Conventional ventilatory support of patients with the adult respiratory distress syndrome (ARDS) consists of volume-controlled ventilation (VCV) with positive end-expiratory pressure (PEEP). This ventilatory mode is commonly used with an inspiratory-to-expiratory time ratio (I/E) lower than one. Inverse ratio ventilation (IRV), defined as a ventilation with I/E higher than one, is proposed as an alternative mode of ventilatory support in this setting. Indeed, IRV was demonstrated to increase Pa02 and to decrease peak airway pressure (pPaw), thus suggesting that this mode can improve pulmonary gas exchange while lowering the risk of barotrauma. In most published studies, IRV was delivered in a pressure-controlled mode (PC-IRV). It is likely that the increase in Pa02 induced by IRV is at least partly due to the occurrence of intrinsic PEEP (PEEPi). Moreover, it is clearly demonstrated that PEEP can induce a decrease in cardiac output (CO) and consequently in oxygen delivery (Do2). The PEEPi may have the same potential detrimental effect as PEEP on cardiac function. Thus, the effects of IRV on CO and Do2 are questionable. Now, Do2 is an important parameter to take into account in ARDS because Do2 was reported as a critical prognostic factor and because oxygen supply dependency was frequently found in this setting. In this way, ventilatory modes in ARDS should be assessed on Do2, rather than on pulmonary gas exchange (ie, Pa02 or shunt fraction). Few studies reported the effects of IRV on CO and Do2 in ARDS, and results were conflicting.
The aim of this study was to assess the cardiorespiratory effects of pressure-controlled ventilation (PCV) and PC-IRV in patients with ARDS. Since end-expiratory pressure is a major determinant of both gas exchange and hemodynamic consequences of mechanical ventilation in ARDS, the different ventilatory modes were evaluated at the same level of total PEEP (PEEPt).
Ten patients (Table 1) suffering from ARDS for 48 h or less were enrolled in the study. Criteria for inclusion were as follows: (1) age greater than 18 years; (2) lung injury score greater than 2.5м in the setting of a known cause of ARDS; and (3) hemodynamic stability; defined as less than 10 percent variation of the heart rate, mean arterial pressure, cardiac index (Cl), and mixed-venous oxygen saturation between 2 sets of measurements performed 1 h and 15 min before the beginning of the study. Pneumothorax was the sole criterion for exclusion. All patients were sedated (benzodiazepines), paralyzed (pancuronium bromide), and ventilated by the means of a ventilator (Siemens-Elema Servo 900c). Prior to the study, all patients were receiving VCV Eight patients were receiving vasoactive drugs (dopamine or dobutamine or both) at the time of the study. The rates of infusion of these drugs were kept constant throughout the study.
Table 1—Characteristics of Patients
|Patient, Sex, Age (yr)*||Diagnosis||Outcome||PaO/FIo2||Applied PEEP, cm H20||Cst(rs), ml/cm H2Of||LISt||Vt,ml/kg||RR,breaths/min|
|1, M, 76||Mesenteric infarction||Died||103||9||52||2.75||8.2||20|
|2, F, 61||Sepsis||Died||68||11||33||3.25||8.8||20|
|4, M, 62||Pneumonia||Survived||113||12||50||3||8.4||20|
|5, M, 38||Acute pancreatitis||Survived||94||9||38||3.25||9.3||18|
|7, M, 46||Pneumococcal pneumonia||Survived||131||8||39||2.75||14.1||20|
|8, F, 60||Peritonitis||Died||86||11||32||3.25||12.3||21|
|9, F, 53||Nosocomial pneumonia||Died||103||12||24||3.25||9.6||21|
|10, F, 61||Aspiration pneumonia||Survived||160||10||35||3||8.7||18|
|Mean ± SEM||102 ±9||10±1||36±3||3.0 ±0.1||9.5±0.7||20 ±0.3|