ABG Interpretation

08/04/23

ABG Interpretation

• First, does the patient have an acidosis or an alkalosis

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

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

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ABG Interpretation

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

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

compensatory acid/base balance, evaluate the effectiveness of oxygenation

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Normal Values

• pH 7.35 to 7.45

• paCO2 36 to 44 mm Hg

• HCO3 22 to 26 meq/L

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Abnormal Values

pH < 7.35• Acidosis (metabolic

and/or respiratory)pH > 7.45• Alkalosis (metabolic

and/or respiratory)

paCO2 > 44 mm Hg• Respiratory acidosis

(alveolar hypoventilation)

paCO2 < 36 mm Hg• Respiratory alkalosis

(alveolar hyperventilation)

HCO3 < 22 meq/L• Metabolic acidosis

HCO3 > 26 meq/L• Metabolic alkalosis

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Putting It Together - Respiratory

SopaCO2 > 44 with a pH < 7.35 represents a

respiratory acidosispaCO2 < 36 with a pH > 7.45 represents a

respiratory alkalosisFor a primary respiratory problem, pH and

paCO2 move in the opposite direction– For each deviation in paCO2 of 10 mm Hg in either

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

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Putting It Together - Metabolic

AndHCO3 < 22 with a pH < 7.35 represents a

metabolic acidosis HCO3 > 26 with a pH > 7.45 represents a

metabolic alkalosisFor a primary metabolic problem, pH and

HCO3 are in the same direction, and paCO2 is also in the same direction

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Compensation

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

Primary Problem Compensationrespiratory acidosis metabolic alkalosisrespiratory alkalosis metabolic acidosismetabolic acidosis respiratory alkalosismetabolic alkalosis respiratory acidosis

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Classification of primary acid-base disturbances and

compensation• Acceptable ventilatory and metabolic acid-

base status• Respiratory acidosis (alveolar

hypoventilation) - acute, chronic • Respiratory alkalosis (alveolar

hyperventilation) - acute, chronic• Metabolic acidosis – uncompensated,

compensated • Metabolic alkalosis – uncompensated,

partially compensated

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Respiratory Alkalosis

• Causes– Pain– Anxiety– Hypoxemia– Restrictive lung

disease– Severe congestive

heart failure– Pulmonary emboli

– Drugs– Sepsis– Fever– Thyrotoxicosis– Pregnancy– Overaggressive

mechanical ventilation

– Hepatic failure

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Metabolic Acidosis Elevated Anion Gap

• Causes– Ketoacidosis - diabetic, alcoholic,

starvation– Lactic acidosis - hypoxia, shock, sepsis,

seizures– Toxic ingestion – salicylates, methanol,

ethylene glycol, ethanol, isopropyl alcohol, paraldehyde, toluene

– Renal failure - uremia

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Metabolic Acidosis Normal Anion Gap

• Causes– Renal tubular

acidosis– Post respiratory

alkalosis– Hypoaldosteronism– Potassium sparing

diuretics– Pancreatic loss of

bicarbonate

– Diarrhea– Carbonic anhydrase

inhibitors– Acid administration

(HCl, NH4Cl, arginine HCl)

– Sulfamylon– Cholestyramine– Ureteral diversions

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Summary

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

• Second, what is the primary problem – metabolic or respiratory– Look at the pCO2

– If the pCO2 change is in the opposite direction of the pH change, the primary problem is respiratory

Assessing Oxygenation

• Normal value for arterial blood gas 80-100mmHg

• Normal value for venous blood gas 40mmHg

• Normal SaO2 – Arterial: 97%– Venous: 75%

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Summary

• Third, is there any compensation by the patient - do the calculations– For a primary respiratory problem, is the

pH change completely accounted for by the change in pCO2

• if yes, then there is no metabolic compensation• if not, then there is either partial compensation

or concomitant metabolic problem

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Summary

– For a metabolic problem, calculate the expected pCO2

• if equal to calculated, then there is appropriate respiratory compensation

• if higher than calculated, there is concomitant respiratory acidosis

• if lower than calculated, there is concomitant respiratory alkalosis

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Summary

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

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

– metabolic acidosis with a concomitant respiratory acidosis is concerning

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Case 1

Little Billy got into some of dad’s barbiturates. He suffers a significant

depression of mental status and respiration. You see him in the ER 3

hours after ingestion with a respiratory rate of 4. A blood gas is obtained (after doing the ABC’s, of course). It shows

pH = 7.16, pCO2 = 70, HCO3 = 22

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Case 1

What is the acid/base abnormality?1. Uncompensated metabolic acidosis2. Compensated respiratory acidosis3. Uncompensated respiratory acidosis4. Compensated metabolic alkalosis

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Case 1

Uncompensated respiratory acidosis• There has not been time for metabolic

compensation to occur. As the barbiturate toxicity took hold, this child slowed his respirations significantly, pCO2 built up in the blood, and an acidosis ensued.

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Case 2

Little Suzie has had vomiting and diarrhea for 3 days. In her mom’s words, “She can’t keep anything down and she’s

runnin’ out.” She has had 1 wet diaper in the last 24 hours. She appears lethargic and cool to touch with a

prolonged capillary refill time. After addressing her ABC’s, her blood gas

reveals: pH=7.34, pCO2=26, HCO3=12

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Case 2

What is the acid/base abnormality?1. Uncompensated metabolic acidosis2. Compensated respiratory alkalosis3. Uncompensated respiratory acidosis4. Compensated metabolic acidosis

08/04/23

Case 3You are evaluating a 15 year old female in the

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

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Case 3

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

severe hypoxia• Uncompensated metabolic alkalosis• Combined metabolic acidosis and respiratory

acidosis with severe hypoxia• Metabolic acidosis with respiratory

compensation

12 year old diabetic presents with Kussmaul breathing

• pH : 7.05• pCO2: 12 mmHg • pO2: 108 mmHg• HCO3: 5 mEq/L• BE: -30 mEq/L

– Severe partly compensated metabolic acidosis without hypoxemia due to ketoacidosis

17 year old w/severe kyphoscoliosis, admitted for pneumonia

• pH: 7.37• pCO2: 25 mmHg• pO2: 60 mmHg• HCO3: 14 mEq/L• BE : -7 mEq/L

– Compensated respiratory alkalosis due to chronic hyperventilation secondary to hypoxia

9 year old w/hx of asthma, audibly wheezing x 1 week, has not slept in 2 nights; presents sitting up and using accessory

muscles to breath w/audible wheezes

• pH: 7.51• pCO2: 25 mmHg• pO2 35 mmHg• HCO3: 22 mEq/L• BE: -2 mEq/L

– Uncompensated respiratory alkalosis with severe hypoxia due to asthma exacerbation

7 year old post op presenting with chills, fever and hypotension

• pH: 7.25• pCO2: 32 mmHg• pO2: 55 mmHg• HCO3: 10 mEq/L• BE: -15 mEq/L

– Uncompensated metabolic acidosis due to low perfusion state and hypoxia causing increased lactic acid

Post test

• Ph – 7.25 • PCO2 – 55• HCO3 - 29

• Ph – 7.05• PCO2 – 20• HCO3 - 15

• 7.48• 32• 12

• 7.36• 52• 34

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References

• The ICU Book – Paul L. Marino, 1991, Algorithms for acid-base interpretations, p415-426

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

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

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