A 75-year-old female insulin-dependent diabetic presents to the Emergency Department semi-comatose. She has been unwell for several days and has a past medical history of left ventricular failure treated with digoxin and a thiazide diuretic.

The following data are from arterial blood gas analysis on admission:

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• FiO2 - 0.4
• pH - 7.4
• pO2 - 82.4mm Hg
• pCO2 - 32 mmHg
• HCO3 - 19mmol/l
• K - 2.7mmol/l
• glucose - 67mmol/l
• Na - 144mmol/l
• Cl - 91 mmol/l

interpret the acid base balance

there are 5 answers

1. high anion gap

2. Delta Ratio > 3

3. pCO2 is lower than expected

4. Hypokalemia

5. severe compensated metabolic acidosis

A-a ratio is high
PAO2 = (0.4 x 713) - (32 x 1.25) = 173.8
thus A-a =
173.8 - 82 = 91.8 mmHg

2. pH is normal

pCO2 is low

HCO3 is low suggesting respiratory alkalosis

Respiratory compensation is excessive

anion gap is raised

delta ratio

pure high anion gap acidosis

case no 2

The following arterial blood gas report was obtained from a 75-year-old female admitted to hospital with gastric outlet obstruction. She has tachypnoea with a diagnosis of aspiration pneumonia

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• fiO2 0.3
• pH 7,53
• pCO2 31mmHg
• pO2 83,7 mmHg
• HCO3 25
• SBE 3.3

a) Comment on the acid-base status


b) Give an explanation for these results

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• Let us dissect these results systematically. The A-a gradient is high; ~91mmHg
• PAO2 = (0.3 x 713) - (31 x 1.25) = 175.1
• A-a difference: 175.1- 83.7 = 91.45

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• There is alkalaemia The PaCO2 is contributing The SBE is 3.3, suggesting a mild metabolic alkalosis

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• The respiratory compensation is inadequate - the expected PaCO2 (25 × 0.7) + 20 = 37.5mmHg, and so there is also a respiratory alkalosis according to the Boston rules

Boston Rules

• The Boston Method: Six bicarbonate-based bedside rules to assess compensation

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• These rules represent an attempt to predict what an appropriate "complete" compensation would be to a given acid-base disorder.

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• Acute Respiratory Acidosis For every 10 mmHg increase in PaCO2, the HCO3- will rise by 1 mmol/L In other words, expected HCO3 = 24 + ((PaCO2-40) / 10)

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• Chronic Respiratory Acidosis For every 10 mmHg increase in PaCO2, the HCO3- will rise by 4 mmol/L In other words, expected HCO3 = 24 + (4 × (PaCO2-40) / 10)

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• Acute Respiratory Alkalosis For every 10 mmHg increase in PaCO2, the HCO3- will fall by 2 mmol/L In other words, expected HCO3 = 24 + (2 ×(PaCO2-40) / 10)

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• Chronic Respiratory Alkalosis For every 10 mmHg increase in PaCO2, the HCO3- will fall by 5 mmol/L In other words, expected HCO3 = 24 + (5 ×(PaCO2-40) / 10)

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• Metabolic Acidosis For complete compensation, expected PaCO2 = (1.5 × HCO3-) + 8

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• Metabolic Alkalosis For complete compensation, expected PaCO2 = (0.7 × HCO3-) + 20

The Copenhagen Method: Four SBE-based bedside rules to assess compensation

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• Acute Respiratory Acidosis or Alkalosis An acute change in PaCO2 will not change the Standard Base Excess. So, expected SBE = 0... in other words, if there is any change in SBE then it cannot be due to acute respiratory acid-base disturbances - it must be of metabolic origin. Working backwards, expected CO2 = 40 + (0 × SBE)

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• Chronic Respiratory Acidosis or Alkalosis Expected change in SBE = 0.4 times the change in PaCO2 In other words, expected SBE = 0.4 × (40 - PaCO2) Working backwards, expected CO2 = 40 + (0.4 × SBE)

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• Metabolic Acidosis Compensatory change in PaCO2 will be proportional to the SBE. In other words, expected CO2 = 40 + (1.0 × SBE)

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• Metabolic Alkalosis Compensatory change in PaCO2 will be proportional to 0.6 times the SBE. In other words, expected CO2 = 40 + (0.6 × SBE)

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