Medicine of the Future in America

Category Archives: ARDS - Part 2

Cardiorespiratory Effects of Pressure-controlled Ventilation With and Without Inverse Ratio in the Adult Respiratory Distress Syndrome: Discussion

The main findings of this study were that (1) the effects of PCV on gas exchange and hemodynamics were not different from those of VCY and (2) PC-IRV failed to improve Pa02 while inducing a decrease in Cl and hence in Do2. Few studies have reported the effects of PCV in ARDS. Abraham and Yoshihara reported an improvement in Pa02 with this mode. This beneficial effect might be ascribed to improvement in alveolar recruitment, gas diffusion, and ventilation of alveolar units with slow time constants, induced by the decelerating flow pattern that occurs in PCV Nevertheless, like other authors, we did not observe any difference in Pa02 between VCV and PCV.
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Cardiorespiratory Effects of Pressure-controlled Ventilation With and Without Inverse Ratio in the Adult Respiratory Distress Syndrome: Results

Cardiorespiratory Effects of Pressure-controlled Ventilation With and Without Inverse Ratio in the Adult Respiratory Distress Syndrome: ResultsTable 1 summarizes the main characteristics of the population studied. All patients suffered from ARDS for 48 h or less, with a lung injury score ranging from 2.75 to 3.25. Patient 7 had a history of chronic bronchitis. No other patient had experienced previous pulmonary disease. Main results are expressed in Table 2. The only significant difference between VCV and PCV was a lower pPaw in PCV (p<0.05); this difference was found in all patients. The use of PC-IRV induced an increase in PEEPi and mPaw (p<0.05) and a decrease in pPaw (p<0.05) without change in Pplat. No significant differences in Pa02 were observed among the three modes. Notably, we did not observe a significant increase in Pk02 with PC-IRV. In 3 patients, we observed a more than 20 percent difference in Pa02 between VCV and PC-IRV; Pa02 was higher with PC-IRV in 2 and lower in 1.

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Cardiorespiratory Effects of Pressure-controlled Ventilation With and Without Inverse Ratio in the Adult Respiratory Distress Syndrome: Methods

In all patients a flow-directed pulmonary artery catheter (Edwards Swan-Ganz 7F) and a radial artery catheter had been placed previously for hemodynamic monitoring. The CO was calculated with a computer (Edwards 9250 A), as the mean of 5 measurements obtained by injecting 10 ml of dextrose solution. All vascular pressures were measured via quartz transducer (Hewlett-Packard 1290). Arterial blood and mixed-venous blood gas levels were measured (Corning 178 and CO-Oximeter 2500) immediately after CO measurements. The Do and oxygen consumption (Vo) were calculated using standard formulas. Blood lactate concentration was determined using an enzymatic method. Airway pressures were measured with a differential pressure transducer (Validyne DP 45, ±56 cm HaO) connected close to the proximal end of the endotracheal tube. Plateau pressure (Pplat) and PEEPi were measured 2 s after end-inspiratory and end-expiratory occlusions, respectively. The mean airway pressure (mPaw) was measured by electronic filtering of the airway pressure signal. Airway pressure tracing was recorded on a multichannel polygraph (Could ТА 550). Tidal volume (Vt) was measured by means of the inspiratory flow transducer of the ventilator.
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Cardiorespiratory Effects of Pressure-controlled Ventilation With and Without Inverse Ratio in the Adult Respiratory Distress Syndrome: Patients

Cardiorespiratory Effects of Pressure-controlled Ventilation With and Without Inverse Ratio in the Adult Respiratory Distress Syndrome: PatientsConventional 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.
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