Abg Interpretations

ABG Interpretation

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 08/10/08

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 08/10/08

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) 08/10/08

paCO2 < 36 mm Hg • Respiratory alkalosis (alveolar hyperventilation) HCO3 < 22 meq/L • Metabolic acidosis HCO3 > 26 meq/L • Metabolic alkalosis

Putting It Together Respiratory So paCO2 > 44 with a pH < 7.35 represents a respiratory acidosis paCO2 < 36 with a pH > 7.45 represents a respiratory alkalosis For 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 08/10/08

Putting It Together - Metabolic And HCO3 < 22 with a pH < 7.35 represents a metabolic acidosis HCO3 > 26 with a pH > 7.45 represents a metabolic alkalosis For a primary metabolic problem, pH and HCO3 are in the same direction, and paCO2 is also in the same direction 08/10/08

Compensation • The body’s attempt to return the acid/base status to normal (i.e. pH closer to 7.4) Primary Problem Compensation respiratory acidosis metabolic alkalosis respiratory alkalosis metabolic acidosis metabolic acidosis respiratory alkalosis metabolic alkalosis respiratory acidosis 08/10/08

Classification of primary acidbase disturbances and compensation

• Acceptable ventilatory and metabolic acidbase status • Respiratory acidosis (alveolar hypoventilation) - acute, chronic • Respiratory alkalosis (alveolar hyperventilation) - acute, chronic • Metabolic acidosis – uncompensated, compensated • Metabolic alkalosis – uncompensated, partially compensated 08/10/08

Respiratory Alkalosis • Causes – – – –

Pain Anxiety Hypoxemia Restrictive lung disease – Severe congestive heart failure – Pulmonary emboli 08/10/08

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Drugs Sepsis Fever Thyrotoxicosis Pregnancy Overaggressive mechanical ventilation – Hepatic failure

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 08/10/08

Metabolic Acidosis Normal Anion Gap • Causes – Renal tubular acidosis – Post respiratory alkalosis – Hypoaldosteronism – Potassium sparing diuretics – Pancreatic loss of bicarbonate 08/10/08

– Diarrhea – Carbonic anhydrase inhibitors – Acid administration (HCl, NH4Cl, arginine HCl) – Sulfamylon – Cholestyramine – Ureteral diversions

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

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Assessing Oxygenation • Normal value for arterial blood gas 80-100mmHg • Normal value for venous blood gas 40mmHg • Normal SaO2 – Arterial: 97% – Venous: 75%

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 08/10/08

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 08/10/08

Case 1 What is the acid/base abnormality? 2. Uncompensated metabolic acidosis 3. Compensated respiratory acidosis 4. Uncompensated respiratory acidosis 5. 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. 08/10/08

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 08/10/08

Case 2 What is the acid/base abnormality? 2. Uncompensated metabolic acidosis 3. Compensated respiratory alkalosis 4. Uncompensated respiratory acidosis 5. Compensated metabolic acidosis

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Case 3 You 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. 08/10/08

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 08/10/08

12 year old diabetic presents with Kussmaul breathing

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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

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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

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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

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pH: pCO2: pO2: HCO3: BE:

7.25 32 mmHg 55 mmHg 10 mEq/L -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

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. 08/10/08