Ventilation

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Chapter 4

Ventilation

Objectives

After reading this chapter, you will be able to:

• Calculate the partial pressures of gases under dry and 100% relative humidity conditions

• Explain how minute ventilation, alveolar ventilation, and dead space ventilation are interrelated

• Determine whether hyperventilation or hypoventilation is present by reviewing the arterial carbon dioxide pressure (PaCO₂)

• Explain why alveolar ventilation affects PaCO₂

• Explain the rationale underlying the calculation of anatomical dead space, alveolar ventilation, and physiological and anatomical dead space-to-tidal volume ratios

• Predict the effects of minute ventilation changes on alveolar ventilation, dead space ventilation, and PaCO₂

• Explain how ventilatory pattern affects dead space-to-tidal volume ratio and, subsequently, alveolar ventilation and PaCO₂

• Describe the theoretical basis for measuring and calculating dead space and alveolar ventilation

Ventilation is the process of moving each breath, the tidal volume (V_T), in and out of the lungs. During tidal ventilation, fresh atmospheric air mixes with and replaces some of the air already in the lungs. Each exhalation removes some of the carbon dioxide (CO₂) the blood brings to the alveoli. Each inhalation replenishes the oxygen (O₂) the blood removes from the alveoli. Because not all the inspired V_T reaches the alveoli, only a portion of it participates in the gas exchange that occurs between blood and alveolar air, a process called external respiration. The depth and frequency of tidal breathing (i.e., the ventilatory pattern) greatly affects this gas exchange. Respiratory therapists and other health care clinicians must clearly understand factors affecting ventilation and respiration to assess the benefits and hazards of treatment options.

Partial Pressures of Respiratory Gases

The measurement of respiratory gas pressures must be understood to comprehend the function of the lung as a gas-exchange organ. Air is a gas mixture of mostly nitrogen (N₂) and O₂, with traces of argon, CO₂, and other gases (Figure 4-1, A). The total combined pressure exerted by these atmospheric gases, the barometric pressure (P_B), can be measured by a mercury (Hg) barometer, as shown in Figure 4-1, B. At sea level, atmospheric pressure exerts a force equal to the weight of a mercury column 760 mm high. By convention, standard atmospheric pressure at sea level is expressed simply as the height of the mercury column it supports, or 760 mm Hg. (The term torr is equivalent to mm Hg. In this book mm Hg is used.)

Figure 4-1 Partial pressures of gases in atmospheric air. A, Relative concentrations of atmospheric gases. B, Mercury barometer. Atmospheric pressure pushes mercury down in the dish and up the tube. Sea-level pressure supports a column of mercury 760 mm high in the tube. (From Patton KT, Thibodeau GA: *Anatomy & physiology,* ed 7, St Louis, 2010, Mosby.)

According to Dalton’s law, the pressure of each gas that makes up air exerts a partial pressure proportional to its fractional concentration in air. The partial pressure of any gas (P_gas) in air is equal to its fractional concentration (F_g) multiplied by total atmospheric pressure: P_B (P_gas = F_g × P_B).

CONCEPT QUESTION 4-1

Why is the PO₂ of tracheal gas less than the PO₂ of inspired atmospheric gas?

Because oxygen constitutes 20.93% of dry atmospheric air, its partial pressure (PO₂) at sea level is calculated as follows:

PO2=0.2093×760mmHg=159mmHg

The partial pressure of carbon dioxide (PCO₂) is similarly calculated:

PCO2=0.0003×760mmHg=0.228mmHg

In physiological calculations, the PCO₂ of inspired air is considered to be 0 mm Hg.

The act of inspiring heats air to body temperature (37° C) and saturates it with water vapor (100% relative humidity [RH]). The partial pressure of water vapor (P_H2O) is determined by only temperature and RH. Because the body maintains lung temperature and RH at 37° C and 100% RH, the P_H2O of gas in the lung is constant; under body temperature and humidity conditions, P_H2O is always 47 mm Hg. At sea level, the total pressure of all gases in the lung, including water vapor, is 760 mm Hg. Water vapor accounts for 47 mm Hg, which means the rest of the atmospheric gases account for the remaining 713 mm Hg. Therefore, 47 mm Hg first must be subtracted from P_B to calculate P_gas in the lung or blood. In the lung’s airways, PO₂ is calculated as follows:

PO2=0.2093×(760−47)mmHg=149mmHg

Classification of Ventilation

Minute or Total Ventilation

Minute ventilation (V._E) is defined as the volume of air either entering or leaving the lung each minute; that is, the volume of air inhaled each minute must be equal to the volume exhaled each minute. In clinical practice, V._E is usually measured by adding together the exhaled tidal volumes obtained over 1 minute—hence the subscript E. Theoretically, the same result would be obtained by summing the inhaled tidal volumes over 1 minute, but exhaled volumes are easier to measure clinically. Regardless of the measuring method, V._E is the product of V_T and breathing frequency (f) per minute (V._E = V_T × f). If V_T is 500 mL and breathing frequency is 12 breaths per minute, V._E is calculated as follows:

V.E=500mL×12/min

V.E=6000mL/minor6.0L/min

V._E is easy to measure but not particularly useful in evaluating the amount of ventilation that participates in gas exchange (respiration) at the alveolar level. Not all the V._E reaches the alveoli because the last part of each inspiration (about one third of the V_T) stays in the conducting airways and is eliminated with the next expiration (Figure 4-2). Likewise, the first one third of the next inspired V_T is reinspired exhaled air; it consists of the air left in the conducting airways from the last exhalation. Thus, only part of the V._E ventilates the alveoli; this part is called alveolar ventilation (V._A). The remaining part ventilates conducting airways, which is called anatomical dead space ventilation (V._Danat). V._E can be expressed in terms of its components (V.E=V.Danat+V.A). This equation can be rearranged to solve for any of its components (V.A=V.E−V.Danat or V.Danat=V.E−V.A). A single V_T can be similarly expressed (V_T = V_Danat + V_A), where V_Danat is anatomical dead space volume and V_A is alveolar volume.

Dead Space Ventilation

Anatomical Dead Space

The conducting airways from the mouth and nose down to and including terminal bronchioles constitute anatomical dead space (V_Danat). Ventilation of these airways is necessary to move gas to and from the alveoli, but no gas exchange occurs between blood and air across their walls. The fresh gas filling the conducting airways at the end of inspiration (see Figure 4-2, left) is exhaled and is sometimes called “wasted” ventilation because it takes no part in respiration. The term wasted ventilation is synonymous with dead space ventilation. Dead space is defined as airspaces that are ventilated but do not exchange gases with the pulmonary circulation.

The volume in the conducting airways (V_Danat) does not change unless surgery removes part of a lung or an artificial airway (endotracheal or tracheostomy tube) bypasses the upper airway dead space. Anatomical dead space increases slightly during a deep inspiration and with administration of drugs that relax smooth airway muscle because these factors increase the airway diameter. Diseases characterized by hyperinflation of the lungs (e.g., emphysema) increase V_Danat for the same reason. For clinical monitoring purposes, V_Danat is considered to be constant.

CONCEPT QUESTION 4-2

What effect does increasing the tidal volume have on V_Danat and V_A?

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.

Figure 4-3 Dead space and alveolar gas compositions at end-inspiration and end-expiration. Dead space gas is identical to room air except it is saturated with water vapor (*left*). The anatomical dead space is filled with alveolar gas at end-expiration (*right*). (Redrawn from Fraser RG, Paré JA: *Organ physiology: structure and function of the lung,* ed 2, Philadelphia, 1977, Saunders.)

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.