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Effects of Body Position on Ventilation/Perfusion Matching

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Effects of Body Position on Ventilation/Perfusion Matching Chapter 1 Effects of body position on ventilation/perfusion matching G. HEDENSTIERNA A new interest in body positioning emerged when it was shown that placing the ARDS patient in the prone position frequently improved arterial oxygenation over that observed in the supine patient. Thus, body positioning can be used as a tool for improving oxygenation in severely ill patients. A lateral position with the ‘good lung down’ is a frequent component of treatment in unilateral lung disease, and a combination of lateral position and selective PEEP to the lower lung has been tried in bilateral lung disease. The improvement in oxygenation frequently seen raises the question of what the mechanisms underlying potential improvement are? The awake, healthy subject Ventilation distribution The air that is inspired is not then evenly distributed in the lung. During quiet breathing, most air goes to the lower, dependent regions, i.e. to basal, diaphragma- tic areas for patients in the upright or sitting position and to dorsal units for those in the supine position [l]. This also means that the lower lung will receive most of the air if the subject is in the lateral position. When the patient is in the prone position, more of the air will presumably go to anterior than to dorsal regions. The reason for this seemingly gravitational orientation of something as light as gas is the combined effect of the curved shape of the pressure-volume curve for the lung and the decreasing transpulmonary pressure (Ptp) down the lung (Ptp = airway minus pleural pressure). In the upright position, apical lung regions are exposed to a higher transpulmonary pressure than dependent, basal ones. Thus, upper and lower lung regions are positioned at different levels of the pressure-volume curve (Fig. 1). During an inspiration, pleural pressure is lowered and causes lower lung regions to inflate more than upper ones for a similar change in transpulmonary pressure. (It is assumed that pleural pressure changes uniformly in the pleural space [1].) When the body position is changed from upright to supine, lateral or prone, FRC is reduced by 0.7–0.8 l, down to an average of 2.5 l (there is considerable variation between subjects, depending on sex, age and body configuration). The fall in FRC promotes airway closure in dependent lung regions. This is a normal phenomenon that occurs during expiration, with reopening of airways during the 4 G. Hedenstierna Fig. 1. Alveolar expansion at three vertical levels of the lung (A upper lung, B midlung, C lower, dependent, lung) and pressure–volume curves at end–expiration and end–inspira- tion. Note the pleural pressure difference with more markedly subatmospheric values at upper parts. Since alveolar pressure is the same from top to bottom, the distending, transpulmonary pressure is higher in the upper regions, which is why alveoli at the bottom are less highly aerated and may even be collapsed during expiration. During inspiration the pressure differences between alveoli remain, but all regions have been expanded and are higher up, on the flatter part of the pressure–volume curve succeeding inspiration. The older the subject is the more likely it is that airways will close during breathing [2]. It can be expected during normal breathing in 65- to 70-year-old subjects when they are upright, and even in 50-year-old subjects when in the horizontal position. Airway closure will thus decrease ventilation in the dependent regions. Since lung blood flow passes preferentially to dependent regions, matching between ventilation and perfusion is impeded. The most marked decrease in FRC can be seen in the Trendelenburg position and head-down positions, the decrease in FRC being caused by cranial displace- ment of the diaphragm. These body positions may also be accompanied by more severe decreases in arterial oxygenation than others. Effect of body position on ventilation/perfusion matching 5 Lung blood flow The blood flow through the lung is governed by the driving pressure and the vascular resistance. Pulmonary artery pressure increases from top to bottom in the lung, an effect of the hydrostatic pressure that builds up on the way from top to bottom of the lung. There is therefore less driving pressure at the top of the lung. It may approach zero in the apex of the lung in persons in the upright position. Moreover, if alveolar pressure is increased, as it is during positive pressure venti- lation, it may exceed that in the pulmonary artery and compress the pulmonary capillaries. No blood will then flow through the vessels (zone I, according to the nomenclature introduced by West et al. [3]. Further down, arterial pressure exceeds alveolar pressure (zone II), and still further down both arterial and venous pressu- res exceed alveolar pressure (zone III). The increasing driving pressure from top to bottom of the lung increases perfusion, and most of the blood flow passes through the dependent regions. This gravitational orientation of blood flow results in a preference for caudal regions in persons in an upright position, for dorsal regions in those lying supine, for the lower lung in persons lying in a lateral position and for anterior regions when those lying prone. Blood flow thus has a roughly similar distribution to ventilation. In dog experiments, groups at the Mayo Clinic and subsequently in Seattle noted that the vertical lung blood flow distribution was rather even and did not change when position was altered between supine and prone [4]. This led theSeattle group to conclude that gravity was of minor importance in determining perfusion distribution. The same group also showed that perfusion at a given vertical level was unevenly distributed on that horizontal plane, with an inhomogeneity that far exceeded that in the vertical direction [5]. This suggests that there are morpholo- gical and/or functional differences between lung vessels that are also involved in determining blood flow distribution, and perhaps to a more significant degree than gravity. In their hypothesis, they postulate that blood flow in the lung varies between lung regions, and that the variation becomes larger with decreasing size of the lung unit under scrutiny. Moreover, there may be regional differences in vascular resistance. Thus, vascular resistance seems to be lower in dorsal regions of horse lungs than in the anterior part [6]. This will also affect the distribution of blood flow, opposing the effect of gravity. To what extent this phenomenon may exist in the human lung is not known. Ventilation-perfusion match Since both ventilation and perfusion increases from top to bottom in the lung, the ventilation-lung blood flow match is fairly similar in the gravitational direction, with a ventilation-to-perfusion ratio (V/Q) close to 1. In the upper regions V/Q may approach 4–5; that is to say that ventilation exceeds perfusion by a factor of 4–5. At the bottom of the lung the V/Q is typically 0.6–0.8, indicating relative underventi- lation. The former situation with excess of ventilation over perfusion causes a dead-space-like effect (‘wasted’ ventilation), while relative underventilation causes 6 G. Hedenstierna some impairment of the oxygenation of blood. There is also a mismatch at an isogravitational level, which may be larger than that in the gravitational direction. However, the mechanisms are not known. The anaesthetised subject Ventilation distribution In addition to the fall in FRC with adoption of the horizontal position, anaesthesia causes further reduction of 0.4-0.5 l in FRC. This brings FRC close to the waking residual volume. This is an abnormal lung volume to breathe at, as can be expe- rienced by expiring to the maximal degree and then trying to breathe at that level. The reduction in FRC occurs with spontaneous breathing and regardlessofwhether the anaesthetic is inhaled or given intravenously [7]. Muscle paralysis and mecha- nical ventilation cause no further decrease in FRC. Thus, anaesthetics in general reduce tonic muscle activity even though they allow spontaneous breathing. This loss of tonic activity is the cause of reduced FRC. The fall in FRC during anaesthesia promotes airway closure. It can be expected in almost all adult patients when in the supine position (from 30–35 years of age onward). Moreover, atelectasis appears in 90% of all patients who are anaesthetised [8]. Ventilation is distributed more to the upper half of the lung than to the lower, which is the opposite of the distribution in the awake subject. Major causes of this shift must be airway closure and atelectasis. Moreover, with small lung volume airway resistance increases, and especially in dependent regions that are less inflated than upper regions. There are few studies on atelectasis formation and distribution in the lateral position during anaesthesia. Atelectasis that was seen in both lungs in the anaesthe- tised patient in the supine position decreased in size in the upper, nondependent lung when the patient was turned into a lateral position [9]. However, the atelectasis remained in the dependent lung. The resolution in the upper lung is a consequence of the increase in the upper lung volume when the patient is rotated. This is similar to a PEEP applied to the upper lung, but without the need for external pressure. When anaesthesia was induced in the lateral position, atelectasis appeared in the lower, dependent lung whilst no collapse was seen in the upper, nondependent lung (Fig. 2). When the patient was turned to the supine position no atelectasis appeared in the previously upper lung, whereas it remained in the other lung [9]. The absence of atelectasis formation in the lung that had previously been the upper lung may appear surprising. However, atelectasis formation is promoted by ventilation with high inspiratory oxygen fraction at decreased lung volume (FRC), the decrease being caused by most anaesthetics [10, 11].
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