Optimal operating conditions for many cardiothoracic procedures require collapse of one lung, producing a challenge for the anesthesiologist who must maintain arterial PO2, PCO2, and hemodynamics within tolerable levels while ventilating the single remaining lung. One-lung ventilation (OLV) for thoracic surgery is usually performed while the patient is in the lateral decubitus position, with the nondependent lung collapsed. For other types of procedures, for example whole lung lavage, the patient may be in the supine position and the nonventilated lung remains inflated with saline resulting in different physiologic consequences. Therefore, a thorough understanding of pulmonary physiology will help facilitate the delivery of anesthesia for procedures requiring OLV.
Flow of gas into the lungs is dependent upon the pressure difference between the upper airway (or endotracheal tube) and the alveoli. This pressure change can be induced either by inspiratory muscle contraction (spontaneous breathing) or positive pressure ventilation. Time-related variation in lung and chest wall mechanical properties induce variable tidal volumes and airway pressures during both spontaneous breathing and in some modes of positive pressure ventilation. To expand the lungs and overcome elastic recoil and resistive loads, a sufficient transpulmonary pressure gradient must either be generated with the use of positive pressure breathing or from negative pressure by the contraction of the diaphragm and muscles of inspiration as occurs during spontaneous breathing.
The lungs can be considered as many small, interconnected balloons inside the chest cavity, the elastic properties of which produce a recoil force. During lung expansion, recoil force of the lung is generated due to smooth muscle, elastin, and collagen as well as from other surface forces. The latter are due to surface tension generated by liquid within the alveoli. This is attenuated by surfactant produced by type II pneumocytes present at the alveolar-air interface. Changes in lung volume are accompanied by parallel changes in the volume of the chest wall, which has its own elastic properties. There is also a resistive component, mostly due to the properties of the airways, discussed below.
The lungs expand due to an increase in transpulmonary pressure (Ptp), equal to the difference between alveolar pressure (PA) and pleural pressure (Ppl) (Ptp = PA − Ppl). The pressure generated within the lungs to produce lung inflation must also be great enough to overcome forces independent of the lung including the chest wall, diaphragm, and abdomen (defined collectively as "chest wall").
The change in a given unit volume (eg, lung volume) per unit change in pressure is termed compliance. Compliance (C) is calculated by the following equation:
Where V is intrapulmonary gas and P is transthoracic pressure. Lung compliance (CL) in a normal awake human is typically in the range 150 to 250 mL/cm H2O. Because elastic recoil forces increase at ...