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In a self-ventilating individual in the upright position, ventilation will be preferential in the dependent regions as:

the apex of the lung is more inflated and therefore has less potential to expand

the bases of the lung are compressed by the weight of the lungs and the blood vessels and therefore have more potential to inflate.

Perfusion to the alveoli is approximately equal to that of the systemic circulation but as the pressure is far less, the distribution is gravity dependent. The variability in the distribution of perfusion throughout the lung is far greater than that of ventilation.

Evidence-based approaches

Principles of care

In a self-ventilating upright position, V/Q is not exactly matched even in a healthy lung but is regarded as optimal in the bases (Figure 7.12) as there is the greatest perfusion and ventilation. Similarly in a side-lying position, the effect of gravity alters the distribution of perfusion and ventilation so that the dependent area of lung, that is, the bottom of the lung, has the best V/Q ratio.

Figure 7.12 Effect of gravity on the distribution of ventilation and perfusion in the lung in the upright and lateral positions.

Reproduced with permission from Nelson Thornes Ltd from Hough (2001).

In a patient receiving mechanical ventilation, especially in a mandatory mode (where the ventilator rather than the patient initiates and terminates the breath), the distribution of ventilation and perfusion will alter (Figure 7.13). As ventilation is driven by a positive pressure, rather than the negative pressure when self-ventilating, air will take the path of least resistance. Ventilation will therefore be optimal in the apex of the lungs in the upright position or the non-dependent/uppermost lung in side-lying. This can, however, be altered further in the presence of lung pathology. Perfusion will remain preferentially delivered to the bases (in the upright position) or the dependent/lowermost lung (in side-lying) and have a higher gradient (variability from apex to bases) than in self-ventilating patients as the positive pressure displaces blood from areas of highest ventilation. These two situations mean that the V/Q ratio of those receiving mechanical ventilation can have a higher degree of mismatch. Strategies such as positive end-expiratory pressure (PEEP) and a higher oxygen delivery will help to overcome this.

Figure 7.13 Effect of controlled mandatory ventilation on ventilation and perfusion gradients. In contrast to spontaneous respiration, the perfusion gradient increases downwards and the ventilation gradient is reversed.

Reproduced with permission from Nelson Thornes Ltd from Hough (2001).

Preprocedural considerations

The general principles of care mentioned earlier in the chapter are all relevant to this section. However, there are also some other general principles that need to be considered for these patients.

Pharmacological support

Oxygen requirements

Repositioning can cause a temporary fall in oxygen saturation or a raised respiratory rate. If the fall is greater than 4% or recovery time is protracted, supplemental oxygen delivery may be required for several minutes before, during and after moving.

Procedure guideline 7.7 Positioning the patient to maximize V/Q matching with unilateral lung disease in a self-ventilating patient

Equipment

Pillows/towels

Sliding sheets/manual handling equipment if indicated following risk assessment in accordance to local manual handling policy

Bed extension for tall patients

Preprocedure

Action Rationale

1 Explain and discuss the procedure with the patient. To ensure that the patient understands the procedure and gives their valid consent (NMC 2008, C).

2 Wash hands thoroughly or use an alcohol-based handrub. To reduce the risk of contamination and cross-infection (Fraise and Bradley 2009, E).

3 Ensure that the bed is at the optimum height for handlers. If two handlers are required try to match handlers’ heights as far as possible. To minimize