5 MEDLINE search of papers published from 1966 to January 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 arterio–venous difference in pH was 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 arterio–venous 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 arterio–venous difference is mmol/L (n = 103).
6 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 testbut venous pCO2 might be useful to screen for arterial hypercarbia or to monitor trends in pCO2 for selected patients.
7 5 steps to analyse acid base Look 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.
8 Step 1 – Look at the pH Acidaemia pH < 7.35 Alkalaemia pH > 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 disorders
9 Step 1 – Look at the pHWhat 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.
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.
11 Step 2 – Determine primary process Respiratoryalkalosis ↑ pH, ↓ PaCO2acidosis ↓ pH, ↑ PaCO2Metabolicalkalosis ↑ pH, ↑ HCO3acidosis ↓ pH, ↓ HCO3Or you can remember the acronym ROME. R=RespiratoryO=Oppositemeaning that if PaCO2 and pH are of opposite values, there is a respiratory disorderM=MetabolicE=Equalmeaning that if HCO3 and pH both increased or both decreased, there is a metabolic disorder
12 PracticeA 70 year-old smoker presents with an acute onset of shortness of breath:pH 7.30, PaCO2 60 mmHg, HCO3 30 mmol/LA 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/LA 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/LA 20 year old student presents with excessive vomiting after binge drinking:pH 7.50, PaCO2 44 mmHg, HCO3 30 mmol/L
14 Next time in Part II And later in Part III Step 3 Calculate 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.