INI-CET 2021 — Physiology

Question 11: Acid-base imbalance is suspected in a patient. Which of the following parameters would you use for initial determination of acid-base status?

• A. pH, PaCO2, and Base excess
• B. pH, PaCO2, and Bicarbonate (Correct Answer)
• C. pH and PaCO2
• D. pH, PaCO2, Bicarbonate, and Base excess

Explanation: ***pH, PaCO2, and Bicarbonate*** - The **pH** provides immediate assessment of overall acid-base status (acidemia if <7.35 or alkalemia if >7.45) - The **PaCO2** reflects the respiratory component - elevated in respiratory acidosis or compensated metabolic alkalosis; decreased in respiratory alkalosis or compensated metabolic acidosis - The **HCO3- (bicarbonate)** reflects the metabolic component - essential for determining whether the primary disorder is metabolic or respiratory - This triad forms the **standard approach** to arterial blood gas (ABG) interpretation taught in all major medical textbooks - Together, these three parameters allow complete initial classification of acid-base disorders using the Henderson-Hasselbalch relationship *pH and PaCO2* - While pH and PaCO2 are critical measurements, **without bicarbonate**, you cannot differentiate between metabolic and respiratory disorders or assess metabolic compensation - For example, a low pH with normal PaCO2 could indicate metabolic acidosis, but you need HCO3- to confirm this diagnosis - Incomplete for initial acid-base determination *pH, PaCO2, and Base excess* - Base excess is a **calculated parameter** used to quantify the metabolic component of acid-base disturbances - While useful, it is considered a **secondary parameter** for more detailed metabolic analysis rather than essential for initial determination - Standard ABG interpretation uses bicarbonate, not base excess, as the primary metabolic parameter *pH, PaCO2, Bicarbonate, and Base excess* - While this includes all relevant parameters, **base excess is redundant** for initial determination - Base excess adds quantitative information about metabolic component but is not required for the initial classification of acid-base status - The essential triad for initial assessment is pH, PaCO2, and HCO3-