Diaphragm dimensions of the healthy term infant.

AIM: Human neonatal diaphragm development has not been extensively studied. Previous work in children and adults suggests that diaphragm thickness (t(di)) is in scale with body size such that maximal transdiaphragmatic pressure (P(dimax)) remains relatively constant. Such assessments have not been made in healthy term infants. This study was designed to evaluate the relationships among t(di), body dimensions and P(dimax) in healthy term infants. METHODS: It was hypothesized that in healthy term infants 1) t(di) is positively correlated with body size and 2) calculated P(dimax) is independent of body weight and length. Fifteen clinically stable term infants (8 males and 7 females) were recruited [birthweight (BW), 3.3 +/- 0.7 kg, (mean +/- SD); head circumference (HC), 33.7 +/- 2 cm; body length (BL) 50 +/- 3 cm; gestational age (GA) 39 +/- 1 wk; and postnatal age 1.7 +/- 0.8 day]. Ultrasound was used to visualize the diaphragm at the level of the zone of apposition and measure t(di). Standard techniques were used to measure the anthropometric dimensions of the rib cage. P(dimax) was calculated using the piston-in-cylinder model of diaphragm function. RESULTS: Significant correlations were found among t(di) and BW (R = 0.58), BL (R = 0.58) and HC (R = 0.65) but not between GA (R = 0.20). Larger infants tended to have thicker diaphragms and larger cross-sectional areas of the lower rib cage (A(ZAP)). For the group, calculated P(dimax) was independent of either body weight or length and was greater than that calculated for adults. CONCLUSION: It is concluded that diaphragm mass in healthy term infants is proportional to body size, whereas calcuated P(dimax) is independent of body size. Since calculated P(dimax) is greater than that predicted for adults, there may be perinatal diaphragm strengthening. This may assist the infant in generating sufficient pressure to overcome the enormous elastic and resistive loads imposed during perinatal pulmonary transition.

Diaphragm dimensions of the healthy preterm infant.

BACKGROUND: The diaphragm is the major inspiratory muscle in the neonate; however, human neonatal diaphragm development has not been extensively studied. We hypothesized that diaphragm thickness (t(di)) would be positively related to postmenstrual age (PMA), body weight, body length, head circumference, and nutritional intake. OBJECTIVES: To evaluate the evolution of diaphragm growth and motion in the healthy, preterm infant. METHODS: We used ultrasound to measure t(di) at the zone of apposition to the rib cage and diaphragm excursion (e(di)) during inspiration. Thirty-four stable, preterm infants (16 males and 18 females) between 26 and 37 weeks' PMA were studied during quiet sleep at weekly intervals until the time of discharge or transfer from the neonatal intensive care unit. All infants were clinically stable and not receiving ventilatory support. RESULTS: We found that 1) t(di) increased from 1.2 +/- 0.1 to 1.7 +/- 0.05 mm between 26 to 28 and 35 to 37 weeks' PMA; 2) t(di) was positively correlated with PMA (r = 0.40), body weight (r = 0.52), body length (r = 0.53), and head circumference (0.49), but not with postnatal nutritional intake (r = 0.09); and 3) e(di) decreased with increasing PMA. CONCLUSIONS: Our findings suggest that diaphragm development in premature infants scales with body dimensions. We speculate that the increase in t(di) with age is likely attributable to increased diaphragm muscle mass, and the reduced e(di) with age may be resulting from a reduction in chest wall compliance.

Leak compensation in positive pressure ventilators: a lung model study.

Leak compensating abilities of six different positive pressure ventilators commonly used to deliver noninvasive positive pressure ventilation, including the bilevel positive airway pressure (BiPAP) S/T-D and Quantum (Respironics Inc, Murrysville, PA, USA), 335 and O'NYX (Mallinckrodt Inc, St Louis, MO, USA), PLV 102 (Respironics), and Siemens Servo 900C (Siemens Inc, Danvers, MA, USA). Using a test lung model, compensatory capabilities of the ventilators were tested for smaller and larger leaks using the assist/control or timed modes. Back-up rate was 20 min(-1), inspiratory pressure was 18 cmH2O, and expiratory pressure was 5 cmH2O. It was found that even in the absence of air leaking, delivered tidal volume differed substantially between the ventilators during use of pressure-targeted modes, depending on inspiratory flows, inaccuracies in set versus delivered pressures, and inspiratory duration. Also during pressure-targeted ventilation, increasing the tI/ttot up to, but not beyond, 0.5 improved compensation by lengthening inspiratory duration, whereas use of a sensitive flow trigger setting tended to cause autocycling during leaking, interfering with compensation. Leaking interfered with cycling of the BiPAP S/T, inverting the I:E ratio, shortening expiratory time, and reducing delivered tidal volume. Volume-targeted modes achieved limited compensation for small air leaks, but compensated poorly for large leaks. To conclude, leak-compensating capabilities differ markedly between ventilators but pressure-targeted ventilators are preferred for noninvasive positive pressure ventilation in patients with substantial air leaking. Adequate inspiratory flows and durations should be used, triggering sensitivity should be adjusted to prevent autocycling, and a mechanism should be available to limit inspiratory time and avoid I:E ratio inversion.

Effects of continuous positive airway pressure on diaphragm dimensions in preterm infants.

OBJECTIVE: The use of continuous positive airway pressure (CPAP) in the treatment of a variety of neonatal respiratory conditions is associated with improvement in arterial oxygen saturation, decreased long-term morbidity, and an overall improvement in infant survival. We reasoned that CPAP might change diaphragm length by increasing end-expiratory lung volume (EEV), but the extent to which this occurs has not been assessed. This study was designed to evaluate (1) the extent to which CPAP shortens the diaphragm and (2) the relationship of diaphragm thickness and excursion with arterial oxygen saturation in spontaneously breathing preterm infants. STUDY DESIGN: Ultrasonographically (7.5 MHz transducer), diaphragm thickness and diaphragm excursion were measured in 12 stable preterm infants [birth weight 1120+/-225 g (mean+/-SD); study weight 1187+/-400 g; gestational age 29+/-1 week; postnatal age 10+/-8 days, six males and six females] at three levels of CPAP [1-3, 4-6, and 7-9 cm H(2)O (low, medium, and high, respectively)]. Heart rate, respiratory rate, and arterial oxygen saturation were simultaneously recorded. RESULTS: We found that diaphragm thickness and arterial oxygen saturation increased, and diaphragm excursion decreased significantly at higher levels of CPAP (p<0.05). The shortening of the diaphragm at the high levels of CPAP, calculated from the increase in diaphragm thickness, was 36% at EEV and 31% at end-inspiratory volume. CONCLUSION: We conclude that the improvement in arterial oxygen saturation with CPAP occurred despite the presence of a shorter and a less mobile diaphragm, and that other physiological and mechanical alterations accompanying the application of CPAP offset its negative effects on diaphragm function. We speculate that with excessive CPAP, however, diaphragm dysfunction along with the previously described adverse hemodynamic effects may outweigh its benefits on oxygenation.

Diaphragm recruitment during nonrespiratory activities.

We previously found that weight lifters could generate greater inspiratory pressures and had more diaphragm mass than control subjects. We postulated that the weight-lifting activity itself provided a strength-training stimulus to the diaphragm. To evaluate the extent to which the diaphragm is recruited during strenuous nonrespiratory activities, we measured transdiaphragmatic pressure (Pdi) in six healthy subjects during biceps curls, bench press, power lift, and sit-ups. Each maneuver was performed with and without added weight (control), and with and without an abdominal binder. The weighted maneuvers were performed either during inspiration or expiration. Maximal static transdiaphragmatic pressure (Pdi(max)) was measured during a combined inspiratory and expulsive maneuver. Group mean values of Pdi increased during all activities when compared with control (57 +/- 24 versus 18 +/- 10 cm H(2)O [mean +/- SD] [p < 0.001]), as task intensity increased (98 +/- 14 versus 35 +/- 13 cm H(2)O for high- and low-intensity activities, respectively) (p < 0.001), and with abdominal binding (75 +/- 25 versus 59 +/- 25 cm H(2)O) (p < 0.05). Peak levels of Pdi attained during the activities were 126 +/- 11 cm H(2)O or 0.65 +/- 0.09 of Pdi(max). Changes in gastric pressure accounted for 85 +/- 4% of the increase in Pdi during the activities whereas it accounted for only 58 +/- 9% of the rise in Pdi during the control activities (p < 0.001). We conclude that during a range of weight-lifting activities, the diaphragm is recruited and Pdi is raised to a level that may provide a significant strength-training stimulus to the diaphragm.

Effects of the supine and prone position on diaphragm thickness in healthy term infants.

BACKGROUND: The physiological basis underlying the decline in the incidence of sudden infant death syndrome (SIDS) associated with changing the sleep position from prone to supine remains unknown. AIMS: To evaluate diaphragm thickness (t(di)) and shortening in healthy term infants in the prone and supine positions in order to determine whether changes in body position would affect diaphragm resting length and the degree of diaphragm shortening during inspiration. METHODS: In 16 healthy term infants, diaphragm thickness at the level of the zone of apposition on the right side was measured using ultrasonography. Heart rate (HR), breathing frequency (f), and transcutaneous oxyhaemoglobin saturation (SaO(2)) were recorded simultaneously during diaphragm imaging with the infants in the supine and prone positions during quiet sleep. RESULTS: At end expiratory (EEV) and at end inspiratory lung volumes (EIV), t(di) increased significantly in the prone position. The change in t(di) during tidal breathing was also greater when the infant was prone. SaO(2), HR, and f were not significantly different at EEV and at EIV in both positions. CONCLUSION: In healthy term infants, placed in the prone position, the diaphragm is significantly thicker and, therefore, shorter, both at EEV and EIV. Diaphragm shortening during tidal breathing is greater when the infant is prone. In the prone position, the decreased diaphragm resting length would impair diaphragm strength, and the additional diaphragm shortening during tidal breathing represents added work performed by the diaphragm. This may compromise an infant's capacity to respond to stressful situations when placed in the prone position and may contribute to the association of SIDS with prone position.

Prevention of human diaphragm atrophy with short periods of electrical stimulation.

We determined whether prolonged complete inactivation of the human diaphragm results in atrophy and whether this could be prevented by brief periods of electrical phrenic nerve stimulation. We studied a subject with high spinal cord injury who required removal of his left phrenic nerve pacemaker (PNP) and the reinstitution of positive-pressure ventilation for 8 mo. During this time, the right phrenic nerve was stimulated 30 min per day. Thickness of each diaphragm (tdi) was determined by ultrasonography. Maximal tidal volume (VT) was measured during stimulation of each diaphragm separately. After left PNP reimplantation, VT and tdi were measured just before the resumption of electrical stimulation and serially for 33 wk. On the previously nonfunctioning side, there were substantial changes in VT (from 220 to 600 ml) and tdi (from 0.18 to 0.34 cm). On the side that had been stimulated, neither VT nor tdi changed appreciably (VT from 770 to 900 ml; tdi from 0.25 to 0.28 cm). We conclude that prolonged inactivation of the diaphragm causes atrophy which may be prevented by brief periods of daily phrenic nerve stimulation.

Inspiratory muscle adaptations following pressure or flow training in humans.

Skeletal muscle adapts differently to training with high forces or with high velocities. The effects of these disparate training protocols on the inspiratory muscles were investigated in ten healthy volunteers. Five subjects trained using high force (pressure) loads (pressure trainers) and five trained using high velocity (flow) loads (flow trainers). Pressure training entailed performing 30 maximal static inspiratory efforts against a closed airway. Flow training entailed performing 30 sets of three maximal dynamic inspiratory efforts against a minimal resistance. Training was supervised and carried out 5 days a week for 6 weeks. Inspiratory flow rates and oesophageal pressure-time curves were measured before and after training. Peak inspiratory pressures during maximal static and dynamic efforts and peak flows during the maximal dynamic efforts were calculated. The time-to-peak pressure and rate of rise in peak pressure during maximal static and dynamic manoeuvres were also calculated before and following training. Maximal static pressure increased in the pressure training group and maximal dynamic pressure increased in the flow training group. Both groups increased the rate of pressure production (dP/dt) during their respective maximal efforts. The post-training decrease in time-to-peak pressure was proportionately greater in the flow trainers than in the pressure trainers. The differences in time-to-peak pressure between the two groups were consistent with the different effects of force and velocity training on the time-to-peak tension of skeletal muscle.

Diaphragm thickening during inspiration.

Ultrasound has been used to measure diaphragm thickness (Tdi) in the area where the diaphragm abuts the rib cage (zone of apposition). However, the degree of diaphragm thickening during inspiration reported as obtained by one-dimensional M-mode ultrasound was greater than that predicted by using other radiographic techniques. Because two-dimensional (2-D) ultrasound provides greater anatomic definition of the diaphragm and neighboring structures, we used this technique to reevaluate the relationship between lung volume and Tdi. We first established the accuracy and reproducibility of 2-D ultrasound by measuring Tdi with a 7.5-MHz transducer in 26 cadavers. We found that Tdi measured by ultrasound correlated significantly with that measured by ruler (R2 = 0.89), with the slope of this relationship approximating a line of identity (y = 0.89x + 0.04 mm). The relationship between lung volume and Tdi was then studied in nine subjects by obtaining diaphragm images at the five target lung volumes [25% increments from residual volume (RV) to total lung capacity (TLC)]. Plots of Tdi vs. lung volume demonstrated that the diaphragm thickened as lung volume increased, with a more rapid rate of thickening at the higher lung volumes [Tdi = 1.74 vital capacity (VC)2 + 0.26 VC + 2.7 mm] (R2 = 0. 99; P < 0.001) where lung volume is expressed as a fraction of VC. The mean increase in Tdi between RV and TLC for the group was 54% (range 42-78%). We conclude that 2-D ultrasound can accurately measure Tdi and that the average thickening of the diaphragm when a subject is inhaling from RV to TLC using this technique is in the range of what would be predicted from a 35% shortening of the diaphragm.

Air leaking through the mouth during nocturnal nasal ventilation: effect on sleep quality.

Air leaking through the mouth has been reported in kyphoscoliotic patients receiving nasal ventilation via volume-limited ventilators. This study accessed the frequency of occurrence and effect on sleep quality of air leaking through the mouth during nocturnal nasal ventilation in patients with chest wall and neuromuscular disease using pressure-limited ventilation. Overnight and daytime polysomnography was performed in six stable experienced users of nocturnal nasal noninvasive positive-pressure ventilation (NPPV) who had chronic respiratory failure due to neuromuscular disease or chest wall deformity. All patients used the BiPAP S/T-D ventilatory support system (Respironics, Inc., Murrysville, PA). Measures included sleep scoring, leak quantitation, diaphragm and submental electromyograms (EMGs), and tidal and leak volumes. All patients had air leaking through the mouth for the majority of sleep. Sleep quality was diminished because of poor sleep efficiency and reduced percentages of slow-wave and rapid eye movement (REM) sleep. Air leaking through the mouth was associated with frequent arousals during stages 1 and 2 and REM sleep that contributed to sleep fragmentation, but arousals were infrequent during slow-wave sleep. Despite prevalent leaking, oxygenation was well maintained in all but one patient. Patients used a-combination of passive and active mechanisms to control air leaking. Although nasal ventilation improves nocturnal hypoventilation and symptoms in patients with restrictive thoracic disorders, air leaking through the mouth is very common during use. The leaking is associated with frequent arousals during lighter stages of sleep that interfere with progression to deeper stages, compromising sleep quality. Portable pressure-limited ventilators compensate for leaks, maintaining ventilation and oxygenation, but further studies are needed to determine which interfaces and ventilator techniques best control air leaking and optimize sleep quality.

Ultrasound evaluation of the paralyzed diaphragm.

Ultrasound has been used to evaluate diaphragm thickness in the zone of apposition of the diaphragm to the rib cage. The purpose of this study was to determine if ultrasonography could distinguish between a paralyzed and normally functioning diaphragm. We predicted that a paralyzed diaphragm would be atrophic and not shorten, therefore it would be thin and not thicken during inspiration. Thirty subjects (five with bilateral diaphragm paralysis, seven with unilateral diaphragm paralysis, three with inspiratory weakness but normally functioning diaphragms, and 15 healthy control subjects) had diaphragm ultrasound performed with a 7.5 to 10.0 MHz transducer placed over the lower rib cage in the mid-axillary line. The thickness of the diaphragm (tdi) was measured to the nearest 0.1 mm at FRC (t(di)FRC) and TLC (t(di)TLC). Diaphragm thickening during inspiration (delta t(di)) was calculated as (t(di)TLC - t(di)FRC)/t(di)FRC. In patients with unilateral paralysis, t(di) and delta t(di) for the paralyzed hemidiaphragm were significantly less than those values for the normally functioning hemidiaphragm (1.7 +/- 0.2 mm versus 2.7 +/- 0.5 mm [mean + SD] p < 0.01 for t(di), and -8.5 +/- 13% versus 65 +/- 26% [p < 0.001] for delta t(di)). The t(di) and delta t(di) for patients with bilateral diaphragm paralysis were significantly less than those values for the healthy volunteers (1.8 +/- 0.2 versus 2.8 +/- 0.4 and -1 +/- 15% versus 37 +/- 9% for t(di) and delta t(di), respectively) (p < 0.001). We conclude that ultrasound measurements of t(di) and delta t(di) can be used to determine if a diaphragm is paralyzed and confirm our predictions that a chronically paralyzed diaphragm is atrophic and does not thicken during inspiration.

Variability of diaphragm structure among healthy individuals.

The ratio of the muscular cross-sectional area of the diaphragm (CSA(di)) to the axially projected area of the thorax (A(thor)) theoretically determines the strength of the inspiratory pump. We studied these dimensions in 37 healthy subjects by ultrasonography and anthropometry. In 21 subjects who did not train with weights, thickness of the diaphragm (t(di)), circumference of the rib cage (c(di)), and CSA(di) increased with height and with body weight. The increase of thoracic cavity dimensions with weight was similar to that described across a wide range of mammals and was consistent with the scaling principle of elastic similarity. CSA(di)/A(thor) showed considerable variability and was not systematically dependent on height or weight. The 15 adults who trained with weight-lifting had thicker diaphragms for comparable height and greater CSA(di)/A(thor) than the adults who did not train. We conclude that (1) the structural dimensions of the diaphragm and thorax show substantial variability, some of which is systematic with stature; (2) the variations of structure predict substantial variation of inspiratory strength which is not systematic with stature; (3) the muscular cross-section of the diaphragm is increased by general or specific training.

Maximal inspiratory pressures and dimensions of the diaphragm.

We postulated that the variation of maximal voluntary inspiratory pressures (PI,max and Pdi,max) among individuals largely reflects the variation of the structural attributes of the inspiratory muscles, in particular the muscular cross-sectional area of the diaphragm (CSAdi) and its axially projected area (A(thor)). To test this postulate, we measured PI,max in 36 healthy subjects, including 3 children and 15 weight-lifters, and Pdi,max in 11 subjects. Structural measurements by ultrasonography and anthropometric calipers were available as reported in the companion manuscript. We found a high degree of correlation of Pdi,max with diaphragm thickness (tdi), CSAdi, and CSAdi/A(thor) (r2 = 0.89, 0.89, and 0.77, respectively). PI,max was also correlated with diaphragm structural measurements, although less well. The weight-lifters had greater pressures, thicker diaphragms, and greater diaphragm maximal stress (sigma(max)) than adults of similar stature who had not trained with weights. We conclude (1) that both Pdi,max and PI,max reflect in part structural attributes of the respiratory muscles; (2) that the variation of maximal transdiaphragmatic pressures is largely attributable to the normal variation of diaphragm structure; (3) weight lifting increases diaphragm structure and pressures.

Changes in breathing and ventilatory muscle recruitment patterns induced by lung volume reduction surgery.

Patients with chronic obstructive pulmonary disease have abnormal breathing and ventilatory muscle recruitment patterns at rest and during exercise, and these alterations may contribute to the limited exercise capacity seen in this disease. Lung volume reduction surgery (LVRS), a recently described treatment for emphysema, is reported to improve exercise performance. We studied the breathing and ventilatory muscle recruitment (VMR) patterns in eight patients with severe chronic obstructive lung disease (median FEV1 = 0.79 L, range 0.46 to 1.13 L) by measuring esophageal and gastric pressure measurements as well as tidal volumes (VT), respiratory rates (f), inspiratory (TI) and expiratory (TE) times, and watts at rest and during maximal exercise, before and 3 mo after lung volume reduction surgery. Maximal exercise capacity increased a median of 49% (median increase 17 watts, range 6 to 44 watts, p < 0.05) and maximal minute ventilation (VEmax) increased by a median of 22% (median increase 6.5 L/min, range 3 to 25 L/min, p < 0.05). At isowatt exercise after surgery, VT increased 0.31 L (range 0.07 to 0.69 L) and f decreased four breaths/min (range +0.5 to -15 breaths/min). Dyspnea scores as measured by a visual analog scale (VAS) decreased significantly at rest and at peak exercise after surgery. End-expiratory esophageal (Pes) and gastric (Pga) pressures at rest and at isowatt exercise decreased. A rightward shift in the slope of the Pes versus Pga plot was also observed suggesting increased use of the diaphragm after surgery. Our data indicate that LVRS improves the mechanics of breathing both at rest and during exercise.

Transmission of pressure within the abdomen.

The extent to which transmission of pressure within the abdomen is accomplished in accordance with the laws of fluid mechanics, i.e., homogeneous transmission to all portions of the abdomen, is controversial. To examine the cranial-to-caudal as well as side-to-side transmission of pressure within the abdomen in humans, we measured intra-abdominal pressure at four sites in five subjects undergoing colonoscopy. Liquid-filled catheters were inserted into the colon, and intracolonic pressure was measured in the rectum and in transverse, descending, and sigmoid colon. Differences in intracolonic pressure were recorded during breaths to total lung capacity and brief expulsive maneuvers. Measurements were taken in the supine, right lateral, and seated position. Comparison of pressure swings at all sites showed that the pressure changes were nearly equal during both inspiratory and expulsive maneuvers. The changes in pressure were uniform in the cephalocaudal axis as from side to side. We conclude that transmission of abdominal pressure in humans is nearly homogeneous. Our findings provide support for the hydraulic model of abdominal mechanics.

Inspiratory muscle training in the patient with neuromuscular disease.

Pulmonary complications due to respiratory muscle dysfunction are commonly a source of morbidity and mortality in patients with neuromuscular diseases. This review discusses the adverse effects of respiratory muscle weakness on pulmonary mechanics and examines the role that inspiratory muscle training may play in reversing pulmonary dysfunction in these individuals. In asymptomatic persons, it is well established that the inspiratory muscles can be trained to increase both force and endurance. In patients with neuromuscular diseases, the effects of training protocols on force and endurance are more controversial. This article reviews seven studies that have evaluated respiratory muscle training in a total of 75 patients with varied neuromuscular disorders. Training regimens included breathing through inspiratory resistive loads and isocapnic hyperpnea. Despite methodologic differences among studies, investigators have generally shown that the inspiratory muscles are similar to other skeletal muscle groups in that they can be trained for both force and endurance in these patients. The training-related improvements in inspiratory muscle performance are more pronounced in patients who are less severely affected by their disease. In those patients who have disease to the extent that they are already retaining carbon dioxide, there is little change in force or endurance with training. In these individuals, the inspiratory muscles may already be working at a level sufficiently severe to provide a training stimulus with each breath. No adverse effects of inspiratory muscle training were reported. Inspiratory muscle training can improve force and endurance in patients with neuromuscular weakness.(ABSTRACT TRUNCATED AT 250 WORDS)

Lung volume specificity of inspiratory muscle training.

We examined the extent to which training-related increases of inspiratory muscle (IM) strength are limited to the lung volume (VL) at which the training occurs. IM strength training consisted of performing repeated static maximum inspiratory maneuvers. Three groups of normal volunteers performed these maneuvers at one of three lung volumes: residual volume (RV), relaxation volume (Vrel), or Vrel plus one-half of inspiratory capacity (Vrel + 1/2IC). A control group did not train. We constructed maximal inspiratory pressure-VL curves before and after a 6-wk training period. For each group, we found that the greatest improvements in strength occurred at the volume at which the subjects trained and were significantly greater for those who trained at low (36% for RV and 26% for Vrel) than at high volumes (13% for Vrel + 1/2IC). Smaller increments in strength were noted at volumes adjacent to the training volume. The range of vital capacity (VC) over which strength was increased was greater for those who trained at low (70% of VC) than at high VL (20% of VC). We conclude that the greatest improvements in IM strength are specific to the VL at which training occurs. However, the increase in strength, as well as the range of volume over which strength is increased, is greater for those who trained at the lower VL.