02 blood gas

Marc D. Berg, MD – DeVos Children’s Hospital

Rita R. Ongjoco, DO – Sinai Hospital of Baltimore

12/30/02 ABG Interpretation 1

ABG InterpretationABG Interpretation

First, does the patient have an acidosis or an First, does the patient have an acidosis or an alkalosisalkalosis

Second, what is the primary problem – Second, what is the primary problem – metabolic or respiratorymetabolic or respiratory

Third, is there any compensation by the Third, is there any compensation by the patient – respiratory compensation is patient – respiratory compensation is immediate while renal compensation takes immediate while renal compensation takes timetime


ABG InterpretationABG Interpretation

It would be extremely unusual for either the It would be extremely unusual for either the respiratory or renal system to respiratory or renal system to overcompensateovercompensate

The pH determines the primary problemThe pH determines the primary problem After determining the primary and After determining the primary and

compensatory acid/base balance, evaluate compensatory acid/base balance, evaluate the effectiveness of oxygenationthe effectiveness of oxygenation


Normal ValuesNormal Values

pH 7.35 to 7.45pH 7.35 to 7.45 paCOpaCO22 36 to 44 mm Hg 36 to 44 mm Hg

HCOHCO33 22 to 26 meq/L 22 to 26 meq/L


Abnormal ValuesAbnormal Values

pH < 7.35pH < 7.35 Acidosis (metabolic and/or Acidosis (metabolic and/or

respiratory)respiratory)

pH > 7.45pH > 7.45 Alkalosis (metabolic and/or Alkalosis (metabolic and/or

respiratory)respiratory)

paCOpaCO22 > 44 mmHg > 44 mmHg Respiratory acidosis Respiratory acidosis

(alveolar hypoventilation)(alveolar hypoventilation)

paCOpaCO22 < 36 mmHg < 36 mmHg Respiratory alkalosis Respiratory alkalosis

(alveolar hyperventilation)(alveolar hyperventilation)

HCOHCO33 < 22 meq/L < 22 meq/L Metabolic acidosis Metabolic acidosis

HCOHCO33 > 26 meq/L > 26 meq/L Metabolic alkalosisMetabolic alkalosis


Putting It Together - Putting It Together - RespiratoryRespiratorySoSo paCOpaCO22 > 44 with a pH < 7.35 represents a > 44 with a pH < 7.35 represents a

respiratory acidosisrespiratory acidosis paCOpaCO22 < 36 with a pH > 7.45 represents a < 36 with a pH > 7.45 represents a

respiratory alkalosisrespiratory alkalosis For a primary respiratory problem, pH and For a primary respiratory problem, pH and

paCOpaCO22 move in the opposite direction move in the opposite directionFor each deviation in paCOFor each deviation in paCO22 of 10 mm Hg in either of 10 mm Hg in either

direction, 0. 08 pH units change in the opposite direction, 0. 08 pH units change in the opposite directiondirection


Putting It Together - Putting It Together - MetabolicMetabolicAndAnd HCOHCO33 < 22 with a pH < 7.35 represents a < 22 with a pH < 7.35 represents a

metabolic acidosis metabolic acidosis HCOHCO33 > 26 with a pH > 7.45 represents a > 26 with a pH > 7.45 represents a

metabolic alkalosismetabolic alkalosis For a primary metabolic problem, pH and For a primary metabolic problem, pH and

HCOHCO33 are in the same direction, and paCO are in the same direction, and paCO22 is also in the same directionis also in the same direction


CompensationCompensation

The body’s attempt to return the acid/base The body’s attempt to return the acid/base status to normal (i.e. pH closer to 7.4)status to normal (i.e. pH closer to 7.4)

Primary ProblemPrimary Problem CompensationCompensation

respiratory acidosisrespiratory acidosis metabolic alkalosismetabolic alkalosis

respiratory alkalosisrespiratory alkalosis metabolic acidosismetabolic acidosis

metabolic acidosismetabolic acidosisrespiratory alkalosisrespiratory alkalosis

metabolic alkalosismetabolic alkalosis respiratory acidosisrespiratory acidosis


Expected CompensationExpected Compensation

Respiratory acidosisRespiratory acidosis Acute – Acute –

the pH decreases 0.08 units for every 10 mm Hg the pH decreases 0.08 units for every 10 mm Hg increase in paCOincrease in paCO22; ;

HCOHCO33 0.1-1 mEq/liter per 0.1-1 mEq/liter per 10 mm Hg paCO10 mm Hg paCO22

Chronic – Chronic – the pH decreases 0.03 units for every 10 mm Hg the pH decreases 0.03 units for every 10 mm Hg

increase in paCOincrease in paCO22; ;

HCOHCO33 1.1-3.5 mEq/liter per 1.1-3.5 mEq/liter per 10 mm Hg paCO10 mm Hg paCO22



Respiratory alkalosisRespiratory alkalosis Acute – Acute –

the pH increases 0.08 units for every 10 mm Hg the pH increases 0.08 units for every 10 mm Hg decrease in paCOdecrease in paCO22; ;

HCOHCO33 0-2 mEq/liter per 0-2 mEq/liter per 10 mm Hg paCO10 mm Hg paCO22

Chronic – Chronic – the pH increases 0.17 units for every 10 mm Hg the pH increases 0.17 units for every 10 mm Hg

decrease in paCOdecrease in paCO22; ;

HCOHCO33 2.1-5 mEq/liter per 2.1-5 mEq/liter per 10 mm Hg paCO10 mm Hg paCO22



Metabolic acidosisMetabolic acidosis paCOpaCO22 = 1.5(HCO = 1.5(HCO33) + 8 () + 8 (2)2)

paCOpaCO22 1-1.5 per 1-1.5 per 1 mEq/liter HCO1 mEq/liter HCO33

Metabolic alkalosisMetabolic alkalosis paCOpaCO22 = 0.7(HCO = 0.7(HCO33) + 20 () + 20 (1.5)1.5)

paCOpaCO22 0.5-1.0 per 0.5-1.0 per 1 mEq/liter HCO1 mEq/liter HCO33


Classification of primary acid-Classification of primary acid-base disturbances and base disturbances and compensationcompensation Acceptable ventilatory and metabolic acid-base Acceptable ventilatory and metabolic acid-base

statusstatus Respiratory acidosis (alveolar hypoventilation) - Respiratory acidosis (alveolar hypoventilation) -

acute, chronic acute, chronic Respiratory alkalosis (alveolar hyperventilation) Respiratory alkalosis (alveolar hyperventilation)

- acute, chronic- acute, chronic Metabolic acidosis – uncompensated, Metabolic acidosis – uncompensated,

compensated compensated Metabolic alkalosis – uncompensated, partially Metabolic alkalosis – uncompensated, partially

compensatedcompensated


Acute Respiratory Acute Respiratory AcidosisAcidosis paCOpaCO22 is elevated and pH is acidotic is elevated and pH is acidotic

The decrease in pH is accounted for entirely The decrease in pH is accounted for entirely by the increase in paCOby the increase in paCO22

Bicarbonate and base excess will be in the Bicarbonate and base excess will be in the normal range because the kidneys have not normal range because the kidneys have not had adequate time to establish effective had adequate time to establish effective compensatory mechanismscompensatory mechanisms


Acute Respiratory Acute Respiratory AcidosisAcidosis CausesCauses

Respiratory pathophysiology - airway obstruction, Respiratory pathophysiology - airway obstruction, severe pneumonia, chest trauma/pneumothoraxsevere pneumonia, chest trauma/pneumothorax

Acute drug intoxication (narcotics, sedatives)Acute drug intoxication (narcotics, sedatives)Residual neuromuscular blockadeResidual neuromuscular blockadeCNS disease (head trauma)CNS disease (head trauma)


Chronic Respiratory Chronic Respiratory AcidosisAcidosis paCOpaCO22 is elevated with a pH in the is elevated with a pH in the

acceptable rangeacceptable range Renal mechanisms increase the excretion of Renal mechanisms increase the excretion of

HH++ within 24 hours and may correct the within 24 hours and may correct the resulting acidosis caused by chronic resulting acidosis caused by chronic retention of COretention of CO2 2 to a certain extentto a certain extent


Chronic Respiratory Chronic Respiratory AcidosisAcidosis CausesCauses

Chronic lung disease (BPD, COPD)Chronic lung disease (BPD, COPD)Neuromuscular diseaseNeuromuscular diseaseExtreme obesityExtreme obesityChest wall deformityChest wall deformity


Acute Respiratory Acute Respiratory AlkalosisAlkalosis paCOpaCO22 is low and the pH is alkalotic is low and the pH is alkalotic

The increase in pH is accounted for entirely The increase in pH is accounted for entirely by the decrease in paCOby the decrease in paCO22

Bicarbonate and base excess will be in the Bicarbonate and base excess will be in the normal range because the kidneys have not normal range because the kidneys have not had sufficient time to establish effective had sufficient time to establish effective compensatory mechanismscompensatory mechanisms


Respiratory AlkalosisRespiratory Alkalosis

CausesCausesPainPainAnxietyAnxietyHypoxemiaHypoxemiaRestrictive lung Restrictive lung

diseasediseaseSevere congestive Severe congestive

heart failureheart failurePulmonary emboliPulmonary emboli

DrugsDrugsSepsisSepsisFeverFeverThyrotoxicosisThyrotoxicosisPregnancyPregnancyOveraggressive Overaggressive

mechanical ventilationmechanical ventilationHepatic failureHepatic failure


Uncompensated Metabolic Uncompensated Metabolic AcidosisAcidosis Normal paCONormal paCO22, low HCO, low HCO33, and a pH less , and a pH less

than 7.30than 7.30 Occurs as a result of increased production of Occurs as a result of increased production of

acids and/or failure to eliminate these acidsacids and/or failure to eliminate these acids Respiratory system is not compensating by Respiratory system is not compensating by

increasing alveolar ventilation increasing alveolar ventilation (hyperventilation)(hyperventilation)


Compensated Metabolic Compensated Metabolic AcidosisAcidosis paCOpaCO22 less than 30, low HCO less than 30, low HCO33, with a pH of , with a pH of

7.3-7.47.3-7.4 Patients with chronic metabolic acidosis are Patients with chronic metabolic acidosis are

unable to hyperventilate sufficiently to lower unable to hyperventilate sufficiently to lower paCOpaCO22 for complete compensation to 7.4 for complete compensation to 7.4


Metabolic AcidosisMetabolic Acidosis Elevated Anion Gap Elevated Anion Gap CausesCauses

Ketoacidosis - diabetic, alcoholic, starvationKetoacidosis - diabetic, alcoholic, starvationLactic acidosis - hypoxia, shock, sepsis, seizuresLactic acidosis - hypoxia, shock, sepsis, seizuresToxic ingestion – salicylates, methanol, ethylene Toxic ingestion – salicylates, methanol, ethylene

glycol, ethanol, isopropyl alcohol, paraldehyde, glycol, ethanol, isopropyl alcohol, paraldehyde, toluenetoluene

Renal failure - uremiaRenal failure - uremia


Metabolic AcidosisMetabolic Acidosis Normal Anion Gap Normal Anion Gap CausesCauses

Renal tubular acidosisRenal tubular acidosisPost respiratory Post respiratory

alkalosisalkalosisHypoaldosteronismHypoaldosteronismPotassium sparing Potassium sparing

diureticsdiureticsPancreatic loss of Pancreatic loss of

bicarbonatebicarbonate

DiarrheaDiarrheaCarbonic anhydrase Carbonic anhydrase

inhibitorsinhibitorsAcid administration Acid administration

(HCl, NH(HCl, NH44Cl, arginine Cl, arginine

HCl)HCl)SulfamylonSulfamylonCholestyramineCholestyramineUreteral diversionsUreteral diversions


Effectiveness of Effectiveness of OxygenationOxygenation Further evaluation of the arterial blood gas Further evaluation of the arterial blood gas

requires assessment of the effectiveness of requires assessment of the effectiveness of oxygenation of the bloodoxygenation of the blood

Hypoxemia – decreased oxygen content of Hypoxemia – decreased oxygen content of blood - paOblood - paO22 less than 60 mm Hg and the less than 60 mm Hg and the

saturation is less than 90%saturation is less than 90% Hypoxia – inadequate amount of oxygen Hypoxia – inadequate amount of oxygen

available to or used by tissues for metabolic available to or used by tissues for metabolic needsneeds


Mechanisms of Mechanisms of HypoxemiaHypoxemia Inadequate inspiratory partial pressure of Inadequate inspiratory partial pressure of

oxygenoxygen HypoventilationHypoventilation Right to left shuntRight to left shunt Ventilation-perfusion mismatchVentilation-perfusion mismatch Incomplete diffusion equilibriumIncomplete diffusion equilibrium


Assessment of Gas Assessment of Gas ExchangeExchange Alveolar-arterial OAlveolar-arterial O22 tension difference tension difference

A-a gradientA-a gradientPAOPAO22-PaO-PaO22

PAOPAO22 = FIO = FIO22(PB - PH(PB - PH22O) - PaCOO) - PaCO22/RQ*/RQ* arterial-Alveolar Oarterial-Alveolar O22 tension ratio tension ratio

PaOPaO22/PAO/PAO22

arterial-inspired Oarterial-inspired O22 ratio ratioPaOPaO22/FIO/FIO22

P/F ratioP/F ratio*RQ=respiratory quotient= 0.8*RQ=respiratory quotient= 0.8


Assessment of Gas Assessment of Gas ExchangeExchange

ABGABG A-a gradA-a grad

PaOPaO22 PaCOPaCO22 RARA 100%100%

Low FIOLow FIO22 N*N* NN

Alveolar hypoventilationAlveolar hypoventilation NN NN

Altered gas exchangeAltered gas exchange

Regional V/Q mismatchRegional V/Q mismatch /N//N/ N/N/ Intrapulmonary R to L shuntIntrapulmonary R to L shunt N/N/ Impaired diffusionImpaired diffusion N/N/ NN

Anatomical R to L shuntAnatomical R to L shunt

(intrapulmonary or intracardiac)(intrapulmonary or intracardiac) N/N/ * N=normal* N=normal


SummarySummary

First, does the patient have an acidosis or an First, does the patient have an acidosis or an alkalosis alkalosis Look at the pHLook at the pH

Second, what is the primary problem – Second, what is the primary problem – metabolic or respiratorymetabolic or respiratoryLook at the pCOLook at the pCO22

If the pCOIf the pCO22 change is in the opposite direction of the change is in the opposite direction of the

pH change, the primary problem is respiratorypH change, the primary problem is respiratory


SummarySummary

Third, is there any compensation by the Third, is there any compensation by the patient - do the calculationspatient - do the calculationsFor a primary respiratory problem, is the pH For a primary respiratory problem, is the pH

change completely accounted for by the change change completely accounted for by the change in pCOin pCO22

○ if yes, then there is no metabolic compensationif yes, then there is no metabolic compensation○ if not, then there is either partial compensation or if not, then there is either partial compensation or

concomitant metabolic problemconcomitant metabolic problem


SummarySummaryFor a metabolic problem, calculate the expected For a metabolic problem, calculate the expected

pCOpCO22

○ if equal to calculated, then there is appropriate if equal to calculated, then there is appropriate respiratory compensationrespiratory compensation

○ if higher than calculated, there is concomitant if higher than calculated, there is concomitant respiratory acidosisrespiratory acidosis

○ if lower than calculated, there is concomitant if lower than calculated, there is concomitant respiratory alkalosisrespiratory alkalosis


SummarySummary

Next, don’t forget to look at the effectiveness Next, don’t forget to look at the effectiveness of oxygenation, (and look at the patient)of oxygenation, (and look at the patient)your patient may have a significantly increased your patient may have a significantly increased

work of breathing in order to maintain a “normal” work of breathing in order to maintain a “normal” blood gasblood gas

metabolic acidosis with a concomitant respiratory metabolic acidosis with a concomitant respiratory acidosis is concerningacidosis is concerning


Case 1Case 1

Little Billy got into some of dad’s barbiturates. Little Billy got into some of dad’s barbiturates. He suffers a significant depression of mental He suffers a significant depression of mental status and respiration. You see him in the status and respiration. You see him in the

ER 3 hours after ingestion with a respiratory ER 3 hours after ingestion with a respiratory rate of 4. A blood gas is obtained (after rate of 4. A blood gas is obtained (after

doing the ABC’s, of course). It shows pH = doing the ABC’s, of course). It shows pH = 7.16, pCO7.16, pCO22 = 70, HCO = 70, HCO33 = 22 = 22


Case 1Case 1

What is the acid/base abnormality?What is the acid/base abnormality?

1.1. Uncompensated metabolic acidosisUncompensated metabolic acidosis

2.2. Compensated respiratory acidosisCompensated respiratory acidosis

3.3. Uncompensated respiratory acidosisUncompensated respiratory acidosis

4.4. Compensated metabolic alkalosisCompensated metabolic alkalosis


Case 1Case 1

Uncompensated respiratory acidosisUncompensated respiratory acidosis There has not been time for metabolic There has not been time for metabolic

compensation to occur. As the barbiturate compensation to occur. As the barbiturate toxicity took hold, this child slowed his toxicity took hold, this child slowed his respirations significantly, pCOrespirations significantly, pCO22 built up in the built up in the

blood, and an acidosis ensued.blood, and an acidosis ensued.


Case 2Case 2

Little Suzie has had vomiting and diarrhea for 3 Little Suzie has had vomiting and diarrhea for 3 days. In her mom’s words, “She can’t keep days. In her mom’s words, “She can’t keep anything down and she’s runnin’ out.” She anything down and she’s runnin’ out.” She has had 1 wet diaper in the last 24 hours. has had 1 wet diaper in the last 24 hours.

She appears lethargic and cool to touch with She appears lethargic and cool to touch with a prolonged capillary refill time. After a prolonged capillary refill time. After addressing her ABC’s, her blood gas addressing her ABC’s, her blood gas

reveals: pH=7.34, pCOreveals: pH=7.34, pCO22=26, HCO=26, HCO33=12=12


Case 2Case 2

What is the acid/base abnormality?What is the acid/base abnormality?

1.1. Uncompensated metabolic acidosisUncompensated metabolic acidosis

2.2. Compensated respiratory alkalosisCompensated respiratory alkalosis

3.3. Uncompensated respiratory acidosisUncompensated respiratory acidosis

4.4. Compensated metabolic acidosisCompensated metabolic acidosis


Case 2Case 2

Compensated metabolic acidosisCompensated metabolic acidosis The prolong history of fluid loss through The prolong history of fluid loss through

diarrhea has caused a metabolic acidosis. The diarrhea has caused a metabolic acidosis. The mechanisms probably are twofold. First there is mechanisms probably are twofold. First there is lactic acid production from the hypovolemia and lactic acid production from the hypovolemia and tissue hypoperfusion. Second, there may be tissue hypoperfusion. Second, there may be significant bicarbonate losses in the stool. The significant bicarbonate losses in the stool. The body has compensated by “blowing off” the CObody has compensated by “blowing off” the CO22

with increased respirations.with increased respirations.


Case 3Case 3

You are evaluating a 15 year old female in the You are evaluating a 15 year old female in the ER who was brought in by EMS from school ER who was brought in by EMS from school because of abdominal pain and vomiting. because of abdominal pain and vomiting. Review of system is negative except for a 10 Review of system is negative except for a 10 lb. weight loss over the past 2 months and lb. weight loss over the past 2 months and polyuria for the past 2 weeks. She has no polyuria for the past 2 weeks. She has no other medical problems and denies any sexual other medical problems and denies any sexual activity or drug use. On exam, she is alert and activity or drug use. On exam, she is alert and oriented, afebrile, HR 115, RR 26 and regular, oriented, afebrile, HR 115, RR 26 and regular, BP 114/75, pulse ox 95% on RA.BP 114/75, pulse ox 95% on RA.


Case 3Case 3

Exam is unremarkable except for mild abdominal Exam is unremarkable except for mild abdominal tenderness on palpation in the midepigastric tenderness on palpation in the midepigastric region and capillary refill time of 3 seconds. region and capillary refill time of 3 seconds. The nurse has already seen the patient and has The nurse has already seen the patient and has sent off “routine” blood work. She hands you sent off “routine” blood work. She hands you the result of the blood gas. pH = 7.21 pCOthe result of the blood gas. pH = 7.21 pCO22==

24 pO24 pO22 = 45 HCO = 45 HCO33 = 10 BE = -10 saturation = = 10 BE = -10 saturation =

72%72%


Case 3Case 3

What is the blood gas interpretation?What is the blood gas interpretation? Uncompensated respiratory acidosis with Uncompensated respiratory acidosis with

severe hypoxiasevere hypoxia Uncompensated metabolic alkalosisUncompensated metabolic alkalosis Combined metabolic acidosis and respiratory Combined metabolic acidosis and respiratory

acidosis with severe hypoxiaacidosis with severe hypoxia Metabolic acidosis with respiratory Metabolic acidosis with respiratory

compensationcompensation


Case 3Case 3

Metabolic acidosis with respiratory compensationMetabolic acidosis with respiratory compensation This is a patient with new onset diabetes This is a patient with new onset diabetes

mellitus in ketoacidosis. Her pulse oximetry mellitus in ketoacidosis. Her pulse oximetry saturation and clinical examination do not reveal saturation and clinical examination do not reveal any respiratory problems except for tachypnea any respiratory problems except for tachypnea which is her compensatory mechanism for the which is her compensatory mechanism for the metabolic acidosis. The nurse obtained the metabolic acidosis. The nurse obtained the blood gas sample from the venous stick when blood gas sample from the venous stick when she sent off the other labs.she sent off the other labs.


ReferencesReferences The ICU Book – Paul L. Marino, 1991, The ICU Book – Paul L. Marino, 1991,

Algorithms for acid-base interpretations, p415-Algorithms for acid-base interpretations, p415-426426

Textbook of Pediatric Intensive Care 3Textbook of Pediatric Intensive Care 3 rdrd Edition Edition – edited by Mark C. Rogers, 1996, Respiratory – edited by Mark C. Rogers, 1996, Respiratory Monitoring: Interpretation of clinical blood gas Monitoring: Interpretation of clinical blood gas values, p355-361values, p355-361

Pediatric Critical Care – Bradley Fuhrman and Pediatric Critical Care – Bradley Fuhrman and Jerry Zimmerman, 1992, Acid-Base Balance Jerry Zimmerman, 1992, Acid-Base Balance and Disorders, p689-696and Disorders, p689-696

Critical Care Physiology – Robert Bartlett, 1996, Critical Care Physiology – Robert Bartlett, 1996, Acid-Base physiology p165-173.Acid-Base physiology p165-173.


02 blood gas

Health & Medicine

metabolic alkalosis

abg interpretation

hg paco

meql metabolic acidosis

ph units

primary respiratory

meql metabolic alkalosis

hg hco