Estimating lung volume during high frequency ventilation

The primary goals of lung ventilation are to bring oxygen into the alveoli from where it can enter the blood, and to eliminate carbon dioxide dissolved in the blood. Since CO2 is much more soluble in blood than oxygen, O2 transport is diffusion-limited and a large alveolar surface area is the primary factor controlling its delivery. CO2 transport is flow-limited, so that efficient gas transport and mixing in the lung primarily controls its elimination from the blood. The lung has approximately 20 generations of bifurcating airways which terminate in elastic sacs known as alveoli, where the majority of gas exchange occurs. During inspiration, the volume of the thoracic cavity increases and air is drawn into the lung. The lung is elastic and returns passively to its pre-inspiratory volume during expiration. Figure 1 shows a pressure-volume curve for a lung, which may be found using the apparatus shown on the left of figure 1. When the pressure within the jar is reduced below atmospheric pressure, the lung expands and its volume change can be measured. The quasi-static pressure-volume curves for inflation and deflation are different. This hysteresis arises in part from the effects of surface tension of the thin liquid film lining the airways of the lung. The slope of the pressure-volume curve (volume change per unit pressure change) is known as the compliance. The lung has low compliance at high lung volumes. Artificial ventilators are often used to support premature infants suffering from respiratory distress, which is often associated with a deficiency of pulmonary surfactant. Under Conventional Mechanical Ventilation (CMV), which operates at normal tidal frequencies, the lungs are actively inflated at each breath, and they recoil passively to expel inhaled air. A clinician can control six ventilator variables: