INTRODUCTION

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Keywords: diaphragmatic function; maturation; transdiaphragmatic pressure

The diaphragm is the most important respiratory muscle throughout life, yet there are a number of disorders that may impair its function in the neonatal period. In addition, it may function less well in infants than in adults, as the former have a higher chest wall compliance and the angle of insertion of the diaphragm in neonates is almost horizontal (1). Performance is likely to be further impaired in infants born prematurely, in whom growth and development of skeletal muscle is incomplete and the proportion of fatigue resistance fibers is only 20% of that found in the diaphragm of an adult (2). In a previous study (3), we demonstrated that the maximal inspiratory pressure generated during crying (PImax) did correlate significantly with gestational age (GA) and postconceptional age (PCA). Although PImax does not permit discrimination between inspiratory muscle groups (4), it seems likely that maturity would affect diaphragmatic function. That hypothesis is important to test, as it would influence interpretation of the results of infants with presumed abnormalities. Diaphragmatic strength can be assessed specifically by measurement of transdiaphragmatic pressure during crying (cPdi) (5, 6). Assessment of transdiaphragmatic pressure is invasive as it requires the placement of an esophageal and a gastric balloon, whereas maximal inspiratory pressure is measured noninvasively by simply recording the pressure generated inside a face mask (3). The latter, however, may be inaccurate if airway patency is not maintained during the measurement. The aim, therefore, of this study was to assess if maturation affects diaphragmatic function, as assessed by measurement of cPdi. Our subsidiary aim was to determine if PImax correlated with cPdi, and therefore, the less invasive procedure could be useful in the clinical setting.

	METHODS

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Infants without evidence of respiratory distress or supplementary oxygen requirement were eligible for entry into the study. The study was approved by the hospital's Research Ethics Committee, and infants were studied if their parent(s) gave informed written consent.

No sedation was given, and the infants were studied in the supine position at least 1 h after a feed. Diaphragmatic function was assessed by measuring transdiaphragmatic pressure during crying (cPdi). Balloon catheters (P.K. Morgan, Rainham, UK) were placed in the mid-esophagus and stomach. The esophageal balloon was 2.5 cm long (inflation volume, 0.2 ml), and the gastric balloon was 4 cm long (inflation volume, 0.8 ml). Mouth pressure was measured from a side port of a pneumotachograph inserted into a face mask. Esophageal (Pes), gastric (Pgas), and mouth pressures were measured using Validyne differential pressure transducers (MP45 range ± 100 cm H₂O, Validyne, Northridge, CA). The pressure signals were displayed in real time on a Macintosh Centris 650 computer (Apple Computer Company, Cupertino, CA) running Labview 2.2 software with analog-to-digital sampling at 100 Hz (NB-M1016; National Instruments, Austin TX).

The esophageal balloon was positioned in the stomach and then withdrawn until a negative pressure deflection was recorded during inspiration. A temporary occlusion of the airway was then effected by occluding the distal end of a pneumotachograph sited in a mask placed over the infant's nose and mouth. The total dead space of the pneumotachograph and face mask in situ was approximately 5 ml. The esophageal balloon catheter was confirmed to be correctly sited when the pressure recorded by the esophageal catheter was 90 to 110% of the pressure recorded inside the face mask (7, 8). The gastric balloon catheter was assumed to be correctly sited when there were positive pressure swings during inspiration.

To measure cPdi the face mask was held firmly over the infant's nose and mouth during crying. The airway was occluded at the end of a crying effort, i.e., at end-expiration. The infants were observed for evidence of chest wall distortion during occlusions, and none was witnessed. The timing of the occlusions were determined by observation of the real time display of the pressure signals. The baselines of cPdi, Pes during crying (cPes), and Pgas during crying (cPgas) were identified during quiet breathing. Two or three sets of five or more occlusions, giving at least 10 airway occlusions, were performed and the maximum cPdi achieved for an individual noted. PImax was measured in the second half of the study population. During end-expiratory occlusions, pressure changes during crying were measured within the face mask. As with cPdi, two or three sets of at least five occlusions, giving at least 10 airway occlusions, were performed and the greatest PImax recorded.

Lung volume was assessed by measurement of FRC using a helium gas dilution technique and a specially designed infant circuit with a volume of 95 ml (9). The FRC system contained a rebreathing bag, the system reservoir, which was enclosed in an airtight cylinder. The infant breathed through a face mask that was connected to the rebreathing bag via a three-way valve. The FRC system contained a helium analyzer (Series 7700; Equilibrated Biosystems Inc, Melville, NY) with a digital display. Helium concentration was recorded prior to and at 15-s intervals during the measurement. Equilibration was assumed when there had been no change in the helium concentration over a 30-s interval. The initial and equilibration helium concentrations were used in the calculation of FRC. FRC results were corrected for oxygen consumption, assumed to be 7 ml/kg/min (10) and the results corrected to body temperature under pressure-saturated conditions. FRC was estimated twice in each infant, with an interval of 10 min between measurements. An individual's FRC was the mean of the paired measurements and related to body weight. The coefficient of repeatability of FRC measurements in spontaneously breathing infants is 3.9 ml/kg (11). The median FRC of term infants is 30 ml/kg (range, 24 to 36) (12); thus, hyperinflation was defined as an FRC in excess of 36 ml/kg.

Patients

Twenty-eight infants (14 male) were studied with a median GA of 35.5 wk (range, 25 to 42 wk), a PCA of 37.6 wk (32 to 44 wk), and a birthweight of 2.554 kg (range, 0.952 to 4.634 kg). Twelve infants had a PCA less than or equal to 37 wk; that is, they were studied prior to term (preterm). Eighteen infants were born prematurely. The original diagnoses and reason for admission to the neonatal intensive care unit (NICU) of the 28 infants were respiratory distress syndrome (n = 6), meconium aspiration syndrome (n = 6), transient tachypnea of the newborn (n = 3), hypoglycemia (n = 4), birth depression (n = 4), prematurity (n = 4), and infection (n = 1). One infant had developed chronic lung disease (that is, had remained oxygen-dependent until 28 d after birth), but none had required supplementary oxygen for at least 4 d prior to testing. In 14 of the 28 infants, PImax was also measured. The 14 infants did not differ significantly from the rest of the group and had a median GA of 37 wk (range, 30 to 42 wk), a birthweight of 2.623 kg (range, 1.230 to 3.260 kg), and a PCA of 38.4 wk (range, 33.1 to 42.8 wk); six had a PCA less than or equal to 37 wk at the time of study.

Analysis

Data were tested for normality using the Kolmogorov-Smirnov and D'Agostino skewness tests. Logarithmic transformation was performed for data that were not normally distributed. Differences were assessed for statistical significance using Student's t test. The relationships between cPdi, cPes, and Pgas with GA, birthweight, PCA and postnatal age (PNA), and length and weight at the time of study were examined using regression analysis. Forward stepwise linear regression analysis was then undertaken to determine which factors independently related to the pressure measurement results. To assess the relationship between cPdi and PImax, simple regression analysis was used. StatView 5.0 (SAS Institute Inc, Cary, NC) and SPSS for Windows, Release 6.1 (SPSS Inc, Chicago, IL) were the statistical packages used.

	RESULTS

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The median FRC of the study population was 25 ml/kg (range, 23 to 35). The median cPdi of the study population was 62 cm H₂O (range, 30 to 101). Median cPdi (56 cm H₂O; range, 30 to 76) and cPes (median, 42 cm H₂O; range, 18 to 60) was significantly lower in infants tested at a PCA of less than term compared with those tested at a PCA greater than term (median, 67 cm H₂O; range, 40 to 101; p < 0.05; median, 54 cm H₂O; range, 31 to 82; p < 0.05, respectively) (Figures 1 and 2). There were no significant differences in the median cPgas of the infants tested at a PCA less than term versus those studied at a PCA greater than term. Comparison of the results from infants born prematurely with those born at term revealed the median cPdi was lower in the prematurely born infants (56 cm H₂O; range, 30 to 88 versus 71 cm H₂O; range, 51 to 101) p < 0.05, as was the median cPes (42 cm H₂O; range, 18 to 60 versus 58 cm H₂O; range, 34 to 82), p < 0.01. There were no significant differences in the median cPgas of the preterm versus the term group. The mean intrapatient coefficients of variation of cPdi and PImax were 7% and 12%, respectively.

View larger version (10K): [in this window] [in a new window]   Figure 1.   Comparison of cPdi results of infants measured at a PCA of less than term (preterm) with those of infants measured at a PCA greater than term (term). Box and whisker plot: the fine horizontal lines represent the 10th, 25th, 50th, 75th, and 90th centiles of cPdi. Outliers are plotted as discrete data points.

View larger version (12K): [in this window] [in a new window]   Figure 2.   Comparison of Pes and Pgas results of infants measured at a PCA of less than term (preterm) with those of infants measured at a PCA greater than term (term). Box and whisker plot: the fine horizontal lines represent the 10th, 25th, 50th, 75th, and 90th centiles of cPdi. Outliers are plotted as discrete data points.

cPdi (Figure 3) and cPes, but not cPgas, significantly correlated with GA (r = 0.46, p < 0.02; r = 0.56, p < 0.01; respectively) and PCA (r = 0.44, p < 0.02; r = 0.43, p < 0.03; respectively). Forward stepwise regression analysis demonstrated that, independently of birthweight, sex, PNA, and PCA, weight and length at the time of measurement, both cPdi (p < 0.02) and cPes (p < 0.005) related to gestational age (Table 1).

View larger version (15K): [in this window] [in a new window]   Figure 3.   Relationships of cPdi to PCA (top panel ) and gestational age (bottom panel ). Individual data and the regression line are demonstrated.

In the subgroup of 14 infants in which both cPdi and PImax were measured, the median PImax (52 cm H₂O; range, 28 to 83) was lower than the median cPdi (64 cm H₂O; range, 51 to 101), p < 0.001. The difference expressed as a percentage between PImax and cPdi, tended to be greater in those whose PCA was less than term at the time of measurement compared with those with a PCA of greater than term although this did not reach statistical significance (p = 0.07). Linear regression analysis indicated that there was a significant relationship between PImax and cPdi (r = 0.79, p < 0.001) (Figure 4).

View larger version (13K): [in this window] [in a new window]   Figure 4.   Linear regression analysis of the relationship between PImax and cPdi. The results from preterm infants (open triangles) and the results from term infants (open circles) are shown.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Our results suggest that maturity at birth affects diaphragmatic function as assessed by measurement of cPdi. Developmental patterns of increasing diaphragmatic force output have been suggested by others (13, 14), but their studies were based on few very immature infants (13) or ventilated infants (14). We examined infants who did not need supplementary oxygen nor had respiratory problems when studied. A proportion had had respiratory problems but were studied at least 4 d after their supplementary oxygen requirement ceased. Hypoxia and hypercapnia reduce diaphragmatic contractility in neonates (15, 16), but our patients had neither. In addition, none of our patients was hyperinflated. With hyperinflation, diaphragmatic pressure generation falls as muscle fibers are shortened (1, 17). cPdi measurement is a volitional test, and one explanation for the reduced pressure in premature infants could be less voluntary effort. We, however, measured cPdi during crying, which has been shown to produce the highest values, regardless of maturity (13). In addition, cPdi was measured after occlusion at the end of a crying effort when an inspiratory effort from residual volume produces maximal effort and diaphragmatic force (18)

Chest wall distortion affects the reliability of pressure measurement as it is associated with an uneven distribution of pleural pressure such that esophageal pressure measurements may not accurately reflect the mean pleural pressure (19). Chest wall muscle contraction helps stabilize the infant's compliant rib cage, minimizing inward displacement of the rib cage by diaphragmatic contraction (20). When the stabilizing effect of the intercostal muscles is inhibited, paradoxical inward motion of the rib cage occurs during inspiration (20). If the chest wall is highly compliant, distortion of the rib cage can occur during an occluded inspiratory effect (21). Chest wall distortion is most likely to occur during rapid eye movement (REM) sleep (20) and in premature infants with a large inspiratory load, and therefore respiratory distress. The infants in the present population were studied when awake, without respiratory distress and at least 4 d after supplementary oxygen requirement had ceased. No chest wall distortion was observed during the occlusions. Chest wall distortion is most likely to occur in infants with loaded breathing or neuromuscular disorders, and in such infants thoracoabdominal motion should be monitored when cPdi is measured. Chest wall distortion results in an increase in diaphragmatic displacement (22) and therefore not only a decrease in pleural or airway pressure, but an increase in gastric pressure (23). Chest wall distortion is most likely in preterm infants (21), and if significant chest wall distortion had occurred in our population, we would have expected to see a correlation of both cPes and cPgas with gestational age. The lack of correlation of cPgas with gestational age supports our clinical observation of no chest wall distortion during occlusion.

Neither cPes nor cPgas reflect diaphragmatic contraction alone. Pes is influenced by intercostal muscle activity and chest wall compliance and Pgas by abdominal muscle tone, abdominal wall compliance, and the position of the diaphragm. The lack of correlation between cPgas and gestational age suggests that abdominal muscle tone and/or abdominal wall compliance is little affected by maturity, whereas the positive correlation of cPes with gestational age indicates an effect of maturity on chest wall compliance. The strong correlation of cPes with gestational age and lack of correlation with cPgas with gestational age suggest that maturational influences on chest wall compliance are more important in influencing cPdi than are changes in diaphragmatic function.

cPdi correlated significantly with PCA as well as gestational age, suggesting that diaphragm strength is influenced by ongoing development postnatally. The rise in maximal cPdi with PCA may relate to improvement in respiratory muscle performance (24) and/or structural changes (25, 26). Progressive increases in the amount of contractile protein occur with increasing postnatal age in skeletal muscles (25). In piglets, muscle fiber cross-sectional area growth continues along with body growth (26), although diaphragmatic muscle is differentiated into its adult form by 6 mo of postnatal age. In prematurely delivered baboons, however, diaphragm fibers are considerably smaller at equivalent PCAs than in baboons completing intrauterine development (27). The growth arrest of the diaphragm muscle fibers persists for at least 10 d after premature delivery (27). Other factors such as increased central drive, maturation of the neuromuscular junction, respiratory muscle conditioning, and ossification of the rib cage may also contribute to the increase of diaphragm pressure generation with increasing maturity (28).

Scott and colleagues (13) suggested that the human infant diaphragm has strength similar to that of the adult by 6 mo of age. The mean cPdi reported for infants older than 13 mo, PCA was 85 cm H₂O (13) and similar to the 91 cm H₂O reported in adults (29). In Scott's (13) controls who were of a PCA of less than 10 mo, the mean cPdi was 55.3 cm H₂O, similar to our findings of a mean cPdi of 68 cm H₂O in a population with a PCA of between 32 and 44 wk.

As expected, we found the PImax levels significantly lower than cPdi. The difference between PImax and cPdi tended to be higher in the premature infants than in those born at term. This maturational effect is probably explained by changes in chest wall rigidity. The results of the two measurements, however, were significantly correlated, suggesting that PImax could be used as an alternative to cPdi to assess diaphragm strength in neonates.

In conclusion, maturation affects cPdi. This needs to be taken into account when interpreting results in the clinical setting. Our results further suggest that the noninvasive measurement PImax could be used in neonates as an indication of diaphragm strength.