Chapter 24: Measuring Blood Gases

INTRODUCTION

Blood gas measurement and analysis is an important diagnostic tool used in both the operating room and intensive care unit. Normally drawn from an arterial blood source, they are performed to assess: (1) acid–base balance; (2) pulmonary oxygenation; and (3) alveolar ventilation.

ACID–BASE BALANCE

Normal arterial blood pH is in the range of 7.35–7.45. Through the Henderson–Hasselbalch equation, pH can be calculated as follows:

Acid–base disturbances can result in either acidosis (pH < 7.35) or alkalosis (pH > 7.45) and fall into the following categories: (1) metabolic acidosis; (2) metabolic alkalosis; (3) respiratory acidosis; and (4) respiratory alkalosis.

Table 24-1 lists several medical conditions that produce these acid–base disturbances. To maintain acid–base balance within the normal range, the body has three compensatory mechanisms: pulmonary ventilation to control the arterial carbon dioxide (PaCO₂), renal regulation of the metabolic component (bicarbonate or HCO₃⁻), and weak acid buffers. The primary protein buffer is hemoglobin, which takes up H⁺ ions when pH decreases and releases H⁺ ions when pH increases. With hemoglobin more than 5 g/dL, there is little change in the buffer system with variations in hemoglobin.

In the presence of a metabolic disturbance, the respiratory system will acutely compensate to correct acid–base derangements. For example, in the presence of a metabolic acidosis, the respiratory system will increase ventilation to decrease PaCO₂, thereby minimizing the change in pH. In the presence of a respiratory disturbance, the renally mediated metabolic component will compensate. However, this compensation requires a more prolonged period of at least 6–12 hours to appear, and only develops fully after several days. A mixed acid–base disturbance commonly occurs in clinical practice since the compensatory mechanisms do not necessarily correct these imbalances immediately or completely.

PULMONARY OXYGENATION

Arterial PO₂ (PaO₂) is dependent on several factors: inspired oxygen concentration, alveolar ventilation, mixed venous oxygen saturation (SvO₂), and ventilation–perfusion (V/Q) matching. As a person ages, there is an expected decrease in PaO₂. A normal PaO₂ for age can be determined by the following equation: