9.3.1: The Six Bedside Rules
The method of assessing acid-base disorders discussed here uses a set of six rules which are used primarily to assess the magnitude of the patient's compensatory response. These rules are now widely known and are soundly based experimentally. These rules are used at Step 4 of the method of Systematic Acid-Base Diagnosis outlined in Section 9.2.- (You should read section 9.1 & 9.2 before this section.) These rules are called 'bedside rules' because that can be used at the patient's bedside to assist in the assessment of the acid-base results. The rules should preferably be committed to memory - with practice this is not difficult.
A full assessment of blood-gas results must be based on a clinical knowledge of the individual patient from whom they were obtained and an understanding of the pathophysiology of the clinical conditions underlying the acid-base disorder. Do not interpret the blood-gas results as an intellectual exercise in itself. It is one part of the overall process of assessing and managing the patient.
Know the clinical details of the patient
A set of blood-gas and electrolyte results should NOT be interpreted without these initial clinical details. They cannot be understood fully without knowledge of the condition being diagnosed.
Find the cause of the acid-base disorder
Diagnosing a metabolic acidosis, for example, is by itself, often of little clinical use. What is really required is a more specific diagnosis of the cause of the metabolic acidosis (eg diabetic ketoacidosis, acute renal failure, lactic acidosis) and to initiate appropriate management. The acid-base analysis must be interpreted and managed in the context of the overall clinical picture.
The snapshot problem: Are the results 'current'?
Remember also that a set of blood gas results provides a snapshot at a particular point in time and the situation may have changed since the blood gases were collected so serial assessment of results can be important in assessment (eg of response to therapy).
Determine the major primary process then select the correct rule
The major primary process is usually suggested by the initial clinical assessment and an initial perusal of the arterial pH, pCO2 and [HCO3-] results. Once this major primary process is known, then the appropriate rule is chosen to assess the appropriateness of the patient's compensatory response.
The rules assess compensation and are a guide to detecting the presence of a second primary acid-base disorder: For example in a patient with a metabolic acidosis if the measured pCO2 level was higher than is expected for the severity and duration of the metabolic disorder, than this points to the coexistence of a respiratory acidosis. With a little practice the rules are simple to remember and are quick and easy to apply at the bedside. Rules 1 to 4 are best remembered by the description rather then memorizing the formula. These rules are outlined below
9.3.2: Rules for Respiratory Acid-Base Disorders
9.3.3: Rules for Metabolic Acid-Base Disorders
The combination of a low [HCO3-] and a low pCO2 occurs in metabolic acidosis and in respiratory alkalosis. If only one disorder is present it is usually a simple matter to sort out which is present. The factors to consider are:
- The history usually strongly suggests the disorder which is present
- The net pH change indicates the disorder if only a single primary disorder is present (eg acidaemia => acidosis)
- An elevated anion gap or elevated chloride define the 2 major groups of causes of metabolic acidosis
Remember that only primary processes are called acidosis or alkalosis. The compensatory processes are just that-- compensation. Phrases such as secondary respiratory alkalosis should not be used. (see Section 3.1)
Check Anion Gap and Delta Ratio
An elevated Anion Gap always strongly suggests a Metabolic Acidosis.
If a metabolic acidosis is diagnosed, then the Delta Ratio should be checked
|Delta Ratio Assessment Guidelines in patients with a metabolic acidosis