Gas Composition of Anatomical Dead Space

The composition of V_Danat gas is different at the end of expiration than at the end of inspiration (Figure 4-3). An inspired V_T of 500 mL completely flushes the normal 150 mL of V_Danat with atmospheric air, leaving behind a PCO₂ of 0 mm Hg and a PO₂ of 149 mm Hg (see Figure 4-3, left). Dead space gas is identical in composition to inspired air except that it is 100% humidified. After the 500 mL of V_T is completely exhaled, the 150 mL of V_Danat (see Figure 4-3, right) is wholly occupied by gas that came from the alveoli. The humidified atmospheric air that occupied the dead space at end-inspiration was exhaled ahead of the alveolar gas (i.e., this gas was wasted because it did not participate in respiration). At end-tidal expiration, dead space gas is identical in composition to the average alveolar gas composition. Under normal circumstances, end-tidal expiratory gas is assumed to be identical in composition to alveolar gas.

The alveolar gas residing in the dead space is reinspired with the next breath. Therefore, the first 150 mL of the inspired V_T does not change the alveolar gas composition.

The exhaled V_T is a mixture of dead space gas (identical to inspired gas—containing no CO₂) and alveolar gas (which contains CO₂). The PCO₂ of mixed exhaled gas is lower than alveolar PCO₂ (P_ACO₂) because dead space gas free of CO₂ is a component of exhaled gas. This fact is helpful in understanding the calculation of the dead space-to-tidal volume ratio (V_D/V_T) later in this chapter.

Measuring Anatomical Dead Space

V_Danat is related to lung size; in normal adults, anatomical dead space is approximately 1 mL per pound of ideal body weight.¹ The Fowler technique provides a more precise measurement of V_Danat (Figure 4-4).² In the Fowler technique, the subject first exhales maximally to residual volume (RV), then takes a maximal inhalation of 100% O₂ to total lung capacity, then exhales maximally to RV again. This technique is based on the fact that air is a mixture of N₂ and O₂. An N₂ analyzer is used to measure continuously the exhaled N₂ concentration at the mouth after the single maximal inspiration of 100% O₂. N₂ concentration measured at the mouth decreases abruptly to 0 during the 100% O₂ inspiration (see Figure 4-4). At the end of inspiration, V_Danat contains pure O₂, which is the first gas to be exhaled. This means that N₂ concentration remains at 0% for the first part of the expiration.

As expiration proceeds, alveolar gas (which still contains some N₂) moves up into the conducting airways, and dead space N₂% gradually increases to alveolar levels—note the S-shaped curve in Figure 4-4. The increase in N₂% would be sharp and abrupt if dead space and alveolar gas were completely separated, as illustrated by the hypothetical square front in Figure 4-5, A. If this were the case, N₂% would remain at 0% until all dead space gas was expired, and then it would abruptly increase to alveolar levels when alveolar gas suddenly appeared, as shown by the theoretical thin, solid vertical line in Figure 4-4. V_Danat would simply be the volume exhaled to the sharp increase in N₂%. However, alveolar gas moves through conducting airways in a conical rather than square front (see Figure 4-5, B), mixing with dead space gas. This movement makes it difficult to detect the exact point at which only the V_Danat has been expired. It is necessary to construct a line representing the theoretical square front that would be seen if all N₂-free dead space gas were expired first, followed by only N₂-containing alveolar gas. The hypothetical square front is constructed by placing a vertical line (see Figure 4-4) such that area A equals area B (i.e., in effect, the vertical line averages alveolar and dead space gas compositions). The dead space volume is the volume expired up to the hypothetically constructed square front.

Alveolar and Physiological Dead Space

Alveolar dead space (V_DA) is the volume contained in nonperfused alveoli—that is, alveoli with no blood flow. The presence of V_DA is abnormal. Any factor decreasing pulmonary blood flow, such as extremely low cardiac output or a pulmonary embolus, increases V_DA. V_DA represents a decreased surface area for gas exchange and an increase in total wasted ventilation.

Physiological dead space (V_D) is the sum of V_Danat and V_DA (V_D = V_Danat + V_DA). Similarly, physiological dead space ventilation (V._D) per minute is as follows:

V.D=V.Danat+V.DA

V._D is increased not only by decreased pulmonary blood flow but also by an increased breathing rate. Doubling the breathing frequency doubles the number of times the V_T moves through the V_Danat, doubling the V._Danat.

Hyperventilation and Hypoventilation

If V._A momentarily removes more CO₂ per minute than is metabolically produced, alveolar and blood PCO₂ decrease, and a state of hyperventilation exists. Similarly, if V._A momentarily removes less CO₂ than the body produces, alveolar and blood PCO₂ increase, and a state of hypoventilation exists. Normally, alveolar gas and blood PCO₂ values equilibrate as blood flows past alveoli and enters the left ventricle. As Figure 4-3 shows, arterial blood arising from the left ventricle has the same PCO₂ as the alveoli (P_ACO₂ = PaCO₂).

V._A determines PaCO₂ because it controls P_ACO₂. The PaCO₂ obtained clinically through arterial blood gas analysis is the definitive indicator of V._A. Hyperventilation and hypoventilation are defined by V._A relative to CO₂ production, a relationship that can be known only by measuring PaCO₂. If PaCO₂ is above normal (hypercapnia), hypoventilation exists; if PaCO₂ is below normal (hypocapnia), hyperventilation exists. The presence of hyperventilation or hypoventilation cannot be reliably determined by observing only the respiratory rate or V_T depth.

Alveolar Ventilation and P_ACO₂

P_ACO₂ is inversely related to PA