Pulmonary ventilation and perfusion

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Pulmonary ventilation and perfusion

H. Michael Marsh, MB, BS

This chapter examines gas exchange in the normal lung and in the lung under general anesthesia. Maximal gas-exchange efficiency for O2 and CO2 in an ideal single-lung unit has a ventilation/perfusion (image/image) ratio of 1 in a situation of continuous countercurrent flow of gas to blood, with a blood-to-gas exposure of 0.75 sec. Human lung, by contrast, is only relatively efficient, showing a range of image/image ratios for its many alveoli, determined by the distribution of image and image throughout the lungs.

Ventilation

Inspired gas flows into the lungs influenced by pulmonary compliance and airway resistance. Gravity, interacting with posture, and regional alveolar time constants for filling and emptying of lung regions, interacting with the frequency of respiration, are the other two major factors determining the distribution of image within the lungs. The right lung is larger than the left lung, receiving approximately 52% to 53% of a tidal breath in the supine position, during both spontaneous breathing and with mechanical ventilation. These percentages change under the influence of gravity with change in posture. Anesthesia, paralysis, and mechanical ventilation introduce further changes.

At functional reserve capacity, in each slice of lung, from nondependent (apex in sitting position, anterior lung in supine, up lung in lateral decubitus position) to most dependent portion, the alveolar volume decreases. Basal alveoli are one quarter the volume of apical alveoli at end expiration. This puts the basal alveolar characteristics on a steeper portion of their pressure-volume (P-V) curve (Figure 29-1); although the basal alveoli are smaller than apical alveoli at functional reserve capacity, the basal alveoli expand more than do the apical alveoli during inspiration. Therefore, in an awake, spontaneously breathing patient, in all positions, ventilation per unit of lung volume is smallest at the highest portion (e.g., the apex in an upright patient) and increases with vertical distance down the lung.

In the supine patient, general anesthesia with paralysis and mechanical ventilation decreases the difference between the ventilation of the dependent and nondependent alveoli, causing nearly uniform distribution of ventilation throughout the lung. This is attributed to a decreased functional reserve capacity, shifting alveolar characteristics downward on their P-V curves (see Figure 29-1). When the patient is in the lateral decubitus position, anesthesia reverses the distribution of ventilation so that the nondependent (upper) part of the lung receives more ventilation than does the dependent (lower) part of the lung. This arrangement holds for both spontaneous and mechanical ventilation and is clinically significant because the dependent lung has greater perfusion, which causes increased image/image mismatch. The change in distribution of image to lung regions in the lateral decubitus position is attributed to (1) decreased functional reserve capacity, causing a shift along the P-V curve (which can be partially reversed by positive end-expiratory pressure); (2) more compression of the dependent lung by the mediastinum and abdominal contents; and (3) increased compliance of the nondependent hemithorax.

The time constant for filling and emptying of a lung region is determined by the product of compliance and resistance of the region. If respiratory frequency is such that complete emptying of a region does not occur before the next inspiratory effort is applied, gas trapping will occur. This is a concern when obstructive airways disease is present. Incomplete filling or emptying of lung regions may also increase image/image mismatching. Anesthesia may reverse bronchoconstriction and favorably impact this factor.

Pulmonary blood flow

The two major determinants of distribution of pulmonary blood flow (image) within the lung are (1) gravity and (2) hypoxic pulmonary vasoconstriction (HPV). Pulmonary artery pressure (PPA) decreases by 1 mm Hg or 1.35 cm H2O for every cm of vertical distance up the lung. Because the pulmonary circulation is a low-pressure system, this causes significant differences in image between the lower and higher regions of the lung, with greater image going to the lower lung regions. The actual image to an alveolus also depends on the alveolar pressure (PALV), which opposes the PPA and pulmonary venous pressure (PPV). This interaction is summarized in Figure 29-2. All of these relationships are dynamic, varying throughout the cardiac and respiratory cycles. There are four defined zones of blood flow in the lung. In zone 1, at the apex of an upright lung, PALV is greater than PPA, preventing any blood flow and thereby creating alveolar dead space. Zone 1 is negligible in healthy lungs. In zone 2, PPA is greater than PALV, which is greater than PPV, so that image depends only on PPA minus PALV. In zone 3, PPA is greater than PPV, which is greater than PALV, and image is a function of PPA minus PPV independent of PALV

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