Which Mode Is More Comfortable: Pressure Support or Volume-control Continuous Mandatory Ventilation?
Abstract & Commentary
By David J. Pierson, MD, Editor, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle, is Editor for Critical Care Alert.
Synopsis: In a small group of fully-alert, clinically stable patients with modest ventilation requirements, who were recovering from acute respiratory failure, most of them preferred pressure-support ventilation targeted to a tidal volume of 8 mL/kg over volume-control continuous mandatory ventilation at the same tidal volume.
Source: Betensley AD, et al. Respir Care. 2008;53(7):897-902.
In this study from Henry Ford Hospital in Detroit, investigators sought to determine whether pressure support ventilation (PSV) targeted at the same delivered tidal volume was more comfortable for patients than volume-control continuous mandatory ventilation (VC-CMV). In their 44-bed medical ICU and over a 13-month period, they screened 118 patients who were recovering from acute respiratory failure, of whom 19 met their criteria of being awake and alert, off all sedation for 12 hours or more, receiving a stable inspired oxygen fraction of 0.40 or less for at least 24 hours, with oxyhemoglobin saturation of 90% or more, and hemodynamically stable without requirement for pressors. Five of the 19 qualifying patients declined to participate, and 14 patients were studied.
All patients were initially ventilated using pressure-regulated volume control (PRVC). They were asked to quantitate their baseline level of comfort using a 100-mm visual analog scale extending between 0 ("Not at all comfortable") and 100 ("Very comfortable"). They were then ventilated for 30 minutes using either PSV (set to match end-inspiratory plateau pressure on PRVC and producing a tidal volume of 8 mL/kg) or VC-CMV (at the same tidal volume, with inspiratory:expiratory ratio 1:3 and square-wave inspiratory flow), in random sequence with a 30-minute interval period on PRVC. Patients rated their comfort levels on the visual analog scale at the conclusion of each 30-minute experimental period.
The 14 patients (mean age 65, 79% female) were recovering from congestive heart failure (5 patients), pneumonia (3), COPD or asthma (3), and other medical illnesses, and had been ventilated for 5.4 ± 5.0 days prior to being studied. Eleven patients indicated that they found PSV more comfortable than VC-CMV. Mean (SD) analog scale values for all the patients were 62.2 (18.0) on the baseline PRVC, 69.6 (17.8) on VC-CMV, and 83.2 (11.0) on PSV (difference between PSV and VC-CMV, p = 0.02). The 3 patients who did not find PSV more comfortable than VC-CMV were the ones who required the highest pressure support levels in the study (21, 26, and 26 cm H2O) in order to achieve the target tidal volume of 8 mL/kg.
Commentary
Previous studies have shown PSV to be more comfortable to patients than VC-CMV during noninvasive ventilation. In the present study, Betensley et al have shown for the first time, in a highly selected group of medical patients, that PSV was perceived by most patients to be more comfortable than VC-CMV during invasive mechanical ventilation.
These findings were obtained in a group of patients with little resemblance to those in whom the issue of comfort (ie, tolerance, and avoidance of dyssynchrony) comes up most often, at least in my practice. Even in the authors' ICU, these patients (less than 10 patients screened each month in a 44-bed ICU, and only 12% of screened patients actually studied) represented a small minority of those receiving mechanical ventilation in the unit. Which mode to use is often an issue early in the ICU course, when patients are most seriously ill. In such circumstances, comfort, or the lack thereof, is usually assessed by the bedside clinician rather than by the patient, using signs such as tachycardia, restlessness, tachypnea, breath-stacking, and the appearance of excessive effort used in triggering breaths or exhaling. Studying patients who were convalescing from critical illness and clinically stable was made necessary by the method used — assessing the patients' perceptions using a visual analog scale—and this needs to be kept in mind as the results of this study are interpreted.
Ever since the initial comparisons of intermittent mandatory ventilation with VC-CMV in the 1970s, studying different ventilator modes has been a tricky business. There are so many potential variables to compare and control — airway pressures (peak, plateau, mean); compliance and other measures of respiratory system mechanics; minute ventilation; arterial PO2 or PCO2 — that it has been difficult to impossible to satisfy critics that the settings were not chosen deliberately (or inadvertently) to favor a particular mode. Blinding is rarely possible, and seldom have such studies been performed by investigators who were not proponents of one of the modes or strategies being examined. It is therefore not surprising that so few good studies have been published in this field.
Betensley and colleagues are to be commended for trying very hard to compare PSV and VC-CMV under conditions that were as equivalent as possible. They have demonstrated that, in the patients selected and under the experimental conditions, PSV was generally the more comfortable mode. Could PSV have been set up differently, making the patients less comfortable breathing on this mode? Probably. Could VC-CMV have been set up in such a way that the patients liked it better than they did — for example, with decelerating inspiratory flow or a different I:E ratio? Possibly. For now, though, this is the best study available to clinicians who must contend regularly with the issue it addresses.
In this study from Henry Ford Hospital in Detroit, investigators sought to determine whether pressure support ventilation (PSV) targeted at the same delivered tidal volume was more comfortable for patients than volume-control continuous mandatory ventilation (VC-CMV).You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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