ABG Interpretation

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Yasser Etman, MD Assistant Professor of Medicine

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ABG Interpretation :

ABG Interpretation Yasser Etman, MD 2010

Acid-Base Disorders:

Acid-Base Disorders Acidosis: presence of a process which tends to lower pH by gaining H + or losing HCO 3 Alkalosis: presence of a process which tends to raise pH by losing H + or gaining HCO 3 - Respiratory: process that lead to a primary alteration in ventilation and resultant excessive elimination or retention of CO 2 Metabolic : processes that affects the kidneys and the consequent disruption of H + and HCO 3 - control

Acid Base Balance:

Acid Base Balance pH is maintained within a narrow range to preserve normal cell function Buffers – minimize the change in pH resulting from production of acid → provides immediate protection from acid The primary buffer system is HCO 3 - HCO 3 - + H +  H 2 CO 3  H 2 O + CO 2

Basics of acid-base disturbances:

Basics of acid-base disturbances Disorder Primary pH Primary Compensated Response Metabolic acidosis   [HCO 3 - ]  Pco 2 Metabolic alkalosis   [HCO 3 - ]  Pco 2 Respiratory acidosis   Pco 2  [HCO 3 - ] Respiratory alkalosis   Pco 2  [HCO 3 - ]

Basics of acid-base disturbances:

Basics of acid-base disturbances SIMPLE vs. MIXED COMPENSATED vs. NOT

Compensation:

Compensation the normal response of the Lungs or K idneys to a change in pH induced by a 1ry acid-base disorder No overcompensation ( except occasionally primary respiratory alkalosis) Kidneys slow , lungs fast Lack of compensation (or over) determines a second primary disorder The degree of appropriate compensation is predictable

Compensation:

Compensation Role of the kidney: To retain and regenerate HCO3- thereby regenerating the buffer with the net effect of eliminating the acid H + secretion HCO 3 - reabsorption Role of the respiratory system eliminate CO 2

Compensatory response to a Primary Acid-Base Disturbance :

Compensatory response to a Primary Acid-Base Disturbance The body’s attempt to return the acid/base status to normal (i.e. pH closer to 7.40) Primary Problem Compensation Respiratory acidosis Metabolic alkalosis Respiratory alkalosis Metabolic acidosis

Compensation:

Compensation

Compensation:

Compensation

PowerPoint Presentation:

pH PCO 2 HCO 3 Interpretation Acidotic >40 Alkalotic Respiratory Acidosis Acidotic Respiratory + Metabolic Acidosis <40 Acidotic Metabolic Acidosis Alkalotic >40 Alkalotic Metabolic Alkalosis <40 Acidotic Respiratory Alkalosis Alkalotic Respiratory + Metabolic Alkalosis

Approach to Acid-Base Problem Solving:

Approach to Acid-Base Problem Solving Is it Valid ? What is the primary disturbance? Is compensation appropriate? What is the anion gap (AG)? Does the change in the anion gap equal the change in the serum bicarbonate concentration (a value called delta-delta )?

Validity of ABG :

Validity of ABG 24 x PaCO 2 = H + HCO 3 ~ PaCO 2 – pH Relationship: H + ~ pH 80 7.20 60 7.30 40 7.40 30 7.50 20 7.60

Normal ABG values:

Normal ABG values pH 7.38 – 7.42 PaCO 2 35 – 45 mmHg PaO 2 80 – 100 mmHg HCO 3 22 – 26 mmol/L BE - 2 - + 2 SaO 2 > 95%

Acidosis Alkalosis:

Acidosis Alkalosis pH < 7.38 Respiratory PaCO 2 > 40 Metabolic HCO 3 < 24 pH > 7.42 Respiratory PaCO 2 < 40 Metabolic HCO 3 > 24

DETERMINE PRIMARY DISORDER:

DETERMINE PRIMARY DISORDER The change that produces the pH is the Primary disorder ACIDOSIS ACIDOSIS ALKALOSIS pH = 7.25 HCO 3 = 12 PaCO 2 = 30 METABOLIC ACIDOSIS pH = 7.25 HCO 3 = 28 PaCO 2 = 60 ACIDOSIS ALKALOSIS ACIDOSIS RESPIRATORY ACIDOSIS

DETERMINE PRIMARY DISORDER:

DETERMINE PRIMARY DISORDER ALKALOSIS ACIDOSIS ALKALOSIS pH = 7.55 HCO 3 = 19 PaCO 2 = 20 RESPIRATORY ALKALOSIS pH = 7.55 HCO 3 = 28 PaCO 2 = 60 ALKALOSIS ALKALOSIS ACIDOSIS METABOLIC ALKALOSIS

DETERMINE PRIMARY DISORDER:

DETERMINE PRIMARY DISORDER S ame trend, check % difference, bigger % difference= 1ry ALKALOSIS ALKALOSIS ALKALOSIS pH = 7.55 HCO 3 = 36 PaCO 2 = 3 0 METABOLIC ALKALOSIS pH = 7.25 HCO 3 = 16 PaCO 2 = 60 ACIDOSIS ACIDOSIS ACIDOSIS RESPIRATORY ACIDOSIS ( 36-24 )/24 = 50% ( 30-40)/40 = 25% ( 16-24)/24 = 33%) (60-40)/40 = 50%)

Expected Compensation:

Expected Compensation Metabolic Acidosis : Acute:  PaCO 2 = (1.5)HCO 3 + 8 Chronic:  PaCO 2 = HCO 3 + 15 Failure of the PaCO 2 to decrease to expected value = Complicating Respiratory Acidosis. Excessive decrease of the PaCO 2 = Complicating Respiratory Alkalosis. Time for adaptation: 12 to 24 hr

Expected Compensation:

Expected Compensation 2. Respiratory Acidosis : Acute: 1 meq/L  in HCO 3 for each 10 mmHg  in PaCO 2 . ( Minutes to hours) ↓ pH by 0.08 for each 10 mm Hg ↑ PaCO2 Chronic: 3.5 meq/L  in HCO 3 for each 10 mmHg  in PaCO 2 . (days) ↓ pH by 0.03 for each 10 mm Hg ↑ PaCO2 Failure of the HCO 3 to increase to expected value = Complicating Metabolic Acidosis. Excessive increase of the HCO 3 = Complicating Metabolic Alkalosis.

Expected Compensation:

Expected Compensation 3. Metabolic Alkalosis : 0.7 meq/L  PaCO 2 for each 1 mmHg  HCO 3 Time for adaptation: 24 to 36 hrs

Expected Compensation:

Expected Compensation 4. Respiratory Alkalosis : Acute: 2 meq/L  in HCO 3 for each 10 mmHg  in PaCO 2 . ( Minutes to hours) Chronic: 4 meq/L  in HCO 3 for each 10 mmHg  in PaCO 2 . (days) Failure of the HCO 3 to decrease to expected value = Complicating Metabolic Alkalosis. Excessive decrease of the HCO 3 = Complicating Metabolic Acidosis.

Anion Gap:

Anion Gap AG = [Na + ]- ( [ HCO 3 + ] - [ Cl - ] ) = 12 ± 2 Should be calculated regardless the 1ry disturbance. When the 1ry disturbance in other than metabolic acidosis, the AG helps reveal a “ hidden” AG Metabolic Acidosis. Narrow AG suggests Multiple Myeloma.

delta-delta  :

delta-delta   In AG acidosis , the expected ratio between  in AG and  in the plasma HCO 3 concentration is 1 to 2.  AG = 12 - AG = 1 to 2  HCO 3 24 - HCO 3 If   < 1 + concurrent Non-AG acidosis If   > 2 + concurrent Metabolic Alkalosis

7 Steps:

7 Steps Validity pH PaCO 2 HCO 3 AG Compensation AG / HCO 3 =  

Case 1 :

Case 1 pH = 7.31; PaCO 2 = 10; HCO 3 = 5 ; Na = 127 ; Cl = 99 . pH PaCO 2 HCO 3 AG= 127 – [99+5] = 23  PaCO 2 = 1.5 x 5 + 8 = 15.5 ± 2 PaCO 2 < Expected PaCO 2 = Respiratory Alkalosis  = [23-12] / [24-5] = 11/19 = 0.57 < 1 Non-AG Metabolic Acidosis

Case 2 :

Case 2 pH = 7.20; PaCO 2 = 23; HCO 3 = 8 ; Na = 134 ; Cl = 80 . pH PaCO 2 HCO 3 AG= 134 – [80+8] = 46  PaCO 2 = 1.5 x 8 + 8 = 20 ± 2  = [46-12] / [24-8] = 34/16 = 2.12 > 2 Metabolic Alkalosis

Tips to diagnose mixed acid-base dis.:

Tips to diagnose mixed acid-base dis. TIP 1. Don’t interpret any blood gas data for acid-base diagnosis without closely examining the serum electrolytes: Na + , K + , Cl - and CO 2 . A serum CO 2 out of the normal range always represents some type of acid-base disorder (barring lab or transcription error). High serum CO 2 indicates metabolic alkalosis &/or bicarbonate retention as compensation for respiratory acidosis Low serum CO 2 indicates metabolic acidosis &/or bicarbonate excretion as compensation for respiratory alkalosis Note that serum CO 2 may be normal in the presence of two or more acid-base disorders.

Tips to diagnose mixed acid-base dis.:

Tips to diagnose mixed acid-base dis. TIP 2 . Single acid-base disorders do not lead to normal blood pH. Although pH can end up in the normal range (7.35 - 7.45) with a mild single disorder, a truly normal pH with distinctly abnormal HCO 3 - and PaCO 2 invariably suggests two or more primary disorders. Example: pH 7.40, PaCO 2 20 mm Hg, HCO 3 - 12 mEq /L, in a patient with sepsis. Normal pH results from two co-existing and unstable acid-base disorders: acute respiratory alkalosis and metabolic acidosis.

Tips to diagnose mixed acid-base dis.:

Tips to diagnose mixed acid-base dis. TIP 3 . Simplified rules predict the pH and HCO 3 - for a given change in PaCO 2 . If the pH or HCO 3 - is higher or lower than expected for the change in PaCO 2 , the patient probably has a metabolic acid-base disorder as well. Acute: 1 meq/L  in HCO 3 for each 10 mmHg  in PaCO 2 . ↓ pH by 0.08 for each 10 mm Hg ↑ PaCO2 Chronic: 3.5 meq/L  in HCO 3 for each 10 mmHg  in PaCO 2 . ↓ pH by 0.03 for each 10 mm Hg ↑ PaCO2

Tips to diagnose mixed acid-base dis.:

Tips to diagnose mixed acid-base dis. Predicted changes in HCO 3 - for a directional change in PaCO 2 can help uncover mixed acid-base disorders. A normal or slightly low HCO 3 - in the presence of hypercapnia suggests a concomitant metabolic acidosis, e.g., pH 7.27, PaCO 2 50 mm Hg, HCO 3 - 22 mEq /L. Based on the rule for increase in HCO 3 - with hypercapnia , it should be at least 25 mEq /L in this example; that it is only 22 mEq /L suggests a concomitant metabolic acidosis.

Tips to diagnose mixed acid-base dis.:

Tips to diagnose mixed acid-base dis. Predicted changes in HCO 3 - for a directional change in PaCO 2 can help uncover mixed acid-base disorders. A normal or slightly elevated HCO 3 - in the presence of hypocapnia suggests a concomitant metabolic alkalosis, e.g., pH 7.56, PaCO 2 30 mm Hg, HCO 3 - 26 mEq /L. Based on the rule for decrease in HCO 3 with hypocapnia , it should be at least 23 mEq /L in this example; that it is 26 mEq /L suggests a concomitant metabolic alkalosis.

Tips to diagnose mixed acid-base dis.:

Tips to diagnose mixed acid-base dis. TIP 4 . In maximally-compensated metabolic acidosis, the numerical value of PaCO 2 should be the same (or close to) the last two digits of arterial pH. This observation reflects the formula for expected respiratory compensation in metabolic acidosis. TIP 5. DO NOT FORGET YOUR CLINICAL ASSESSMENT and JUDJMENT.

INDICATIONS FOR HCO3 THERAPY:

INDICATIONS FOR HCO 3 THERAPY pH < 7.2 and HCO 3 < 5 – 10 mmHg When there is inadequate ventilatory compensation Elderly on beta blockers in severe acidosis with compromised cardiac function Concurrent severe AG and NAGMA Severe acidosis with renal failure or intoxication

Cases:

Cases

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