Lateral Decubitus Position (Awake, Spontaneous, Closed Chest)
Simply moving into the lateral position has several, important effects – first, the hydrostatic gradient moves 90 degrees (occurring laterally), but because the lungs are taller than they are wide, ΔP is smaller and there is relatively less Zone 1 (PA > Pa > Pv). Second, because abdominal contents are no longer pulled away from the diaphragm, the dependent hemidiaphragm will be disproportionately pushed into the thorax – in spontaneously breathing patient, the strength of the diaphragm overcomes this.
It is generally assumed that 60% of blood will flow through the dependent lung, with the remaining 40% flowing through the non-dependent lung. Because total shunt (10% of cardiac output) is roughly equally divided, 55% and 35% of cardiac output participate in gas exchange, respectively.
Lateral Decubitus Position (Awake, Spontaneous, Open Chest)
Opening the chest of a patient in the lateral decubitus position (ex. thorascopy in an awake patient given extensive local anesthesia) can cause three additional changes. First, if the opening is large enough, the non-dependent lung is no longer constrained by the chest wall, and can receive more ventilation, thus worsening V:Q mismatch. Second and third (both of which result from the intact hemithorax’ ability to generate negative pressures), the mediastinum may shift towards the closed hemithorax, possibly resulting in disastrous hemodynamic changes, and the closed hemithorax lung may remove air from the open hemithorax lung, leading to “paradoxical breathing”.
Lateral Decubitus Position (Anesthetized, Spontaneous, Closed Chest)
The main effect of inducing anesthesia in a spontaneously breathing closed chest patient is redistribution of ventilation towards the non-dependent lung, with a relative increase in VT entering the non-dependent lung, thereby leading to a significant V:Q mismatch. Note, however, that overall there is a reduction in both lung volumes and FRC.
Lateral Decubitus Position (Anesthetized, Spontaneous, Open Chest)
Opening the chest of an anesthetized, spontaneously breathing patient also may result in a mediastinal shift and/or paradoxical breathing, but also further increases the V:Q mismatch that occurs in anesthetized patients (largely owing to changes in ventilation, not perfusion).
Lateral Decubitus Position (Anesthetized, Paralyzed, Open Chest)
Paralysis implies in positive pressure ventilation, which further worsens V:Q mismatch as gas moves preferentially into the non-dependent lung, mostly due to decreased abdominal resistance but potentially due to the open chest (if large enough).
Lateral Decubitus Position (Anesthetized, Paralyzed, Open Chest, One Lung)
On first glance, it would appear that ventilating the dependent lung only would result in loss of 35% of cardiac output that participates in gas exchange (the non-dependent lung). Hypoxic pulmonary vasoconstriction can decrease non-dependent blood flow by 50% (or 17.5% globally), thus the amount of cardiac output available for gas exchange should only fall from 90% to 72.5%. That said, because of abdominal contents, paralysis, anesthesia, and the weight of mediastinal structures, the dependent lung has reduced FRC and is relatively non-compliant.