Acid-Base Physiology

9.6 Clinical Examples

9.6.1 The Need for Experience: Practicing on Example Cases

The rules of the Boston approach are useful only if we know how to apply them clinically to patient care. This section provides a series of examples of their use in real patients so you can gain experience in interpretation. Many of these cases are from our own unit but some are based on published cases. These examples provide good practice in the application of the rules.

The central importance of the history and your clinical knowledge of the patient in assessment is emphasised.

In some cases, an enlarged history and serial results are provided. This should provide some experience in:

Some of the assessments are long and perhaps repetitive but previous experience has indicated that this ‘thinking out loud’ approach increases the usefulness of the examples as teaching material. Brief explanations don’t seem to teach much. The index to the Case Histories is at the bottom of this page.

9.6.2 The Prime Directive: Importance of the Clinical Details

But first, to illustrate how the history and examination are of prime importance in correct interpretation of blood gas results, consider the following set of arterial blood gases:


Arterial Blood Gases

pH 7.21

pCO2 70 mmHg

pO2 75 mmHg

HCO3 27 mmol/l

These identical gases were obtained from the following two patients (based on Bernards).

Is the assessment of the results the same even though the clinical situation is very different?

No!

The pattern (pCO2 & HCO3 both elevated) suggests either a respiratory acidosis or a metabolic alkalosis but the severe acidaemia means that it is a respiratory acidosis that is present. This much is common ground to these two cases. The clinical details are necessary to decide if a simple or a mixed acid-base disorder is present.

Assessment of Case 1: This patient is receiving a relaxant anaesthetic for an upper abdominal procedure. His ventilation is fully controlled. A perusal of the results in the light of the clinical details (respiratory acidosis in a well patient on controlled ventilation) suggests strongly that the most likely primary problem is hypoventilation in a patient with previously normal acid-base results. A marked acute respiratory acidosis is present. [In any anaesthetised patient with an acute acidosis, malignant hyperthermia, though rare, should always be considered.]

Is a metabolic disorder also present in this patient? (eg due to lactic acidosis) The [HCO3] would be expected to increase by 1 mmol/l for each 10 mmHg rise in pCO2 above the nominal usual value of 40 mmHg. (Rule 1 in Section 9.3). A rise of 30mmHg predicts a [HCO3] of 27 (ie 24 + 3). The actual value matches the predicted value. There is no metabolic component present.

If the history suggested that the situation may be more complex then a check should be made for any suggestive evidence of a mixed metabolic component (coexistent metabolic acidosis and metabolic alkalosis) as well as the acute respiratory acidosis. This check would include initially anion gap, [K+], [Cl-] and glucose. In this case there is no clinical indication.

Acute respiratory acidosis due to alveolar hypoventilation is the acid-base assessment in this case. The cause for this should be found and corrected. The absence of a metabolic component and the other clinical evidence makes a diagnosis of excessive CO2 production (eg malignant hyperthermia) very unlikely.

Assessment of Case 2: This man has severe chronic obstructive airways disease and has an elevation in his pCO2 which has probably been present for at least 3 or 4 days. He is probably a chronic CO2 retainer with some chronic elevation in his pCO2. Review of previous blood gas results or ‘bicarbonate’ (ie ‘total CO2’) levels on a biochemistry profile may confirm this. In any case, the history suggests chronic respiratory acidosis.

Based on rule 2, the predicted [HCO3] is 36 mmol/l [ie: 24 + { ( 70-40 / 10 ) x 4 } ]. The actual [HCO3] is 9 mmol/l lower then this indicating a coexistent severe metabolic acidosis. Note that the pO2 is not severely depressed. Patients admitted with respiratory distress are almost invariably commenced on oxygen by ambulance and hospital staff. This may be life-saving as the pO2 in increased.

A lactic acidosis related to hypoxaemia and maybe peripheral circulatory failure is the probable cause of the metabolic acidosis. Other causes of metabolic acidosis should be considered. Infection is a potent precipitant of diabetic ketoacidosis. A finger-prick test for glucose and urine tests for glucose and ketones should be performed on arrival in the Casualty department. The anion gap will define the type of metabolic acidosis present and guide further investigation.

This patient has a severe mixed acidosis. An acute severe metabolic acidosis is superimposed on a compensated chronic respiratory acidosis. The metabolic compensation for the respiratory disorder has disguised the magnitude of the metabolic acidosis.

It is noted that the gas results in these two cases are identical, but that the interpretation and therefore management are different.

Commenting on an isolated set of blood gas results without benefit of any pertinent history can lead to serious error.

Remember that the clinician is focusing on the assessment of the patient and here our attention is predominantly on the acid-base assessment.

9.6.3. Clinical Cases

Index to Clinical Examples

Further examples with more extensive discussions can be found in the Gas Archives

1. Postoperative Cardiac Arrest

2. A Sick Diabetic Patient

3. A weak old lady

4. A case of pneumonia

5. A motor vehicle crash

6. A COAD patient with acute abdominal pain

7. A dehydrated man with diarrhoea

8. A diabetic patient with vomiting and polyuria

9. A man with a postop cardiac arrest

10. A semi-comatose diabetic taking diuretics

11. A man with CCF & vomiting

12.A weak patient following a week of diarrhoea 

13. A case with a postop morphine infusion

14. A man with an out-of-hospital cardiac arrest 

15. An old man with abdominal pain & shock

16. A woman with muscle weakness and vomiting

17. An intoxicated baby

18. A smoker with fever and rigors

19. A young man who ingested barium carbonate

20. An alcoholic with GIT bleeding and shock

21. A vague historian with weakness and diarrhoea

22. An old man with hiccoughs and confusion

23. A diabetic using phenformin

24. A man with a leaking aneurysm

25. An old lady with abdominal pain & vomiting 

26. A man with a gunshot wound & a cardiac arrest 

27. To be added

28. A lady with a rigid abdomen

29. A teenage boy with an obstructed colonic bladder

30. A child with ingestion of windscreen washer fluid