A pinhole method for measuring chest wall compliance

We describe a method of measuring chest wall compliance (Cw) that readily detects whether respiratory muscles are relaxed. The method simulates a normal slow sigh, with the subject exhaling through a needle valve. Cw is calculated from the slope of the volume-esophageal pressure line. With relaxed subjects, repeated measurements yield similar slopes. When subjects cannot relax, the volume-pressure line is irregular and variable. In 26 subjects who could relax, Cw averaged 0.208 +/- 0.05 (SD) l/cmH2O.     

Respiratory cross-sectional area-flux measurements of the human chest wall

A new device that utilizes the voltages induced in separate coils encircling the rib cage and abdomen by a magnetic field is described for measurement of cross-sectional areas of the human chest wall (rib cage and abdomen) and their variation during breathing. A uniform magnetic field (1.4 X 10(-7) Tesla at 100 kHz) is produced by generating an alternating current at 100 kHz in two square coils, 1.98 m on each side, parallel to the planes of the areas to be measured and placed symmetrically cephalad and caudad to these planes at a mean distance of 0.53 m. We demonstrated that the accuracy of the device on well-defined surfaces (squares, circles, rectangles, ellipses) was within 1% in all cases. Observed errors are due primarily to small inhomogeneities of the magnetic field and variation of the orientation of the coil relative to the field. Using a second magnetic field (80 kHz) perpendicular to the first, we measured the errors due to nonparallel orientation during quiet breathing and inspiratory capacity maneuvers. In 10 normal subjects, orientation effects were less than 2% for the rib cage and less than 0.7% for the abdomen. In five of these subjects, orientation effects at functional residual capacity in lateral and seated postures were generally less than or equal to 5%, but estimated tidal volume during spontaneous breathing was comparable to measurements in the supine posture. In five curarized patients, we assessed the linearity of volume-motion relationships of the rib cage and abdomen, comparing cross-sectional area and circumference measurements. Departures from linearity using cross-sectional areas were only one-third of those using circumferences. In seven normal subjects we compared cross-sectional area measurements with respiratory inductive plethysmography (RIP) and found comparable estimates of lung volume change over a wide range of relative rib cage contributions to tidal volume (-5 to 105%), with slightly higher standard deviations for the RIP (SD = 10% for RIP; SD = 4% for cross-sectional area).     

Effect of intravenous midazolam on breathing pattern and chest wall mechanics in human

Breathing pattern, thoracoabdominal motion, and separate end-expiratory positions of the rib cage and abdomen were measured noninvasively in eight healthy subjects before and after intravenous administration of either placebo or midazolam, a short-acting benzodiazepine. Compared with placebo, midazolam produced a significant (P less than 0.01) decrease in mean inspiratory flow of 29% from preinjection values, resulting in a 39% reduction in tidal volume (VT). This ventilatory depression was partly compensated by a 35% decrease in expiratory time producing an increase in respiratory rate (+39%). The fall in VT was almost entirely (91%) mediated by a reduction of the abdominal contribution to tidal breathing while sparing rib cage motion. This fact contrasts with the effects of inhalational anesthetics or morphine, which preferentially depress rib cage expansion, indicating that thoracoabdominal motion may selectively be depressed by different pharmacological agents. In addition, continuous recording of end-expiratory levels showed a significant transient fall in the rib cage's end-tidal position 2 min after midazolam administration associated with the occurrence of central apneas.     

The effect of compliance on wall shear in casts of a human aortic bifurcation

Rigid and compliant casts of a human aortic bifurcation were subjected to physiologically realistic pulsatile fluid flows. At a number of sites near the wall in the approximate median plane of the bifurcation of these models, fluid velocity was measured with a laser Doppler velocimeter, and wall motion (in the case of the compliant cast) was determined with a Reticon linescan camera. The velocity and wall motion data were combined to estimate the instantaneous shear rates at the cast wall. Analysis showed that at the outer walls the cast compliance reduced shear rates, while at the walls of the flow divider the shear rate was increased.     

Dynamics of chest wall in preterm infants

The chest wall of the preterm infant has visible paradoxical movement during breathing, because of its greater flexibility than those of older children and adults. We studied the dynamics of the chest wall in 10 preterm infants to describe the interaction of the chest wall volume, as partitioned by the inductance plethysmograph, and the transthoracic and abdominal pressures. There was considerable hysteresis between the chest wall volume and the transthoracic pressure, and it had linear pressure-volume behavior during airway occlusion, late inspiration, and early expiration. The slope of this pressure-volume relationship, or the instantaneous chest wall compliance, averaged 0.89 +/- 0.16 and 0.94 +/- 0.18 ml/cmH2O for the respiratory effort during airway occlusion and early expiration, respectively. The dynamic compliance was considerably greater, averaging 7.8 +/- 2.3 ml/cmH2O. This resistive pressure-volume behavior was not related to the absolute value of or the rate of development of the esophageal or abdominal pressures. This additional degree of freedom of motion of the chest wall suggests that its linkage to the diaphragm is flexible, which provides a braking force for expiration and allows free movement of the diaphragm for breathing movements before birth.     

Compliance of chest wall in obese subjects

Whereas studies in awake subjects have demonstrated that chest wall compliance (Ccw) is low in obese subjects, the one study performed on paralyzed obese subject found Ccw to be normal. The purpose of this study was to measure Ccw in awake obese subjects with the pulse-flow technique, a method which appears to detect respiratory muscle relaxation. Seven normal males, 14 obese males, and 8 obese females [body mass index (BMI) varied from 20 to 83 kg/m2] were studied in the seated position. Ccw was measured by blowing air at a constant flow into the mouth and lungs for approximately 2 s and calculated by dividing airflow in liters per second by the change in esophageal minus body surface pressure in centimeters of water per second. In normal and obese subjects we found no correlation between BMI and Ccw. We conclude that obesity does not decrease Ccw.     

Lung and chest wall mechanics in microgravity.

We studied the effect of 15-20 s of weightlessness on lung, chest wall, and abdominal mechanics in five normal subjects inside an aircraft flying repeated parabolic trajectories. We measured flow at the mouth, thoracoabdominal and compartmental volume changes, and gastric pressure (Pga). In two subjects, esophageal pressures were measured as well, allowing for estimates of transdiaphragmatic pressure (Pdi). In all subjects functional residual capacity at 0 Gz decreased by 244 +/- 31 ml as a result of the inward displacement of the abdomen. End-expiratory Pga decreased from 6.8 +/- 0.8 cmH2O at 1 Gz to 2.5 +/- 0.3 cmH2O at Gz (P less than 0.005). Abdominal contribution to tidal volume increased from 0.33 +/- 0.05 to 0.51 +/- 0.04 at 0 Gz (P less than 0.001) but delta Pga showed no consistent change. Hence abdominal compliance increased from 43 +/- 9 to 70 +/- 10 ml/cmH2O (P less than 0.05). There was no consistent effect of Gz on tidal swings of Pdi, on pulmonary resistance and dynamic compliance, or on any of the timing parameters determining the temporal pattern of breathing. The results indicate that at 0 G respiratory mechanics are intermediate between those in the upright and supine postures at 1 G. In addition, analysis of end-expiratory pressures suggests that during weightlessness intra-abdominal pressure is zero, the diaphragm is passively tensed, and a residual small pleural pressure gradient may be present.     

Nasal wall compliance in vasomotor rhinitis.

Nasal compliance is a measure related to the blood volume in the nasal mucosa. The objective of this study was to better understand the vascular response in vasomotor rhinitis by measuring nasal cross-sectional area and nasal compliance before and after mucosal decongestion in 10 patients with vasomotor rhinitis compared with 10 healthy subjects. Nasal compliance was inferred by measuring nasal area by acoustic rhinometry at pressures ranging from atmospheric pressure to a negative pressure of -10 cmH2O. Mucosal decongestion was obtained with one puff per nostril of 0.05% oxymetazoline. At atmospheric pressure, nasal cross-sectional areas were similar in the vasomotor rhinitis group and the healthy subject group. Mucosal decongestion did not induce any decrease of nasal compliance in patients with vasomotor rhinitis in contrast with healthy subjects. Our results support the hypothesis, already proposed, of an autonomic dysfunction based on a paradoxical response of the nasal mucosa in vasomotor rhinitis. Moreover, the clearly different behavior between healthy subjects and vasomotor rhinitis subjects suggests that nasal compliance measurement may therefore represent a potential line of research to develop a diagnostic tool for vasomotor rhinitis, which remains a diagnosis of exclusion.