Makindo Medical Notes

Related Subjects: Asthma |Acute Severe Asthma |Exacerbation COPD |Pulmonary Embolism |Cardiogenic Pulmonary Oedema |Pneumothorax |Tension Pneumothorax |Respiratory (Chest) infections Pneumonia |Fat embolism |Hyperventilation Syndrome |ARDS |Respiratory Failure |Diabetic Ketoacidosis

Introduction

• The alveolar gas equation is used to estimate alveolar oxygen partial pressure (PAO₂) since it is not feasible to collect gases directly from the alveoli.
• This calculation helps estimate the partial pressure of oxygen (PaO₂) inside the alveoli under various physiological and pathophysiological conditions.
• The respiratory quotient (RQ) can vary depending on diet and metabolic state: carbohydrates, fats, and proteins each yield different RQ values. The average RQ for a typical human diet is approximately 0.82.
• Indirect calorimetry can provide a more precise measurement of RQ by assessing oxygen uptake (VO₂) and carbon dioxide production (VCO₂).

Alveolar Gas Equation:

PAO₂ = (Patm - PH₂O) FiO₂ - PaCO₂ / RQ

At sea level, the alveolar PAO₂ is:
PAO₂ = (760 - 47) × 0.21 - 40 / 0.8 = 99.7 mm Hg

A-a Gradient: A-a gradient = PAO₂ - PaO₂

Definitions

• PAO₂: Partial pressure of oxygen in the alveoli.
• PaO₂: Partial pressure of oxygen in the arterial blood.
• PH₂O: Partial pressure of water (typically around 45 mm Hg).
• FiO₂: Fraction of inspired oxygen (usually 21% in room air).
• PaCO₂: Partial pressure of carbon dioxide in the alveoli (normally 40 to 45 mm Hg).
• RQ: Respiratory quotient, reflecting the ratio of carbon dioxide produced to oxygen consumed.

The primary variables influencing the alveolar gas equation include atmospheric pressure, inspired oxygen fraction, and carbon dioxide levels. These factors have significant clinical relevance, providing insights into various physiological and pathophysiological conditions.