Effects of intrinsic PEEP on pulmonary gas exchange in mechanically-ventilated patients.

The aim of the study was to assess the impact of the intrinsic positive end-expiratory pressure (PEEPi) on pulmonary gas exchange in mechanically-ventilated patients, by comparing the effects of similar levels (0.8-0.9 kPa) of positive end-expiratory pressure (PEEP) and PEEPi. Ten patients with acute respiratory failure, without chronic airway disease, were studied with three ventilatory modes: 1) intermittent positive pressure ventilation with zero end-expiratory pressure (ZEEP mode); 2) continuous positive pressure ventilation with PEEP set by the ventilator (PEEP mode); and 3) intrinsic PEEP elicited by adequate shortening of the expiratory time (PEEPi mode). Cardiorespiratory variables (e.g. respiratory compliance and resistance, arterial and mixed venous blood gases, cardiac output, pulmonary capillary pressure, oxygen delivery) were measured during each ventilatory mode. Compared to ZEEP, both PEEP and PEEPi decreased cardiac output while increasing arterial oxygen tension (PaO2). However, the improvement of PaO2 was more consistent (8 out of 10 patients), and larger (+2.1 kPa, on average, p < 0.05) with PEEP than with PEEPi (5 out of 10 patients, and +1.4 kPa, on average, NS). Since the effects of PEEP and PEEPi on ventilation, lung volume, compliance, cardiac output (QT), mixed venous oxygen tension (PvO2) and oxygen consumption (VO2) were similar, we attributed the less favourable impact of PEEPi on PaO2 to a less homogeneous distribution of PEEPi between lung units with different time constant, and hence to a more uneven distribution of the inspired gas.

Respiratory resistance and intrinsic positive end-expiratory pressure (PEEPi) in patients with the adult respiratory distress syndrome (ARDS).

Fourteen consecutive ARDS patients were examined within 24 h from the onset of mechanical ventilation to determine respiratory resistance (Rrs) and compliance (Cstrs), and to assess the influence of "intrinsic" positive end-expiratory pressure (PEEPi) on the measurement of Cstrs. Flow, pressure, and changes in lung volume were measured with the transducers of the Servo 900C Siemens ventilator. Airway occlusion was performed with the end-inspiratory and end-expiratory buttons of the ventilator. We found PEEPi (3.0 +/- 2.6 cm H2O) in ten of the fourteen patients. Without the correction for PEEPi, Cstrs was underestimated by 13.9 +/- 10% on average in the group as a whole (fourteen patients), and by 19.5 +/- 5.9% in the ten ARDS patients with PEEPi. Maximum and minimum respiratory resistance (Rrsmax and Rrsmin), and frequency-dependence of Rrs were also measured. On average, there was a marked frequency-dependence of resistance, as manifested by the difference between Rrsmax and Rrsmin, with an increase of both Rrsmin (7.7 +/- 4.2 cm H2O.l-1.s) and Rrsmax (14.3 +/- 5.0 cm H2O.l-1.s). The added resistance of the endotracheal tubes and ventilator tubings was flow dependent, and averaged 13.2 +/- 2.9 cm H2O.l-1.s.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory mechanics during the first day of mechanical ventilation in patients with pulmonary edema and chronic airway obstruction.

We investigated the early changes of respiratory mechanics in mechanically ventilated patients with acute respiratory failure (ARF): 8 patients after acute exacerbation of chronic airway obstruction (CAO), 8 patients with cardiogenic pulmonary edema (CPE), and 8 patients with adult respiratory distress syndrome (ARDS). The patients were studied within the first day from the onset of mechanical ventilation. Flow, changes in lung volume, and airway pressure were measured using the 900C Servo Ventilator. End-inspiratory and end-expiratory occlusions of the airway were performed to obtain respiratory compliance and resistance. We found that: (1) acute exacerbation of CAO was characterized by high respiratory resistance (reflecting in part time-constant inequalities within the lung) and severe pulmonary hyperinflation, with "intrinsic" PEEP (PEEPi) up to 22 cm H2O (mean [SD], 13.5 [6.7] cm H2O); (2) PEEPi, even if not high, was present in almost all patients with pulmonary edema, averaging 3.8 and 3.0 cm H2O in ARDS and CPE, respectively; (3) respiratory resistance was increased in patients with CPE and ARDS who had no history of airway disease; (4) patients with ARDS were characterized also by low compliance (mean [SD], 0.035 [0.005] L/cm H2O) and high resistance, the latter also reflecting a substantial component caused by time-constant inequalities; (5) in all 24 patients, static respiratory compliance (and its reciprocal, elastance) was significantly correlated with the pulmonary oxygenation index, i.e., the PaO2/PAO2 ratio. We conclude that early assessment of respiratory mechanics in mechanically ventilated patients with ARF can provide better understanding of the patients' conditions as well as guidelines for therapeutic approach and weaning attempts.

Late, but not early, asthmatic reactions induced by toluene-diisocyanate are associated with increased airway responsiveness to methacholine.

We investigated whether airway responsiveness to methacholine differs in subjects with a history of sensitization to TDI who develop immediate, dual, late, or no asthmatic reactions after exposure to TDI, and also the effect of a TDI inhalation challenge in asthmatic subjects with hyperreactive airways with no history of sensitization to TDI. We measured FEV-1 immediately before and after exposure to TDI (0.018 ppm; 5-30 min) and then hourly for 8 h and the provocative dose (mg) of methacholine that caused a decrease in FEV-1 of 20% (PD20 FEV-1). The results of the present study suggest that the bronchoconstrictor effect of isocyanates is specifically linked to exposure to TDI and subsequent sensitization, excluding a nonspecific irritant effect on the airways. Moreover, they suggest that the increase in airway responsiveness to methacholine associated with the late asthmatic reaction is linked to factors that cause the late component of the asthmatic reaction.