Acid Base Abnormalities

Welcome to your comprehensive MCCQE1 study resource on Acid Base Abnormalities. This guide is tailored for Canadian medical students preparing for the Medical Council of Canada Qualifying Examination Part I (MCCQE1). We'll cover key concepts, Canadian guidelines, and provide practice questions to help you succeed in your exam.

Introduction to Acid Base Disorders

Acid-base disorders are common clinical problems that Canadian physicians encounter regularly. Understanding these disorders is crucial for success in the MCCQE1 and for your future practice in the Canadian healthcare system.

💡

For MCCQE1 preparation, focus on the four primary acid-base disorders: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis.

Key Concepts for MCCQE1

1. Normal Values in Acid-Base Balance

Memorize these normal values for your MCCQE1 exam:

Parameter	Normal Range
pH	7.35 - 7.45
PaCO2	35 - 45 mmHg
HCO3-	22 - 26 mEq/L

2. Compensatory Mechanisms

Understanding compensatory mechanisms is crucial for MCCQE1 success:

Step 1: Identify the primary disorder

Determine which parameter (pH, PaCO2, or HCO3-) is abnormal and in which direction.

Step 2: Look for compensation

Check if the other parameters have changed in a way that would bring the pH closer to normal.

Step 3: Assess adequacy of compensation

Use the following rules of thumb:

For metabolic disorders: PaCO2 = 1.5 (HCO3-) + 8 ± 2
For respiratory disorders: Change in HCO3- = 0.1 × Change in PaCO2

3. Anion Gap

The anion gap is a key concept for MCCQE1 preparation:

Anion Gap = Na+ - (Cl- + HCO3-)
Normal range: 8-12 mEq/L

🇨🇦

In Canadian labs, potassium is often included in the anion gap calculation. Be aware of this difference for your MCCQE1 exam!

Types of Acid-Base Disorders

pH < 7.35
HCO3- < 22 mEq/L
Compensation: ↓ PaCO2

Metabolic Acidosis

For MCCQE1 preparation, remember the mnemonic MUDPILES for causes of metabolic acidosis:

M: Methanol
U: Uremia
D: Diabetic ketoacidosis
P: Paraldehyde
I: Isoniazid, Iron, Inborn errors of metabolism
L: Lactic acidosis
E: Ethylene glycol
S: Salicylates

Metabolic Alkalosis

Key causes to remember for your MCCQE1 exam:

Vomiting or nasogastric suction
Diuretic use (especially loop and thiazide diuretics)
Hyperaldosteronism
Excessive alkali intake

Respiratory Acidosis

Common causes in the Canadian healthcare context:

Chronic obstructive pulmonary disease (COPD)
Severe asthma exacerbation
Opioid overdose (a growing concern in Canada)
Neuromuscular disorders

Respiratory Alkalosis

Important causes for MCCQE1:

Anxiety or panic disorder
High altitude (relevant for patients in mountainous regions of Canada)
Sepsis
Pulmonary embolism

Canadian Guidelines for Acid-Base Management

The Canadian Association of Emergency Physicians (CAEP) provides guidelines for managing severe acidemia in the emergency department:

Identify and treat the underlying cause
Consider sodium bicarbonate therapy for severe metabolic acidosis (pH < 7.1) or in cases of tricyclic antidepressant overdose
Use caution with sodium bicarbonate in lactic acidosis
In diabetic ketoacidosis, focus on insulin therapy and fluid resuscitation rather than bicarbonate administration

Key Points to Remember for MCCQE1

Memorize normal values for pH, PaCO2, and HCO3-
Understand the compensatory mechanisms for each acid-base disorder
Know how to calculate and interpret the anion gap
Be familiar with the causes of each acid-base disorder, especially those common in Canadian practice
Understand the basic principles of managing acid-base disorders in the Canadian healthcare context
Be aware of specific Canadian guidelines for managing severe acidemia

Sample Question

A 68-year-old man presents to the emergency department with shortness of breath. He has a history of COPD and uses a salbutamol inhaler. His arterial blood gas shows:

pH: 7.30 PaCO2: 60 mmHg HCO3-: 29 mEq/L

Which one of the following acid-base disorders is most likely present in this patient?

Explanation

The correct answer is:

B. Chronic respiratory acidosis

This patient's arterial blood gas shows:

Low pH (< 7.35): indicating acidemia
High PaCO2 (> 45 mmHg): indicating respiratory acidosis
Elevated HCO3- (> 26 mEq/L): indicating metabolic compensation

The presence of metabolic compensation (elevated HCO3-) suggests that this is a chronic respiratory acidosis, which is consistent with the patient's history of COPD. In chronic respiratory acidosis, the kidneys have had time to compensate by retaining bicarbonate, leading to an elevated HCO3- level.

This question tests your ability to interpret arterial blood gases and understand the compensatory mechanisms in acid-base disorders, which is crucial for MCCQE1 success.

References

Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-45.
Canadian Association of Emergency Physicians. Position statement on early care of adults with severe acidemia in the emergency department. CJEM. 2018;20(3):331-340.
Seifter JL. Integration of acid-base and electrolyte disorders. N Engl J Med. 2014;371(19):1821-31.
Hamm LL, Nakhoul N, Hering-Smith KS. Acid-Base Homeostasis. Clin J Am Soc Nephrol. 2015;10(12):2232-42.
Kraut JA, Madias NE. Treatment of acute metabolic acidosis: a pathophysiologic approach. Nat Rev Nephrol. 2012;8(10):589-601.

Lower Gastrointestinal Bleeding White Blood Cells, Abnormalities of