Quantitative and/or qualitative analysis of exhaled carbon dioxide (CO2) has become standard practice in many clinical situations. The rationale for measuring the partial pressure of CO2 (PCO2) in exhaled gas is the assumption that end-tidal PCO2 (PETCO2) is a reflection of alveolar PCO2 (PaCO2). Hemoglobin plays an essential role in CO2 transport and elimination. Elimination of CO2 from the lung occurs as a function of gas exchange between the atmosphere and alveoli. In order to gain a greater understanding of the gradient between PaCO2 and PETCO2, an analysis of the interaction of pulmonary ventilation and perfusion is necessary. The alveolar gas equation (AGE) is used to analyze the effect of ventilation on oxygenation of arterial blood. An understanding of the interrelationship between pulmonary perfusion, ventilation, tidal volume, and regional VA/Q will enhance the utility of capnography as a monitor.

The three parts of the respiratory control system are: sensors, central control and respiratory muscles. The different disorders of ventilation include hypoventilation syndromes and hyperventilation syndromes. The three main types of hypoventilation syndromes are: obesity hypoventilation syndrome, respiratory neuromuscular disorders and primary alveolar hypoventilation. Alveolar hyperventilation exists when PaCO2 decreases below 4.5 kPa. It is not the same as hyperpnoea which is increased minute ventilation with a normal PaCO2. Detailed history and clinical examination along with knowledge of coexistent disorders can provide clues to the cause of hyperventilation. An elevated pH is suggestive of a primary respiratory alkalosis; a low pH will show a metabolic acidosis. Widened A-a gradient suggests presence of a primary pulmonary disorder. Low bicarbonate suggests a chronic nature of the disorder and implies an organic cause. Transcutaneous PCO2 or arterial PCO2 during sleep studies can help to exclude psychogenic hyperventilation.