All infants tolerated the measurements well. There were no incidents of hypoxia or bradycardia; in fact, transcutaneous Po2 usually increased during the repeated inflations.
In such cases, pressure and you can frequency where C
The recording of P-V curves (15, 16) always preceded FRC measurements. Before and after obtaining each P-V diagram, the lungs were manually inflated with 10 large breaths to counteract atelectasis and standardize the volume history. P-V measurements were done as follows: the equipment shown in Figure 1 was attached to the endotracheal tube, and the lungs were inflated by a syringe, attempting to reach an airway pressure just above 30 cm H2O, which was maintained for about 2 s. During the following passive deflation to zero airway pressure, the expired flow was interrupted for 80 ms every 160 ms by a fast, computer-controlled, electromagnetic valve (Harting 08300320120, Siemens Agelema, Stockholm, Sweden), so that exhalation occurred in small portions. Airway flow was measured by a heated pneumotachograph (Fleisch no. 00, Lausanne, Switzerland; linearity flow range = 0-100 mL/s) connected to a differential pressure transducer (MP 45-1-871; < ±2.5 cm H2O; Validyne, Northridge, CA), and the expired volume between the interruptions was calculated by integration of the flow signal with a computer (PDP , Digital Equipment Corp., Marlboro, MA). Airway pressure was measured by a pressure transducer (SCX01DN, SenSym antichat, Rugby, UK) at the end of each occlusion and was assumed to equal alveolar pressure. Static P-V curves were drawn by an X-Y-writer (HP 7575A). Measurements were discarded if the curves showed signs of spontaneous breathing. The first few interrupter cycles occurred while the investigator’s finger prevented the lungs from emptying(Fig. 1), and a constant pressure during these cycles assured that no leak was present. Twenty to sixty data points were recorded during each deflation, which took 3-10 s. Two to four technically acceptable recordings were achieved in each case. Because an increase in the amplitude and steepness of the P-V curve was often seen between the first and the final measurement in the RDS patients, the last two P-V curves were taken to represent the infant.
Set-up for recording P-V curves. P = pressure transducer. V?E = flow measured by a pneumotachograph and a differential pressure transducer.
IC was defined as the volume expired between 30 and 0 cm H2O of airway pressure. Because of the short deflation time, volume changes due to oxygen uptake were disregarded. In three infants a maximum airway pressure of only 26-27 cm H2O was reached. In these infants (marked with footnoted in Table 1) IC was calculated by extrapolation, assuming that the uppermost part of the P-V curve could be approximated by the equation (17) :
2O, in the 13 infants in whom a pressure of 30 cm H2O was reached, was -3.6 to +0.7 mL (median -0.1 mL).
Crs-max was calculated as the slope of the steepest segment of the P-V curve. In many infants the P-V curves had no clearly defined inflection point, but rather contained a nearly linear segment (seeFig. 2). rs-max occurred were defined as the mid-point of the linear portion of the curve. TLC was defined as FRC at used PEEP (measured as explained below) plus the exhaled volume from 30 cm H2O to PEEP, i.e. it was assumed that FRC during PEEP ventilation coincided with the volume at the corresponding pressure during the P-V maneuver. Specific compliance was calculated as Crs-max/TLC. Deflation stability (V10) was defined as the fraction of TLC still present after deflation to 10 cm H2O (2) . The volumes at airway pressure 0, 2.5, 5, 15, 20, 25, and 30 cm H2O were also calculated and were used to construct mean P-V curves in each group (Fig. 2).