The effect of postural drainage positioning on ventilation homogeneity in healthy subjects.

Key Words: Bronchial drainage, Posture, Pulmonary, Respiration.

Postural drainage (PD) is often used as a component of chest physical therapy in patients with pulmonary disease. Postural drainage has been demonstrated to be effective in mobilizing secretions, although the addition of manual techniques has not conclusively been shown to provide additional benefit.[1-3] In addition, the bronchospasm and desaturation associated with chest physical therapy in some patients has been attributed to the manual techniques and did not occur when PD was used in isolation.[1-3] Thus, of the conventional techniques used in chest physical therapy, PD has been shown to have the greatest effect on the clearance of secretions.

The effect of body position on the components of oxygen transport (ie, cardiovascular and pulmonary mechanics), the distribution of ventilation and perfusion, and gas exchange is well documented.[4-15] Similarly, the influence of pathology and body position on these aspects of gas transport has been studied extensively.[6,16-27] We do not believe, however, that this body of literature has had an influence on the clinical use of PD positioning when the treatment is focused on secretion clearance.[28,29] Consideration of body position is important if the potential benefits of positioning on oxygen transport, and in particular that associated with PD, are to be clearly understood.

The purpose of this study was to determine the effect of modified PD positions on ventilation homogeneity in healthy individuals. The findings of this study will provide a basis for examining this relationship in patient populations in subsequent studies. Ventilation becomes inhomogeneous when factors such as airway closure and increased time constants (reflecting changes in resistance and compliance characteristics of alveolar units) become prevalent, leading to relative hypoventilation and ventilation-perfusion mismatch, and thus to impaired arterial oxygenation. Although ventilation homogeneity, or the evenness of the distribution of ventilation, is but one of the determinants of oxygen transport, it is one of the components that is most significantly affected by changes in body position, and thus most influenced by physical therapy intervention.

Rationale for Measurentent of Slope of Phase 3 of Single-Breath Nitrogen Washout Test

The tracing from the single-breath nitrogen washout test is a plot of lung volume versus nitrogen concentration when a subject inhales a vital-capacity (VC) breath of 100% oxygen and then expires completely (Figure). During expiration, the anatomical dead space, containing 100% oxygen, is first emptied (phase 1). The initial rise in nitrogen concentration represents mixed dead space and alveolar gas (phase 2), followed by a relative plateau of nitrogen concentration, which represents alveolar emptying (phase 3). A sharper slope, wherein the nitrogen concentration rises sharply (phase 4), represents the onset of closure of the dependent airways, which were relatively better ventilated and thus contained the highest proportion of oxygen that had diluted the nitrogen concentration of the expired air. The homogeneity of ventilation is determined by measuring the slope of phase 3 from the best-fit line drawn through the last two thirds of the curve and is expressed as the change in percentage of nitrogen concentration per liter of expired air ([SBN.sub.2]/L%).(30)

The [SBN.sub.2]/L% primarily represents non-gravity-determined, intraregional ventilation inhomogeneity.[31-33] That is, it is determined by the resistance characteristics of the small airways, alveolar compliance, and diffusion distance in the terminal units. Other washout techniques primarily reflect interregional ventilation inhomogeneity (phase 4), which is determined by the effect of gravity on the pleural pressure gradient, and thus regional volume.[31-33]

In pulmonary disease, the intraregional inhomogeneity increases to a point at which it masks the interregional gradient, such that phase 4 does not occur or is within normal limits.[33-35] Consequently, the [SBN.sub.2]/L% is an accepted indicator of ventilation inhomogeneity in individuals with air flow obstruction.[35,36] The reproducibility of the [SBN.sub.2]/L% measurement, expressed as the coefficient of variation, has been previously determined as 5.9% in our laboratory as measured in 15 young, healthy subjects. Thus, the [SBN.sub.2]/L% was used in this study to determine the effect of PD positioning on ventilation homogeneity in healthy subjects and to refine the procedures for subsequent use in subjects with cardiopulmonary dysfunction.

Method

Subjects

Seventeen healthy subjects of normal height and weight, ranging from 22 to 40 years of age (X [bar]=28.4, SD=5.6), participated in the study. The subjects were students recruited from the University of British Columbia who signed a consent form approved by the Ethics Review Committees of the University of British Columbia and Vancouver General Hospital. All subjects were nonsmokers, free from cardiopulmonary disease, and in good general health, as determined by a screening questionnaire. The subjects had no musculoskeletal abnormalities that would interfere with their ability to comfortably maintain the positions required in the study.

Instrumentation

The single-breath nitrogen washout test was performed with a bag-in-box system, with the bag filled with 100% oxygen. The subject was connected to the system by a mouthpiece and five-way valve. The inspiratory side of the valve was connected to the oxygen bag, and the outlet hose from the box was connected to the spirometer. To analyze the nitrogen concentration, we used a nitrogen gas analyzer(*) and a vacuum pump, with the sampling needle-valve located at the mouthpiece. A resistor inside the expiratory port of the five-way valve was used to assist the subject in maintaining a flow rate of 0.3 to 0.4 L/s, as done previously by Abboud and Morton.34 Volume change was determined with a dry-rolling-seal spirometer,[dagger] The output from both the nitrogen analyzer and spirometer was traced by an X-Y recorder.[double dagger]

A standard hospital stretcher was used for the PD positions. The stretcher allowed for a 15-degree head-down position when the head was lowered to its maximum position. The angle of the position was verified using a goniometer and level.

Procedure

All equipment was calibrated prior to each period of testing, and a linearity check was performed on the nitrogen analyzer at monthly intervals. The nitrogen analyzer and spirometer were calibrated in accordance with National Heart and Lung Institute (NHLI)[37] and American Thoracic Society[38] guidelines. All procedures were conducted by the same experienced individual (JR) to control for potential tester variability.

The subjects were requested not to participate in any exercise or heavy physical activity on the day of a test session. They were requested to avoid eating a heavy meal within 2 hours of the test and to wear comfortable, nonrestrictive clothing. The subjects arrived at the laboratory 15 minutes prior to the commencement of testing procedures to allow for familiarization with the environment and tester and to establish a resting state. During this time, their age and height were determined, and the testing procedures were explained.

All subjects performed the single-breath nitrogen washout test, first in a sitting position and then in one of two modified PD positioning sequences: (1) supine with head down/right side lying with head down/right side lying or (2) right side lying/right side lying with head down/supine with head down. To eliminate any systematic ordering effect of the body positions, the PD positioning sequence was alternated for successive subjects. For the sitting position, an adjustable stool was used to ensure that the subjects sat upright and that their mouths were level with the mouthpiece. For the PD positions, pillows were used to maintain body alignment and specific positions. The subjects rested in each position for 10 minutes prior to the performance of the single-breath nitrogen washout test. Following the PD positioning sequence, the subjects repeated the test in the sitting position, so that the preintervention and postintervention data could be compared.

The single-breath nitrogen washout test was conducted according to the procedures recommended by the NHIA,[37] using the method of Buist and Ross.[30] Each subject performed tidal breathing of room air until a steady state was achieved, then the subject performed a deep inspiration followed by a VC breath before inspiting a VC breath of oxygen from residual volume. The subject then exhaled, maintaining a flow rate of 0.3 to 0.4 L/s. To restore the normal nitrogen gradient, the subject rested 5 minutes between trials and then repeated the test until three acceptable tracings, based on the criteria of the NHIA,[37] were obtained.

Data Analysis

The nitrogen versus volume tracings were coded and arranged randomly. Calculations were then made by an investigator (ED) without knowledge of the coding system. The VCs were measured to confirm that differences in VC between trials in one position did not exceed 10% and that the difference between inspired and expired VC was less than 5%, consistent with the NHIA[37] criteria for acceptance of the tracings. The mean value of the expired VC for the three trials in each position was then used in the data analysis. The [SBN.sub.2]/L% was determined by the "best-fit" line drawn through the last two thirds of the slope of phase 3, according to the method of Buist and Ross.30 The mean value of the [SBN.sub.2]/L% for the three trials in each position was expressed as a percentage of the predicted [SBN.sub.2]/L%, using the prediction equations of Buist and Ross,30 and then used in the data analysis.

Descriptive statistics were used to characterize the measurement of expired VC and [SBN.sub.2]/L% in each of the four positions studied. Within-subject one-way analyses of variance (ANOVAs) were used to compare differences for the VC and [SBN.sub.2]/L% across the four positions. Post hoc comparisons were made using Tukey's tests. An alpha value of .05 was used for all statistical comparisons.

Results

The descriptive statistics for the expired VC and [SBN.sub.2]/L% (expressed as a percentage of the predicted [SBN.sub.2]/L%) for each of the positions are presented in Table 1. An [SBN.sub.2]/L% that is less than or equal to the predicted value represents normal ventilation homogeneity, and an [SBN.sub.2]/L% that is greater than the predicted value represents ventilation inhomogeneity. The lowest mean VC occurred in the right side-lying head-down position, and the highest mean VC occurred in the sitting position. The lowest mean [SBN.sub.2]/L% was recorded in the supine head-down position, and the greatest mean [SBN.sub.2]/L% was recorded in the right side-lying head-down position.

The results of the ANOVAs (Tab. 2) and the post hoc analysis (Tab. 1) showed that VC was significantly greater in the sitting position than in the other positions (significant difference=0.2, P<.05) and not significantly different across the PD positions (P>.05). Ventilation was significantly less homogeneous in the side-lying positions than in the other positions (significant difference= 57.4, P<.05). The VC and [SBN.sub.2]/L% determined in the sitting position were not significantly different before and after the subjects were tested in the PD positions (P>.05).

Compared with the sitting position, the supine head-down position resulted in a significant decrease in VC, with no change in ventilation homogeneity. Both side-lying positions resulted in a significant decrease in VC and ventilation homogeneity.

Discussion

The effect of PD positioning on VC observed in this study is consistent with that previously reported for recumbent positions.[5,8] That is, the recumbent position reduces lung volume compared with the sitting position. Interestingly, the VC in the level right side-lying position did not significantly differ from that in either head-down position. This finding suggests that, in this study, recumbent positioning had the greatest impact on lung volume and the recumbent head-down position induced little effect.

During recumbency, the diaphragm is displaced cephalad in response to the increased abdominal pressure and, together with the increase in thoracic blood volume, accounts for a decreased functional residual capacity.[39,40] Further, the decrease in functional residual capacity predisposes the individual to closure in the dependent airways and an increase in the resistance of the respiratory system.[16,40] Thus, it is possible that the VC would have been significantly lower in the head-down positions assumed in our study if the subjects were older, had pulmonary disease that altered lung compliance or resistance, or had abdominal pathology or obesity that increased intra-abdominal pressure and if they had remained in the position longer prior to performing

the test. As VC largely represents the individual's ability to generate a maximal respiratory maneuver, measurement of static lung volumes in such individuals while in PD positions would help to elucidate the effect of PD positioning on lung volume and ventilation homogeneity.

Table 2. Analysis of Variance Summary for Vital Capacity and Homogeneity of
Ventilation ([SBN.sub.2]/L%)
Source           df      SS          MS        F      P
Vital capacity
  Position        4        1.76        0.44   20.9   <.01
  Error          64        1.35        0.02
  Total          68        3.11
[SBN.sub.2]/L%
  Position        4    86705.50    21676.40   20.9    <.01
  Error          64    66533.30    1039.60
  Total          68   153238.80

The relationship between PD positioning and ventilation homogeneity is complex to interpret. Because the subjects were young, healthy individuals, the presence of airway closure and increased time constants could be expected to be negligible, so that ventilation within a given lung region (intraregional) would be predicted to be relatively homogeneous. Thus, the [SBN.sub.2]/L% would mainly be determined by the interregional ventilation gradient, created by the gradient in pleural pressure along the vertical height of the lung. In the supine head-down position, this prediction is supported. That is, the [SBN.sub.2]/L% tended to be lower in the supine position than in the sitting position, in which the pleural pressure gradient is greater. In the side-lying positions, however, the [SBN.sub.2]/L% was significantly greater than in the sitting position, although the pleural pressure gradient would have been smaller. These observations indicate that the distribution of ventilation was more inhomogeneous in the side-lying positions because of changes in intraregional characteristics. Further, this effect was specific to the horizontal right side-lying position and not to the right side-lying head-down position, as the [SBN.sub.2]/L% was not significantly different between those two positions.

The greater intraregional ventilation inhomogeneity in the side-lying positions could not be attributed to differences in VC, because the VC was not significantly different between the side-lying positions and the supine head-down position. The exact mechanisms for this inhomogeneity are of considerable interest; however, these mechanisms cannot be deduced from the results of this study. Further research is necessary to elucidate whether airway closure, increased airway resistance, or altered compliance was a contributing factor and to what extent any of these factors had a role in altering the subjects' ventilation homogeneity.

The results of this study indicate that the effect of PD positioning on ventilation homogeneity cannot be predicted from the effect of positioning on the pleural pressure gradient alone. Modified PD positions that do not involve a head-down tip can still affect the homogeneity of ventilation. Thus, the effect of PD positioning on arterial oxygenation should be anticipated in the context of the cardiopulmonary status and potential abdominal encroachment of each individual and should be monitored by such means as oximetry. The results of this study were observed in healthy, young individuals and thus are likely to be accentuated in older and patient populations.41 We believe that the prescription of PD positioning needs to be evaluated considering its effect on the distribution of ventilation as a component of oxygen transport, as well as its effect on mobilizing secretions. Thus, the prescription of PD positioning must be weighed against other available approaches that promote secretion clearance while also enhancing oxygen transport overall.[42-47]

Conclusions

Based on data from healthy, young individuals, we conclude that PD positioning influences ventilation homogeneity. Specifically, right side lying elicited intraregional ventilation inhomogeneity, irrespective of whether the individuals were tipped head down or positioned horizontally. These effects were not observed in the supine head-down position. Because of the multitude of both intraregional and interregional determinants of the distribution of ventilation, it is difficult to predict the effect of PD positioning in a specific individual. These findings and those of previous studies demonstrating more efficacious means of promoting secretion clearance support our belief that the implementation of PD positioning must be carefully considered in terms of overall treatment efficacy in optimizing oxygen transport. 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J Ross, PT, is Research Associate, School of Rehabilitation Medicine, University of British Columbia, T325 Koerner Pavilion, 2211 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2B5, and Physiotherapist, Rehabilitation Services, Vancouver General Hospital, 855 W 12th Ave, Vancouver, British Columbia, Canada V5Z 1M9. Address correspondence to Ms. Ross at the first address.

E Dean, PhD, PT is Associate Professor, School of Rehabilitation Medicine, University of British Columbia.

RT Abbound, MD, FRCPC, is Professor, Division of Respiratory Medicine, Faculty of Medicine, University of British Columbia, and Director, Lung Function Laboratory, Vancouver General Hospital.