Regional variation in ventilation and perfusion

In an upright position, bases compressed more than apices by weight of lung and by pleural pressure gradient, therefore bases have higher compliance (on a more favourable part of the compliance/volume curve). This means bases are ventilated more than apices.

In all positions, central lung units are ventilated more than peripheral lung units. The right lung is ventilated more than the left (because of the heart).

Perfusion is heterogenous largely due to West Zones.

West zone 1: In hypotension or high \(P_A\) (e.g. dynamic hyperinflation, high PEEP). \(P_A > P_a > P_v\), such that \(P_A\) abolishes flow. Flow is either absent or phasic (with the respiratory cycle).

West Zone 2: \(P_a > P_A > P_v\) such that \(P_A\) forms a starling resistor and \(\text{Pulmonary blood flow} = \frac{P_a - P_A}{PVR}

West zone 3: \(P_a > P_v > P_A\) so normal flow occurs and \(\text{Pulmonary blood flow} = \frac{P_a - P_v}{PVR}

West zone 4: High interstitial pressure from dependant tissue oedema causes a rise in the interstitial pressure such that \(P_a > P_I > P_v\) and the interstitial pressure forms a starling resistor, \(\text{Pulmonary blood flow} = \frac{P_a - P_I}{PVR}

The net effect is that the bases are perfused much more than the apices, especially when alveolar pressure is high or PASP is low.

Central units and perfused more than peripheral units.

V/Q is optimal at 1

V/Q 0.6 at the apex (over-ventilated; West Zone 2) and 3 at the bases (over-perfused; West zone 4), because perfusion increases more dramatically moving from cranial to caudal.

V/Q matching is best in the prone position, because this eliminates the pleural pressure gradient (there is no appreciable anterior-posterior gradient).

Pleural pressure is -10cmH2O at apex, -2cmH2O at bases, because of weight of lung, mediastinum, and pressure from abdominal contents. Affected by position: when supine the effect is halved, when prone it is minimal. In the lateral position the dependant lung's pleural space is less negative (increasing ventilation in a spontaneously breathing patient, potentially decreasing ventilation of the dependant lung in a ventilated patient due to a fall in FRC putting the dependant lung on a flat part of the compliance/volume curve).