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Part I Introduction Jakub Matera Acid Base Interpretation Slide 2 Normal values pH 7.40 (7.35 7.45) Pa CO 2 40 mmHg (35 45) Pa O 2 80 100 (expected PaO 2 = FiO 2 x 5) HCO 3 24 mmol/L (20 28) BE -2 to +2 mEq/L Slide 3 Venous or arterial sample Slide 4 Venous or arterial sample? Slide 5 MEDLINE search of papers published from 1966 to January 2010 for studies comparing arterial and peripheral venous blood gas values for any of pH, pCO2, bicarbonate and base excess in adult patients with any condition in an emergency department setting. The weighted mean arteriovenous difference in pH was 0.035 pH units (n = 1252), with narrow limits of agreement. For bicarbonate, the weighted mean difference between arterial and venous values was 1.41 mmol/L (n = 905), with 95% limits of agreement of the order of 5 mmol/L. The weighted mean arteriovenous difference for pCO2 was 5.7 mmHg (n = 760), but with 95% limits of agreement up to the order of 20 mmHg. Regarding base excess, the mean arteriovenous difference is 0.089 mmol/L (n = 103). Slide 6 Conclusions There is insufficient data to determine if these relationships persist in shocked patients or those with mixed acid-base disorders. For patients who are not in shock, venous pH, bicarbonate and base excess have sufficient agreement to be clinically interchangeable for arterial values. Agreement between arterial and venous pCO2 is too poor and unpredictable to be clinically useful as a one-off test but venous pCO2 might be useful to screen for arterial hypercarbia or to monitor trends in pCO2 for selected patients. Slide 7 5 steps to analyse acid base Step 1 Look at the pH. Acidaemia or alkalaemia? Step 2 Who is responsible for this change in pH ( primary culprit )? Step 3 Calculate compensatory changes. Adequate compensation? Acute or chronic process? Step 4 Calculate AG and gaps. Is there any additional pathological process? Mixed metabolic or respiratory disturbance? Step 5 Clinical correlation? Find the diagnosis. Slide 8 Step 1 Look at the pH AcidaemiapH 7.45 If the pH is within normal range (7.35 7.45) but PaCO 2 or HCO 3 is abnormal indicating an acid base imbalance use 7.40 as your cutoff point. pH between 7.35 and 7.40 indicates acidosis pH between 7.40 and 7.45 indicates alkalosis Principle: The body does not fully compensate for primary acid base disorders Slide 9 Step 1 Look at the pH What if pH is 7.40 exactly, but PaCO 2 or HCO 3 is abnormal, indicating an acid base imbalance??? Is this possible? We will talk about it in part II. Slide 10 Step 2 Determine primary process If the pH is < 7.40 (acidaemia) then elevated PaCO2 (resp acidosis) or lowered bicarbonate (met acidosis) would be primary abnormalities. If pH is > 7.40 (alkalaemia) then a lowered PaCO2 (resp alkalosis) or higher bicarbonate (met alkalosis) would be primary. Slide 11 Step 2 Determine primary process Respiratory alkalosis pH, PaCO 2 acidosis pH, PaCO 2 Metabolic alkalosis pH, HCO 3 acidosis pH, HCO 3 Or you can remember the acronym ROME. R=Respiratory O=Opposite meaning that if PaCO 2 and pH are of opposite values, there is a respiratory disorder M=Metabolic E=Equal meaning that if HCO 3 and pH both increased or both decreased, there is a metabolic disorder Slide 12 Practice A 70 year-old smoker presents with an acute onset of shortness of breath: pH 7.30, PaCO 2 60 mmHg, HCO 3 30 mmol/L A 22 year-old woman presents with 4 hours of numbness in both hands typical of previous episodes of anxiety: pH 7.48, PaCO 2 30 mmHg, HCO 3 24 mmol/L A 68 year old man who recently took antibiotics for a skin infection presents with 10 episodes of watery diarrhoea per day for the last 5 days: pH 7.34, PaCO 2 35 mmHg, HCO 3 18 mmol/L A 20 year old student presents with excessive vomiting after binge drinking: pH 7.50, PaCO 2 44 mmHg, HCO 3 30 mmol/L Slide 13 No history, just gas 1) pH 7.24, pCO 2 55, HCO 3 32 2) pH 7.48, pCO 2 47, HCO 3 35 3) pH 7.36, pCO 2 34, HCO 3 19 4) pH 7.52, pCO 2 20, HCO 3 28 5) pH 7.44, pCO 2 35, HCO 3 26 6) pH 6.91, pCO 2 29, HCO 3 5 7) pH 7.18, pCO2 80, HCO 3 30 8) pH 7.55, pCO 2 20, HCO 3 33 9) pH 7.35, pCO 2 44, HCO 3 26 10) pH 7.43, pCO 2 32, HCO 3 28 Slide 14 Next time in Part II Step 3 Calculate compensatory changes. Adequate compensation? Acute or chronic process? Step 4 Calculate AG and gaps. Is there any additional pathological process? Mixed metabolic or respiratory disturbance? And later in Part III Step 5 Clinical correlation? Find the diagnosis. Slide 15 Any questions?