Acid-Base Physiology

9.3 Bedside Rules for Assessment of Compensation

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

Rule 1 : The 1 for 10 Rule for Acute Respiratory Acidosis

The [HCO₃] will increase by 1 mmol/l for every 10 mmHg elevation in pCO₂ above 40 mmHg.

Expected [HCO₃] = 24 + { (Actual pCO₂ - 40) / 10 }

Comment:The increase in CO₂ shifts the equilibrium between CO₂ and HCO₃ to result in an acute increase in HCO₃. This is a simple physicochemical event and occurs almost immediately.

Example: A patient with an acute respiratory acidosis (pCO₂ 60mmHg) has an actual [HCO₃] of 31mmol/l. The expected [HCO₃] for this acute elevation of pCO₂ is 24 + 2 = 26mmol/l. The actual measured value is higher than this indicating that a metabolic alkalosis must also be present.

Rule 2 : The 4 for 10 Rule for Chronic Respiratory Acidosis

The [HCO3] will increase by 4 mmol/l for every 10 mmHg elevation in pCO2 above 40mmHg.

Expected [HCO3] = 24 + 4 { (Actual pCO2 - 40) / 10}

Comment: With chronic acidosis, the kidneys respond by retaining HCO3, that is, renal compensation occurs. This takes a few days to reach its maximal value.

Example: A patient with a chronic respiratory acidosis (pCO2 60mmHg) has an actual [HCO3] of 31mmol/l. The expected [HCO3] for this chronic elevation of pCO2 is 24 + 8 = 32mmol/l. The actual measured value is extremely close to this so renal compensation is maximal and there is no evidence indicating a second acid-base disorder.

Rule 3 : The 2 for 10 Rule for Acute Respiratory Alkalosis

The [HCO3] will decrease by 2 mmol/l for every 10 mmHg decrease in pCO2 below 40 mmHg.

Expected [HCO3] = 24 - 2 { ( 40 - Actual pCO2) / 10 }

Comment: In practice, this acute physicochemical change rarely results in a [HCO3] of less than about 18 mmol/s. (After all there is a limit to how low pCO2 can fall as negative values are not possible!) So a [HCO3] of less than 18 mmol/l indicates a coexisting metabolic acidosis.

Rule 4 : The 5 for 10 Rule for a Chronic Respiratory Alkalosis

The [HCO3] will decrease by 5 mmol/l for every 10 mmHg decrease in pCO2 below 40 mmHg.

Expected [HCO3] = 24 - 5 { ( 40 - Actual pCO2 ) / 10 } ( range: +/- 2)

Comments:

It takes 2 to 3 days to reach maximal renal compensation
The limit of compensation is a [HCO3] of about 12 to 15 mmol/l

9.3.3 Rules for Metabolic Acid-Base Disorders

Rule 5 : The One & a Half plus 8 Rule - for a Metabolic Acidosis

The expected pCO2 (in mmHg) is calculated from the following formula:

Expected pCO2 = 1.5 x [HCO3] + 8 (range: +/- 2)

Comments:

Maximal compensation may take 12-24 hours to reach
The limit of compensation is a pCO2 of about 10 mmHg
Hypoxia can increase the amount of peripheral chemoreceptor stimulation

Example: A patient with a metabolic acidosis ([HCO3] 14mmol/l) has an actual pCO2 of 30mmHg. The expected pCO2 is (1.5 x 14 + 8) which is 29mmHg. This basically matches the actual value of 30 so compensation is maximal and there is no evidence of a respiratory acid-base disorder (provided that sufficient time has passed for the compensation to have reached this maximal value). If the actual pCO2 was 45mmHg and the expected was 29mmHg, then this difference (45-29) would indicate the presence of a respiratory acidosis and indicate its magnitude. See Section 5.5 for more details.

Rule 6 : The Point Seven plus Twenty Rule - for a Metabolic Alkalosis

The expected pCO2(in mmHg) is calculated from the following formula:

Expected pCO2 = 0.7 [HCO3] + 20 (range: +/- 5)

Comment: The variation in pCO2 predicted by this equation is relatively large. (The reasons for this are discussed in section 7.5)

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 AG is 20-30 then high chance (67%) of metabolic acidosis If AG is > 30 then a metabolic acidosis is definitely present

If a metabolic acidosis is diagnosed, then the Delta Ratio should be checked

Delta Ratio Assessment Guidelines in patients with a metabolic acidosis < 0.4 - Hyperchloraemic normal anion gap acidosis 0.4 to 0.8 - Combined high AG and normal AG acidosis 1 - Common in DKA due to urinary ketone loss 1 to 2 - Typical pattern in high anion gap metabolic acidosis > 2 Check for either a co-existing Metabolic Alkalosis (which would elevate [HCO3]) or a co-existing Chronic Respiratory Acidosis (which results in compensatory elevation of [HCO3])

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