Chapter 7: Moving and positioning
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Positioning a patient to maximize ventilation/perfusion matching
Anatomy and physiology
For optimal gaseous exchange to take place, it is necessary that the air and the blood are in the same area of lung at the same time. Matching of these is expressed as a ratio of alveolar ventilation to perfusion (V/Q). A degree of mismatch can occur due to either adequate ventilation to an underperfused area (dead space) or inadequate ventilation to a well‐perfused area (shunt) (West [130]).
The function of the lungs is to exchange oxygen and carbon dioxide between the blood and atmosphere. Oxygen from the atmosphere comes into close contact with blood via the alveolar capillary membrane. Here, it diffuses across into the blood and is carried around the body. The amount of oxygen that reaches the blood depends on the rate and depth of the breath, the compliance of the chest and any airway obstruction (Lumb and Nunn [63]).
In a self‐ventilating individual in the upright position, ventilation will be preferential in the dependent regions (lowest part in relation to gravity) 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 to the systemic circulation; however, as the pressure is far less, the distribution is gravity dependent. The variability in the distribution of perfusion throughout the lungs is far greater than that of ventilation (West [130]).
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) because this where is the greatest perfusion and ventilation occur. 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 lowermost lung – has the best V/Q ratio (West [130]).
Figure 7.12 The effect of gravity on the distribution of ventilation and perfusion in the lung in the upright and lateral positions. Source: Reproduced from Hough ([49]) with permission of Oxford University Press.
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 involved in self‐ventilation), 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 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 the side‐lying position) and have a higher gradient (variability from apex to bases) than in self‐ventilating patients as the positive pressure displaces blood from the 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 than in self‐ventilating patients. Strategies such as positive end‐expiratory pressure (PEEP) and a higher rate of oxygen delivery will help to overcome this (West [130]).
Figure 7.13 The 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. Source: Reproduced from Hough ([49]) with permission of Oxford University Press.
The prone position may be used in intensive care when patients are mechanically ventilated to improve oxygenation in severe respiratory failure or to manage acute respiratory distress syndrome (ARDS) (Guerin et al. [40]).
