Ventilation/perfusion (V/Q) relationships
The V/Q relationship varies throughout the lungs. During spontaneous ventilation in the upright position, distribution of ventilation and perfusion is not uniform throughout the lungs. In the normal setting, ventilation and perfusion are matched adequately throughout the lung with the bases receiving substantially more of both than the apices. The distribution of perfusion throughout the lung is partly due to the effects of gravity and partly anatomical with improved flow to the bases. In the upright position, the perfusion pressure at the base of the lung is equal to the mean pulmonary arterial pressure plus the hydrostatic pressure between the pulmonary artery and lung base. At the apices, the hydrostatic pressure difference is subtracted from the pulmonary artery pressure with a resulting low pressure that may at times fall below the alveolar pressures, leading to vessel compression and intermittent cessation of blood flow. This gravity-induced distribution pattern is explained by the West three zone model (Figure 11).
The distribution of ventilation across the lung is related to the position of each area on the compliance curve at the point of FRC. Because the bases are on a more favourable part of the compliance curve than the apices, they gain more volume change for the pressure change applied and thus receive a greater degree of ventilation.
Although the inequality between bases and apices is less marked for ventilation than perfusion, overall there is still good V/Q matching and efficient oxygenation of blood passing through the lungs. Disturbance of this distribution can lead to V/Q mismatching (Figure 12). For an area of low V/Q the blood flowing through it is incompletely oxygenated resulting in a decreased level of arterial oxygenation. Providing some ventilation is occurring in an area of low V/Q, the hypoxaemia can normally be corrected by increasing the FiO2.
V/Q mismatch commonly occurs during anaesthesia because the FRC falls, leading to a change in the position of the lung on the compliance curve. The apices move to the most favourable part of the curve while the bases are located on a less favourable part at the bottom of the curve.
Alveolar dead space
Dead space is an area of lung where no significant gas exchange occurs. The physiological dead space equals anatomical plus alveolar dead space. Anatomical dead space includes the conducting airways not lined with respiratory epithelium. Alveolar dead space describes ventilated lung normally contributing to gas exchange, but not doing so because of impaired perfusion. The ratio (Vd/Vt) of dead space volume (Vd) and the volume of ventilation to perfused alveoli (Vt) is normally 0.3. The Vd/Vt ratio increases in conditions such as chronic obstructive pulmonary disease (COPD) because of increased anatomical dead space from lung parenchymal destruction. Beyond a Vd/Vt ratio of 0.7–0.8, spontaneous breathing is no longer possible because the increased work of breathing gives rise to more CO2 than can be exhaled.
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