Acid-base management

Acid-base management

 

Pretest

 

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.

Keyword

 

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 below:

 

The Six Steps of Systematic Acid-Base Evaluation

 

1. pH: Assess the net deviation of pH from normal

 

2. Pattern: Check the pattern of bicarbonate & pCO2 results

 

3. Clues: Check for additional clues in other investigations

 

4. Compensation: Assess the appropriateness of the compensatory response

 

5. Formulation: Bring the information together and make the acid base diagnosis

 

6. Confimation: Consider if any additional tests to check or support the diagnosis are necessary or available & revise the diagnosis if necessary

 

 

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

 

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.

 

From

http://www.anaesthesiamcq.com/AcidBaseBook/ab9_2.php

 

Post test

 

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.