO22 = >85 mmHg = >85 mmHg

– if pif paaOO22 < 80 mmHg, provide O < 80 mmHg, provide O22 support support

– if pif paaOO22 < 60 mmHg while on O < 60 mmHg while on O22 support, support,

consider ventilator therapyconsider ventilator therapy

– To be accurately assessed, pOTo be accurately assessed, pO22 must be must be

measured from an measured from an arterialarterial sample sample

Blood Gas ParametersBlood Gas Parameters

Carbon dioxide (COCarbon dioxide (CO22))– normal pCOnormal pCO22 = 35-45 mmHG = 35-45 mmHG– if pCOif pCO22 < 35 mmHg then is hyperventilation < 35 mmHg then is hyperventilation– if pCOif pCO22 > 45 mmHg then is hypoventilation > 45 mmHg then is hypoventilation– if pCOif pCO22 > 60 mmHg, consider ventilator > 60 mmHg, consider ventilator

therapytherapy

– Can be appropriately measured on venous Can be appropriately measured on venous or arterial sampleor arterial sample

Acid-base BalanceAcid-base Balance

The acidity of the blood is measured as The acidity of the blood is measured as pHpH

The blood has a very specific pH range The blood has a very specific pH range where everything works adequatelywhere everything works adequately

pH = 7.34 – 7.44pH = 7.34 – 7.44


pH is maintained by multiple methods:pH is maintained by multiple methods:

– use of a buffer system consisting of HCOuse of a buffer system consisting of HCO33--

and COand CO22

– maintenance of electroneutrality (same maintenance of electroneutrality (same number of positive and negative charged number of positive and negative charged particles)particles)

Henderson-Hasselbalch EquationHenderson-Hasselbalch Equation

pH = ppH = pKKaa + log + log [salt][salt]//[acid][acid]

pH = 6.1 + log pH = 6.1 + log [HCO3-][HCO3-]//0.3pCO0.3pCO22

pH is a function of the ratio of the HCOpH is a function of the ratio of the HCO33--

and the pCOand the pCO22

Henderson-Hasselbalch EquationHenderson-Hasselbalch Equation

Derived:Derived:

COCO22 + H + H2200 H H22COCO33 H H++ + HCO + HCO33--

Carbonic Acid Buffer SystemCarbonic Acid Buffer System

Derived:Derived:


respiratoryrespiratory metabolic metabolic

controlcontrol control control


To maintain the blood pH:To maintain the blood pH:

– Kidneys will alter [HCO3-]Kidneys will alter [HCO3-]

– Lungs will alter pCO2Lungs will alter pCO2


When there is an abnormality in the When there is an abnormality in the blood pH, we can often blame it on blood pH, we can often blame it on either:either:

– an abnormality in the [HCOan abnormality in the [HCO33--] or] or

– an abnormality in the pCOan abnormality in the pCO22

Primary Acid-Base AbnormalitiesPrimary Acid-Base Abnormalities

Normal pH = 7.34 – 7.44Normal pH = 7.34 – 7.44

pH < 7.34 = acidemia (“emia”=on the pH < 7.34 = acidemia (“emia”=on the blood)blood)

pH > 7.44 = alkalemiapH > 7.44 = alkalemia

Primary Acid-Base AbnormalitiesPrimary Acid-Base Abnormalities

Metabolic acidosis Metabolic acidosis Metabolic alkalosisMetabolic alkalosis Respiratory acidosisRespiratory acidosis Respiratory alkalosisRespiratory alkalosis

Metabolic AcidosisMetabolic Acidosis

Some acidic substance has built up in the Some acidic substance has built up in the body, causing the HCObody, causing the HCO33

-- to become too to become too

lowlow

low HCOlow HCO33-- = metabolic acidosis = metabolic acidosis

Metabolic alkalosisMetabolic alkalosis

Some acidic substance has been lost from Some acidic substance has been lost from the body, causing the HCOthe body, causing the HCO33

-- to become to become

too hightoo high

high HCOhigh HCO33-- = metabolic alkalosis = metabolic alkalosis

Respiratory AcidosisRespiratory Acidosis

Abnormal breathing has caused COAbnormal breathing has caused CO22 to to

build up in the bodybuild up in the body

high COhigh CO22 = respiratory acidosis = respiratory acidosis

Respiratory AlkalosisRespiratory Alkalosis

Abnormal breathing (hyperventilation) has Abnormal breathing (hyperventilation) has caused too much COcaused too much CO22 to be lost from the to be lost from the

bodybody

Low COLow CO22 = respiratory alkalosis = respiratory alkalosis

Acid-base InterpretationAcid-base Interpretation

When you see an abnormal pH on a When you see an abnormal pH on a blood gas, you can then determine blood gas, you can then determine whether it is abnormal due to metabolic whether it is abnormal due to metabolic processes or respiratory processesprocesses or respiratory processes

This is This is essentialessential to figure out the best to figure out the best way to treat the patientway to treat the patient

CompensationCompensation

Remember that the body will try to fix the Remember that the body will try to fix the abnormal pH itself with the following abnormal pH itself with the following equation:equation:


HOWEVER, compensation rarely returns HOWEVER, compensation rarely returns the pH completely back to normalthe pH completely back to normal


A metabolic acidosis, will always have a A metabolic acidosis, will always have a mild respiratory alkalosis to go with itmild respiratory alkalosis to go with it

A respiratory acidosis will always have a A respiratory acidosis will always have a mild metabolic alkalosis to go with itmild metabolic alkalosis to go with it

etcetc


Respiratory compensation happens very Respiratory compensation happens very quickly … pant, pant, pantquickly … pant, pant, pant

Metabolic compensation takes a few Metabolic compensation takes a few daysdays

Mixed Acid-base ProcessMixed Acid-base Process

When two When two separateseparate processes are processes are happening at the same timehappening at the same time

Is very different from normal Is very different from normal compensationcompensation

i.e. mixed metabolic acidosis and i.e. mixed metabolic acidosis and respiratory acidosisrespiratory acidosis

Anion GapAnion Gap

Based on rule of Based on rule of electroneutralityelectroneutrality

The sum of all cations in the body is the same The sum of all cations in the body is the same as the sum of all anions in the bodyas the sum of all anions in the body

cations = anionscations = anions

Anion GapAnion Gap

Cations = positively charged particles Cations = positively charged particles (positive ions)(positive ions)

– NaNa++, K, K++, Ca, Ca++++, Mg, Mg++++

Anions = negatively charged particles Anions = negatively charged particles (negative ions)(negative ions)

– ClCl--, HCO, HCO33--, Ph, Ph--, proteins, proteins--

Anion GapAnion Gap

all cations = all anionsall cations = all anions

Measured cations + unmeasured cations = Measured cations + unmeasured cations = measured anions + unmeasured anionsmeasured anions + unmeasured anions

(Na(Na++ + K + K++) + unmeasured cations = ) + unmeasured cations =

(Cl(Cl-- + HCO + HCO33--) + unmeasured anions) + unmeasured anions

Anion GapAnion Gap

(Na(Na++ + K + K++) + unmeasured cations = ) + unmeasured cations =

(Cl(Cl-- + HCO + HCO33--) + unmeasured anions) + unmeasured anions

(Na(Na++ + K + K++) - (Cl) - (Cl-- + HCO + HCO33--) = unmeasured anions- ) = unmeasured anions-

unmeasured cationsunmeasured cations

(Na(Na++ + K + K++) - (Cl) - (Cl-- + HCO + HCO33--) = anion gap) = anion gap

Anion GapAnion Gap

Normal anion gap is around 20Normal anion gap is around 20

A high anion gap means there are a lot of A high anion gap means there are a lot of extra unmeasured anions presentextra unmeasured anions present

These are usually: lactic acid, ketoacids, These are usually: lactic acid, ketoacids, uremic acids (BUN, creatinine), ethylene uremic acids (BUN, creatinine), ethylene glycol (antifreeze)glycol (antifreeze)

Anion GapAnion Gap

Normal anion gap is around 20Normal anion gap is around 20

A low anion gap usually means there are A low anion gap usually means there are a lot fewer unmeasured anions presenta lot fewer unmeasured anions present

This is usually low proteinThis is usually low protein

Strong Ion Difference (SID)Strong Ion Difference (SID)

Based on rule of Based on rule of electroneutralityelectroneutrality

Simplified: The difference between strong Simplified: The difference between strong cations and strong anions in plasma is cations and strong anions in plasma is constantconstant

Very, very, very simplified:Very, very, very simplified:NaNa++ - Cl - Cl-- = 36 = 36

Strong Ion Difference (SID)Strong Ion Difference (SID)

Very, very, very simplified:Very, very, very simplified:

NaNa++ - Cl - Cl-- = 36 = 36

If NaIf Na++ - Cl - Cl-- > 36, then is a strong ion alkalosis, > 36, then is a strong ion alkalosis, usually hypochloremic alkalosisusually hypochloremic alkalosis

If NaIf Na++ - Cl - Cl-- <36, then is a strong ion acidosis, <36, then is a strong ion acidosis, usually hyperchloremic acidosisusually hyperchloremic acidosis

Blood Gas InterpretationBlood Gas Interpretation

Looking at anion gap and chloride Looking at anion gap and chloride concentration provide a means of concentration provide a means of identifying a couple of specific causes of identifying a couple of specific causes of metabolic acidosismetabolic acidosis

Ionized Calcium (CaIonized Calcium (Ca++++))

Calcium is important for proper muscle Calcium is important for proper muscle and nerve cell functionand nerve cell function

Of the total body CaOf the total body Ca++++, approximately:, approximately:– 40% is bound to albumin40% is bound to albumin– 10% is associated with other substances10% is associated with other substances– 50% is ionized50% is ionized

OnlyOnly ionized Ca ionized Ca++++ is biologically active is biologically active and therefore immediately and therefore immediately availableavailable to the to the bodybody

Ionized Calcium (CaIonized Calcium (Ca++++))

Hypercalcemia (increased CaHypercalcemia (increased Ca++++) ) causes muscle weaknesscauses muscle weakness

Hypocalcemia (low CaHypocalcemia (low Ca++++) causes ) causes muscle spasm and rigidity, muscle spasm and rigidity, sometimes to the point of seizuresometimes to the point of seizure

the basics of blood gas and acid-base

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