9.6 Case 11

Clinical Details
A 70 year old man was admitted with severe congestive cardiac failure. He has been unwell for about a week and has been vomiting for the previous 5 days. He was on no medication. He was hyperventilating and was very distressed. Admission biochemistry is listed below. He was on high concentration oxygen by mask. Arterial blood gases:

Biochemistry results: Na⁺ 127, K⁺ 5.2, Cl^- 79, HCO3^- 20, urea 50.5, creatinine 0.38 & glucose 9.5 mmols/l. Anion gap 33 mmols/l

Arterial Blood Gases

pH 7.58

pCO2 21 mmHg

pO2 154 mmHg

HCO3 19 mmol/l

Assessment
Firstly, initial clinical assessment : The history suggests the following possibilities:

Secondly, the acid-base diagnosis:
1. pH: pH>7.44 so an alkalaemia is present. The cause is an alkalosis
2. Pattern: pCO2 & bicarbonate are both low suggesting either a metabolic acidosis (with respiratory compensation) or a respiratory alkalosis (with renal compensation). As we know an alkalosis must be present then we would accept a respiratory alkalosis.
3. Clues: The anion gap is noted to be very high so there must be a high-anion gap metabolic acidosis present as well. To explore the causes of a HAGMA:

The creatinine is high enough to be associated with hyperkalaemia and metabolic acidosis (due to renal retention of acid anions).

There is no evidence presented of toxic ingestions & no supportive history (eg neurological symptoms)

4. Compensation: Asessing the compensation for a respiratory alkalosis (using the 5 for 10 rule): The expected HCO3 is (24 - 10) = 14. The actual HCO3 is higher (19) which indicates the presence of a metabolic alkalosis.
5. Formulation: Triple acid-base disorder (respiratory alkalosis, high anion gap metabolic acidosis & a metabolic alkalosis).
6. Confirmation: A lactate level should be checked to exclude a lactic acidosis. A urine test for ketones should be routine.

Finally, the Clinical Diagnosis:
CCF with respiratory distress (respiratory alkalosis, renal failure possibly of pre-renal cause (high anion gap metabolic acidosis) & vomiting (metabolic alkalosis) resulting in a complicated acid-base picture. Interestingly, all of these possibilities were suggested by the history. A co-existing lactic acidosis is not excluded.

A perusal of this set of results by an experienced clinician would indicate several things that really stick out:

Metabolic acidosis is the only cause of a large anion gap but several other situations may cause a minor elevation in anion gap. An example is large doses of carbenicillin or penicillin which deliver the antibiotic anion as the sodium salt. The antibiotic anion is not measured and this increases the anion gap. The anion gap in this case is large and there is no history suggestive of other factors affecting the anion gap so a severe metabolic acidosis can confidently be diagnosed. The chloride level is quite low and it is always worth checking this result with another specimen.

The diagnosis so far is: a high anion gap metabolic acidosis is present. These are 4 categories of causes for this condition:

A slight stress related increase in blood glucose is noted. Euglycaemic diabetic ketoacidosis is uncommon and is clinically extremely unlikely here. Normal urinary ketone levels should exclude it. There are two problems to be aware of with ketone testing and these both tend to give false negative results: the reagent strip may be outdated and give a false negative, or a coexistent lactic acidosis may be present. A lactic acidosis alters the beta-hydroxybutyrate to acetoacetate ratio and acetoacetate levels will fall. The ‘Ketostix’ nitroprusside test detects acetoacetate only and may give a false negative result for ‘ketones’ in this situation. There is no note of the detection of the odour of ketones being detected. Assessment on current data in this case: there is no evidence of ketoacidosis.

Toxin ingestions (eg ethylene glycol, salicylates, methanol, paraldehyde) are excluded on the history so far. Any history of alcoholism or if the patient appears intoxicated should prompt re-investigation of this diagnosis.

The [HCO3] is inappropriately high for the rise in the anion gap. The anion gap reported here was calculated using the formula which includes potassium, so 17 mmols/l can be taken as its upper reference limit. The anion gap is 16 mmol/l higher than this upper limit but the [HCO3] is only 5 mmol/l lower than its reference value of 24 mmol/l. The delta ratio is high (3.2) so lets consider the two possibilities that are suggested by this and how to distinguish between them:

First possibility: A coexisting metabolic alkalosis is present.
Assessment: The history is consistent with this as severe vomiting is present and this is an important cause of metabolic alkalosis.

Second possibility: The patient has a chronic respiratory acidosis with renal retention of bicarbonate.
For example, if the patient had a chronic hypercapnia with pCO2 of 60 mmHg, renal compensation would raise the [HCO3] to 32 mmols/l (based on the ‘4 for 10 rule’). A bicarbonate level of 19 could represent a fall from 32 mmols/l: a decrement of 13 mmols/l (in this hypothetical example). If this hypothesis is correct, then there may be no metabolic alkalosis.

SO: What evidence can be found of a recent or long-standing chronic respiratory acidosis?
None. There is no history of chronic respiratory disease. CO2 retention requires severe lung disease. Current pCO2 is low so the patient is obviously able to hyperventilate enough to drop his pCO2 despite the presence of congestive heart failure! Other useful checks are:

Our assessment is that there is no evidence of chronic respiratory acidosis in this patient.

Conclusion then is that a metabolic alkalosis is present. The majority of causes of metabolic alkalosis are due to diuretic use or loss of acidic gastric juice (vomiting or NG suction). A history of five days of vomiting is the cause here. Diuretic use has been excluded on history.

The rise in the anion gap is normally larger than the fall in bicarbonate because at least half of the buffering for metabolic acid-base disorders occurs intracellularly (minimising the decrement in plasma [HCO3]) but the acid anions remain extracellularly and contribute as excess unmeasured anions to the anion gap.

Metabolic alkalosis (due to persistent vomiting) as the major disorder (net alkalaemia)

There is clearly still a problem here because although there is a significant net alkalaemia (suggesting the metabolic alkalosis is more severe then the metabolic acidosis), the [HCO3] is reduced rather then elevated! A third primary acid-base disorder must be present and adding to the net alkalaemia (ie there must be a respiratory alkalosis). Or using the rules to assess compensation:

Is the respiratory compensation appropriate?
As the [HCO3] is less then 24 mmol/l, rule 5 for a metabolic acidosis (rather than rule 6) is best to assess this. The predicted pCO2 is about 36 mmHg. As the actual pCO2 is much lower than this, the third primary acid-base disorder is a respiratory alkalosis. This is secondary to the heart failure and the hyperventilation stimulated by the dyspnoea. Any condition which decreases lung compliance will cause dyspnoea. If the pulmonary oedema was more severe then a respiratory acidosis would be a likely outcome.

The final acid-base assessment is that this patient has a triple acid-base disorder:

Acute metabolic acidosis probably due to renal failure (?prerenal failure) and possibly to lactic acidosis (hypoperfusion due heart failure and hypovolaemia)

The pO2 is elevated due to administration of a high inspired oxygen concentration.

In complicated cases like this it is very important to check results to avoid errors. For example, in this case if the [Cl^-] was really 99 instead of 79mmol/l, then the anion gap would not be elevated. An error of this size would most likely be a transcription error. Similarly check that the blood gas results are internally consistent (ie put the results in the Henderson-Hasselbalch equation and check they are correct). Results taken over the phone by an intermediary can be written incorrectly. Printed results are generally reliable but of course they may still be in error because of faults with collection and handling of the blood gas sample prior to analysis.