Acid Base Introduction

Part I Introduction

Jakub MateraAcid Base Interpretation

Normal valuespH 7.40 (7.35 7.45)Pa CO2 40 mmHg (35 45) Pa O2 80 100 (expected PaO2 = FiO2 x 5)HCO3 24 mmol/L (20 28) BE -2 to +2 mEq/LVenous or arterial sample

Venous or arterial sample?

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).ConclusionsThere 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.5 steps to analyse acid baseStep 1Look at the pH. Acidaemia or alkalaemia?Step 2Who is responsible for this change in pH ( primary culprit )?Step 3Calculate compensatory changes. Adequate compensation? Acute or chronic process?Step 4Calculate AG and gaps. Is there any additional pathological process? Mixed metabolic or respiratory disturbance?Step 5Clinical correlation? Find the diagnosis.Step 1 Look at the pH AcidaemiapH < 7.35 AlkalaemiapH > 7.45

If the pH is within normal range (7.35 7.45) but PaCO2 or HCO3 is abnormal indicating an acid base imbalance use 7.40 as your cutoff point.pH between 7.35 and 7.40 indicates acidosispH between 7.40 and 7.45 indicates alkalosisPrinciple: The body does not fully compensate for primary acid base disordersStep 1 Look at the pH What if pH is 7.40 exactly, but PaCO2 or HCO3 is abnormal, indicating an acid base imbalance???Is this possible?We will talk about it in part II.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.

Step 2 Determine primary process Respiratory alkalosis pH, PaCO2acidosis pH, PaCO2

Metabolicalkalosis pH, HCO3 acidosis pH, HCO3

Or you can remember the acronym ROME.

R=Respiratory O=Opposite meaning that if PaCO2 and pH are of opposite values, there is a respiratory disorder

M=Metabolic E=Equalmeaning that if HCO3 and pH both increased or both decreased, there is a metabolic disorderPracticeA 70 year-old smoker presents with an acute onset of shortness of breath:pH 7.30, PaCO2 60 mmHg, HCO3 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, PaCO2 30 mmHg, HCO3 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, PaCO2 35 mmHg, HCO3 18 mmol/L

A 20 year old student presents with excessive vomiting after binge drinking:pH 7.50, PaCO2 44 mmHg, HCO3 30 mmol/L12No history, just gaspH 7.24, pCO2 55, HCO3 32pH 7.48, pCO2 47, HCO3 35pH 7.36, pCO2 34, HCO3 19pH 7.52, pCO2 20, HCO3 28pH 7.44, pCO2 35, HCO3 26pH 6.91, pCO2 29, HCO3 5pH 7.18, pCO2 80, HCO3 30pH 7.55, pCO2 20, HCO3 33pH 7.35, pCO2 44, HCO3 26pH 7.43, pCO2 32, HCO3 28

Next time in Part IIStep 3Calculate compensatory changes. Adequate compensation? Acute or chronic process?Step 4Calculate AG and gaps. Is there any additional pathological process? Mixed metabolic or respiratory disturbance?

And later in Part IIIStep 5Clinical correlation? Find the diagnosis.

Any questions?