Airspace Mechanics

Gravity interacts with thoracic structures and transdiaphragmatic forces to modulate regional lung volume, distribution of ventilation, and ventilation-perfusion matching. The local transpulmonary pressure gradient (alveolar pressure–pleural pressure), in concert with the corresponding regional lung compliance, is the major determinant of regional lung volume. Under “relaxed” conditions, the total aerated lung volume is denoted as functional residual capacity (FRC). During active inspiration, the transpulmonary pressure gradient determines the regional distribution of inspiratory flow, an important component of ventilation-perfusion matching and the distribution of peak alveolar strain during positive pressure ventilation. Conversely, at end expiration (or during the expiratory phase in the context of pulmonary disease), an unfavorable transpulmonary pressure gradient arising from abnormal pleural or diaphragmatic mechanics can promote airspace collapse, compromising oxygenation by increasing shunt fraction. Regardless of the position, regional pleural pressure tends to be less negative; therefore alveolar dimensions are smaller in the dependent than in the nondependent lung regions because of the effects of gravity on the adjacent abdominal structure and the heart on the most dependent pleural space.

Positional changes affect the gradients of regional pleural pressure and thus regional lung volume. For example, the heart rests on the lungs in the supine position and primarily on the sternum in the prone position. This partially explains the observation that gravitational pleural pressure gradients are consistently less in the prone than the supine position. In addition, the prone position reduces the pressure the abdominal contents exert on the diaphragm, a pressure that is transmitted to the pleural space. Consequently, when in the supine position, the dorsal lung regions are surrounded by a less negative pleural pressure (and a smaller transpulmonary pressure gradient). The prone position results in a more negative pleural pressure adjacent to the dorsal lung zones. The increased ventral pleural pressure in the prone position has less effect on FRC because there is less lung at risk of compression by the heart. The improved aeration of the dorsal lung regions, combined with the smaller effect of cardiac weight on the ventral lung regions, tends to increase FRC. The effect of position on FRC is significant. In healthy subjects, FRC is reduced by approximately 30% on transition from the sitting to the supine, horizontal posture. Anesthesia or neuromuscular blocking agents tend to enhance this effect, presumably by reducing the tone of the diaphragm. When compared with the horizontal supine position, total FRC is approximately 20% greater in the lateral decubitus and prone positions. Not surprisingly, abdominal distension and obesity reduce FRC further, and prone positioning may help offset the consequences of reclining on lung mechanics and gas exchange.

In healthy, spontaneously breathing adults, ventilation distributes preferentially to the dependent lung regions in the upright, supine, prone, and lateral decubitus position. This effect is partially attributable to the phasic swings in pleural pressure that attend respiratory muscle activity. In contrast, elimination of the normal phasic changes in pleural pressure that accompany the pharmacologic paralysis and mechanical ventilation of healthy patients or the altered parenchymal characteristics in the setting of lung injury can markedly attenuate or even reverse the predominantly dependent distribution of ventilation. The changes in the distribution of ventilation during the positive pressure mechanical ventilation of nonparalyzed, partially assisted patients are complex. They vary with the specifics of the applied ventilatory support and regional lung mechanics. For instance, positive end-expiratory pressure (PEEP) can help redistribute ventilation in the dependent regions in patients with acute respiratory distress syndrome (ARDS) but only if those regions are recruitable and the level of PEEP used is sufficient to maintain alveolar patency. Active diaphragmatic contraction, through its effects on pleural pressure, can increase transpulmonary pressure and help preserve alveolar patency.

Distribution of Blood Flow and Ventilation-Perfusion Ratio

Until recently, gravity was thought to be the main determinant of blood flow distribution within the lungs. It has now been shown that perfusion tends to distribute preferentially to the dorsal regions in the supine and prone positions. This distribution cannot be explained by gravity alone. Regional differences in vascular development and geometry and/or vasoregulation by nitric oxide appear to contribute to regional distribution of perfusion within the lungs.

The modulation of airspace events combined with the less marked effect of gravity on distribution of pulmonary blood flow render the overall ventilation-perfusion ratio ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙V˙/Q˙
V ˙ / Q ˙
) sensitive to position. Overall, the ventilation perfusion relationship is less favorable in the supine than in the upright and prone positions. The effects of recumbency on oxygenation are complex and depend on the interrelationship of closing volume, FRC, and tidal volume. Interindividual variations in the relations between these variables contribute to the variable effects of reclining on the partial pressure of oxygen in arterial blood (Pa o ₂ ) between subjects.

Respiratory Effects of Frequent Posture Changes

In anesthetized dogs, immobility is associated with a deterioration of gas exchange that can be prevented by turning every half hour. Frequent changes in position are likely to be similarly important in maintaining normal respiratory function in humans. The effect of frequent positional changes has been tested in the clinical arena with continuous oscillating beds with promising results. Such “kinetic therapy” appears to be well tolerated hemodynamically and has been reported to improve oxygenation, decrease the risk of atelectasis and pulmonary infections, and reduce the duration of intubation and resource utilization in trauma patients. Kinetic therapy also has been used to treat established atelectasis. A reduction in the incidence of pneumonia and improved oxygenation was observed in medical ICU patients. It has been suggested that this modality may improve outcome in the sickest patients ( P = 0.056 for subgroup with APACHE [Acute Physiology and Chronic Health Evaluation] II score >20), but more studies are needed to conclude that it does. Most available studies are relatively small sized and have limitations, and the results are not always consistent. For example, the use of a kinetic therapy bed has been associated with more frequent infectious complications, respiratory failure, and more ventilator support days in patients with thoracolumbar spinal column injuries. In summary, the efficacy of position changes in protecting pulmonary function or improving outcome remains uncertain.

Lateral Position

Because perfusion and ventilation distribute preferentially to the dependent lung during active breathing, <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>?˙/?˙V˙/Q˙
V ˙ / Q ˙
mismatching and intrapulmonary shunting can be significantly reduced by lateral positioning of patients with unilateral or asymmetrical lung disease with the good lung down (GLD). This therapeutic adjunct may significantly improve Pa o ₂ and even preclude the need for intubation and mechanical ventilation. Arterial and mixed venous oxygen content usually increase, without significant hemodynamic changes, in the GLD position. On occasion, critically ill patients fail to improve with GLD (paradoxically); improve with the bad lung down; or develop arrhythmias, hypotension, or a marked reduction in mixed venous saturation of O ₂ (S v O ₂ ), necessitating prompt return to the supine position. The slight and usually transient decrements in S v O ₂ reported after postural changes in critically ill patients do not explain the occasional persistent failure of blood gases to improve in the lateral position with GLD. Atelectasis due to unusual pressure distributions generated by the abdominal contents or increased pressure transmission to the thorax is more likely responsible. In such circumstances, PEEP may prove beneficial. Fortunately, in patients with predominant unilateral alveolar consolidation or flooding, PEEP is less likely to detrimentally affect the distribution of perfusion in the lateral than when the patient is in the supine position. When the patient is supine, an inappropriately high level of PEEP may redistribute blood flow to the diseased lung by promoting zone 1 conditions in the spared lung. However, in unilateral pneumonia, PEEP may help limit contamination of the good lung by the diseased lung and may theoretically be more effective when used in combination with the lateral position.

The practice of positioning of patients with the GLD has notable exceptions. Children, some patients with chronic airflow obstruction, and anesthetized-paralyzed patients share a tendency to have higher ventilation to the nondependent lung. In the presence of a moderate unilateral pleural effusion, <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>?˙/?˙V˙/Q˙
V ˙ / Q ˙
matching during spontaneous breathing appears to be similar in the lateral position with the affected side up or down, suggesting that moderate pleural effusions have little effect on gas exchange. Studies of regional lung function in seated patients with unilateral pleural effusions demonstrate that although the overall lung volume on the side of effusion is reduced, the residual volume/total lung capacity (RV/TLC) and FRC/TLC ratios on both sides are very similar. This may explain the poor correlations among posture, pleural effusion size, and gas exchange in patients with unilateral pleural effusion without marked underlying infiltrates or hypoxemia. Patients with whole lung collapse secondary to unilateral central airway obstruction may not improve or may even deteriorate when positioned with the spared lung down. Patients with unilateral massive pulmonary embolism requiring mechanical ventilation have been reported to have better gas exchange with the diseased lung down. Finally, lateral positioning with the GLD is contraindicated in hemoptysis and lung abscess because of the risk of spillage into the unaffected lung.

Elevation of the Head of the Bed

Elevating the head of the bed can improve oxygenation in ARDS, probably by promoting lung recruitment at the bases. In 16 patients with ARDS, vertical positioning (trunk elevated at 45 degrees and legs down at 45 degrees) significantly increased Pa o ₂ from 94 ± 33 to 142 ± 49 mm Hg, with an increase higher than 40% in 11 patients. The semirecumbent position may also help reduce gastric content aspiration. Conversely, head position less than 30 degrees in the first 24 hours after intubation was found to be an independent risk factor for development of ventilator-associated pneumonia. In a subsequent randomized prospective trial, the semirecumbent position was reported to significantly reduce the rate of ventilator-associated pneumonia (odds ratio [OR] 6.8 for the supine body position). On the basis of this evidence, head-of-the-bed elevation has been endorsed by medical societies such as the Society of Critical Care Medicine. However, this approach has not been universally endorsed because of persistent questions about efficacy. Many queries remain unanswered, such as how many hours per day the head of the bed must be elevated and what the optimal angle of elevation is for the head of the bed. Nonetheless, given that head-of-the-bed elevation is cheap, benign, and potentially helpful, it seems a reasonable intervention even in the absence of definitive data.

Physiology and Physiopathology of Prone Positioning

In 1976, Piehl and Brown first described improved oxygenation in patients with acute hypoxemic respiratory failure who were ventilated in the prone position. This has been confirmed in subsequent studies; overall, oxygenation improves in approximately two thirds of patients when placed in the prone position. The mechanisms underlying this improvement have been most extensively studied in large animal models. Complex interactions between regional aeration and the modulation of perfusion during positive pressure ventilation determine the effects of prone positioning on gas exchange. These mechanisms have been reviewed by Guerin and colleagues.

The improved oxygenation associated with prone positioning appears to be primarily related to regional differences in FRC alongside relatively unchanged distribution of dorsal-ventral perfusion. The largest proportion of pulmonary blood flow is directed to the dorsal lung regions in the supine and prone positions. The predominance of dorsal perfusion is preserved when the animal is turned prone. In a canine model of lung injury induced by oleic acid, the prone position was found to improve gas exchange by reducing shunt. In the setting of lung injury, both animals and patients with ARDS tend to have less aerated lung in the dependent regions because of the effects of gravity on the edematous lungs. The time constant of the dependent collapsed/flooded lung units is such that tidal ventilation distributes preferentially to the “open” nondependent lung units, namely, to ventral regions when supine and to dorsal regions when prone. Accordingly, the increase in FRC seen when an injured animal or patient is turned prone (because of changes in transpulmonary pressure favoring “opening” of the now nondependent dorsal regions, vide supra) is accompanied by an increase in perfusion to aerated lung units, with an accompanying decrease in shunt fraction.

In addition, positive pressure ventilation tends to create West zone 1 or 2 conditions and can redistribute blood flow from the nondependent region to the dependent regions. Positive airway pressure decreases the vertical perfusion gradient when in the prone position whereas it increases the vertical perfusion gradient in the supine position. Positive pressure ventilation of regionally heterogeneous ARDS lungs creates opposing gradients of ventilation and perfusion along the vertical axis, promoting ventilation-perfusion mismatch and shunting. This effect of positive pressure is more marked in the supine position than in the prone position. Indeed, regional ventilation (V _r ) and regional perfusion (Q _r ) assessed by single-photon emission computed tomography showed that the prone position improved dorsal V _r to a greater extent than ventral V _r , whereas Q _r remained essentially unchanged. In other words, recruitment of dorsal lung units associated with preserved dorsal perfusion largely explains why prone positioning improves gas exchange in experimental models and why an overall increase in FRC is not required for prone positioning to improve <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>?˙/?˙V˙/Q˙
V ˙ / Q ˙
matching (vide infra).

However, additional factors may contribute to the improved gas exchange afforded by prone positioning. The pleural pressure gradient is smaller along the vertical axis and pleural pressure is more negative in the dependent regions in the prone than in the supine position. This favors lung recruitment and accounts for the increase in FRC sometimes observed after turning to the prone position. The effect of prone positioning on gas exchange during positive pressure ventilation of pharmacologically paralyzed subjects appears to be further modulated by changes in thoracicoabdominal compliance that accompany the prone position. Pelosi et al. found that the improvement in oxygenation attending prone positioning correlated with a high supine thoracicoabdominal compliance. A very compliant anterior chest tends to redistribute the tidal volume toward the nondependent, less well perfused lung regions, promoting <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>?˙/?˙V˙/Q˙
V ˙ / Q ˙
mismatching in the supine position. Constraint of the flexible ventral chest wall by contact with the bed during prone positioning “stiffens” the anterior chest wall. Such stiffening redirects tidal ventilation toward the better perfused dorsal regions, improving <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>?˙/?˙V˙/Q˙
V ˙ / Q ˙
matching. These data do not suggest that minimizing abdominal contact, as proposed by some, is a prerequisite for improved gas exchange. Finally, the properties of the lung (e.g., cause of ARDS or phase of the disease [edema vs. fibrosis]) tend to alter the response to prone positioning. In general, patients in the early edematous phase of ARDS are more likely to experience improved gas exchange when turned prone than patients who have pulmonary fibrosis.

Which of these mechanisms prevails in individual patients and best accounts for the improved Pa o ₂ /fraction of inspired oxygen (F io ₂ ) ratio associated with prone positioning is not always clear, but it is potentially important. It has been suggested that a reduction in the partial pressure of carbon dioxide in arterial blood (Pa co ₂ ) after prone positioning may indicate the presence of recruitment and improved outcome. Better recruitment distributes a given tidal volume to a larger number of alveoli, thereby reducing alveolar strain and the risk of epithelial and endothelial injury. Mentzelopoulos et al. measured tidal transpulmonary pressures as a function of end expiratory lung volume as a marker for lung mechanical stress and found this to be reduced during prone positioning. More uniform distribution of blood flow may also be important given the potential importance of ventilation and perfusion interaction in the pathogenesis of ventilator-induced lung injury (VILI). Regardless of the mechanisms, prone positioning has been found to attenuate VILI in large animals with normal or injured lungs. Overall, the protective effect of prone positioning is consistent with the post hoc findings of Gattinoni and colleagues, who reported reduced mortality in a subset of patients who received excessive tidal volume (large tidal volume relative to the size of lung) either because of the large tidal volume used (largest tidal volume subgroup) or the small size of the lungs (severest form of ARDS subgroup).

Prone Position and Outcome

Prone ventilation improves oxygenation in most patients and mortality in those with severe ARDS. Multiple randomized trials addressing the effect of prone positioning on outcomes have been published. Patient characteristics and the main results of these trials are summarized in Table 32-1 . Several initial studies demonstrated either no difference or only a trend toward improved mortality in subsets of patients with ARDS. Taken together, the data suggested that the prone position may improve outcome in subgroups of patients with severe ARDS, and multivariate analysis of data from the study by Mancebo and colleagues showed that randomization to the supine position was an independent risk factor for mortality. This hypothesis has been confirmed by the landmark study by Guerin and colleagues. In this study, a standardized protocol for prolonged daily prone ventilation, neuromuscular blockade, and lung-protective ventilation was instituted by trained medical staff within 36 hours for severe ARDS. This study demonstrated a dramatic improvement in 28-day (16% vs 32.8%, OR, 0.39; confidence interval [CI], 0.25 to 0.63) and 90-day (23.6 vs. 41%, OR, 0.44; CI, 0.29 to 0.70) mortality with prone ventilation.

Table 32-1

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

What Lessons Have Intensivists Learned During the Evidence-Based Medicine Era? What Is the Role of Invasive Hemodynamic Monitoring in Critical Care? Should Fever Be Treated? Are Computerized Algorithms Useful in Managing the Critically Ill Patient? Are Anti-inflammatory Therapies in ARDS Effective? Has Outcome in Sepsis Improved? What Has Worked? What Has Not Worked?