Acid-Base Physiology

9.1 Structured Approach to Assessment

The purpose of this chapter is to teach a structured method for the assessment of acid-base disorders. The three stages involved are outlined in the table below.

Structured Approach to Diagnosis of Patients with Acid-Base Disorders

First: Initial Clinical Assessment

A clinical assessment based on clinical details is an essential first step

From the history, examination and initial investigations, make a clinical decision as to what is the most likely acid-base disorder(s).
This is very important but be aware that in some situations, the history may be inadequate, misleading or the range of possible diagnoses large.
Mixed disorders are often difficult: the history and examination alone are usually insufficient in sorting these out.

Second: Acid-Base Diagnosis

Perform a systematic evaluation of the blood gas and other results and make an acid-base diagnosis

The steps are outlined in Section 9.2

Finally: Clinical Diagnosis

Synthesise the information to make an overall clinical diagnosis

Attempt to produce an overall diagnosis of the patient’s condition to guide therapy. Do not view the acid-base disorder in isolation. The history, examination and results often allow very early diagnosis but it is very useful to systematically check the whole picture.

The essential first step is to assess the available clinical information (history, examination, investigations) and use this to make a clinical decision as to the possible and most likely acid-base diagnosis. A knowledge of the pathophysiology of conditions which cause acid-base disorders is extremely useful in making these initial assessments.

Sometimes these initial assessments are easy but sometimes they are misleading but in all cases they provide an initial clinical hypothesis used to guide the next step. Consider the following clinical scenario as a practical example.

Initial Clinical Assessment : An Example

History: A 23 year old woman with a history of insulin-dependent diabetes mellitus is on holidays and is not using her insulin regularly. She presents with vomiting, polyuria and feels unwell. Clinically she is tachypnoeic and looks ill. Findings on urinalysis are 4+ glucose and 2+ ketones.

Asessment: The diagnosis is obvious on this information: the patient has a significant diabetic ketoacidosis. Further investigations such as arterial blood gases and plasma biochemistry will provide:

confirmation of the diagnosis
assessment of severity of the acid-base disorder
evidence of the presence of other acid-base disorders (ie a mixed disorder)

The clinical assessment provides your initial orientation as to what is most likely. Effectively, you are maximising your use of the available clinical information and setting up a hypothesis about the diagnosis which you then test. You also use your knowledge of the pathophysiology to consider what other disorders or complications may coexist or may develop.

What other acid-base disorders could be present?

If she has pneumonia, respiratory compensation could be inadequate indicating the presence of a respiratory acidosis. These patients are significantly volume depleted and impaired perfusion can lead to a lactic acidosis and prerenal azotaemia. Excessive infusion of normal saline can lead to a hyperchloraemic metabolic acidosis and this has implications for therapy and expectations for the rate of correction of the acidosis. Vomiting can lead to a metabolic alkalosis. Useful investigations to sort out these are arterial blood gases, electrolytes, anion gap, urea and creatinine, glucose and lactate. So the obvious simple diagnosis can turn out to be much more complex.

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