Counterpoint: Is Pressure Assist-Control Preferred Over Volume Assist-Control Mode for Lung Protective Ventilation in Patients With ARDS? No

Abstract

On most modern mechanical ventilators, the gas delivery algorithm is generally one of two types: flow/volume targeting (volume assist-control ventilation [VACV]) or pressure targeting with time or flow cycling (pressure assist-control ventilation [PACV] or pressure support ventilation). With flow/volume targeting, the clinician sets an inspiratory flow along with a volume cycling criteria. Airway pressure is, thus, the dependent variable (ie, varying according to lung mechanics and effort). With pressure targeting, the clinician sets an inspiratory pressure target along with either time or flow cycling criteria. Flow and volume are now the dependent variables (ie, varying with lung mechanics and effort). Changes in compliance, resistance, or patient effort will change airway pressure (but not flow) with flow/volume targeting. In contrast, similar changes in compliance, resistance, or effort will cause a change of flow and tidal volume (Vt) (but not airway pressure) with pressure targeting. An important clinical question is whether the different breath delivery algorithms of newer pressure-targeted modes provide advantages over the more traditional flow/volume-targeted breath. This question generally revolves around two key issues. First, which approach best limits Vt and end-inspiratory stretch to prevent ventilator-induced lung injury (VILI)? Second, which approach best synchronizes with patient breathing efforts to minimize sedation needs? Proponents of pressure targeting usually argue that: (1) pressure settings can be provided (either manually or in a feedback fashion) that maintain safe Vts and guarantee an upper pressure limit, and (2) the variable flow feature of pressure targeting will synchronize with patient inspiratory effort better than fixed flow breaths and, thus (at least theoretically) reduce sedation needs. Proponents of flow/volume targeting would counter: (1) flow/volume targeting guarantees a safe Vt at all times, and (2) the putative synchrony advantages of pressure targeting are overblown. Dr Marini1Marini JJ Point: is pressure assist-control preferred over volume assist-control mode for lung protective ventilation in patients with ARD? Yes.Chest. 2011; 140: 286-290Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar has articulated the pressure-targeting argument in the Point segment of the Point/Counterpoint Editorials in this issue of CHEST. It is now time to look at the argument for flow/volume targeting in more detail. As noted, the clinician-set Vt is guaranteed with a flow/volume-targeted breath. So how important is this? The evidence is strong that limiting tidal stretch is as important (if not more important) as limiting maximal stretch in reducing VILI.2Tremblay LN Slutsky AS Ventilator-induced lung injury: from the bench to the bedside.Intensive Care Med. 2006; 32: 24-33Crossref PubMed Scopus (361) Google Scholar, 3Dreyfuss D Saumon G Ventilator-induced lung injury: lessons from experimental studies.Am J Respir Crit Care Med. 1998; 157: 294-323Crossref PubMed Scopus (1864) Google Scholar Tschumperlin et al4Tschumperlin DJ Oswari J Margulies AS Deformation-induced injury of alveolar epithelial cells. Effect of frequency, duration, and amplitude.Am J Respir Crit Care Med. 2000; 162: 357-362Crossref PubMed Scopus (271) Google Scholar observed in alveolar cell tissue cultures that the lethal effects of overstretch could be mitigated by smaller tidal stretch. Mascheroni et al5Mascheroni D Kolobow T Fumagalli R Moretti MP Chen V Buckhold D Acute respiratory failure following pharmacologically induced hyperventilation: an experimental animal study.Intensive Care Med. 1988; 15: 8-14Crossref PubMed Scopus (175) Google Scholar showed in normal sheep that 3 days of excessive tidal breathing caused significant lung injury even though the end-inspiratory transpulmonary pressures were only modestly elevated and well below physiologic maximums. Dreyfuss et al,6Dreyfuss D Soler P Basset G Saumon G High inflation pressure pulmonary edema. Respective effects of high airway pressure, high tidal volume, and positive end-expiratory pressure.Am Rev Respir Dis. 1988; 137: 1159-1164Crossref PubMed Scopus (1316) Google Scholar in a series of animal experiments, convincingly demonstrated that it was end-inspiratory lung volume, not end-inspiratory lung pressure, that produced injury. All of these observations culminated in the design of the ARDS Network ventilator management trial, in which Vt was clinician controlled along with a secondary peak/plateau pressure limitation.7The Acute Respiratory Distress Syndrome Network Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome.N Engl J Med. 2000; 342: 1301-1308Crossref PubMed Scopus (10005) Google Scholar The results of this trial convincingly showed the importance of limiting volumes and pressures to minimize VILI. Moreover, in secondary analysis of data, Hager et al8Hager DN Krishnan JA Hayden DL Brower RG ARDS Clinical Trials Network Tidal volume reduction in patients with acute lung injury when plateau pressures are not high.Am J Respir Crit Care Med. 2005; 172: 1241-1245Crossref PubMed Scopus (429) Google Scholar showed a beneficial effect of Vt reduction from 12 mL/kg to 6 mL/kg ideal body weight (IBW) regardless of plateau pressure before Vt reduction. The analysis of these data coupled with their review of other clinical studies and animal experiments led these investigators to conclude that there was no “safe” plateau pressure below which the benefit of Vt reduction disappears. These findings support a strategy in which control of Vt should take precedence over control of inspiratory pressure. So why not simply use pressure-targeting modes and set the pressure to deliver the desired Vt? Although attractive conceptually, the reality is that Vts will vary in most patients receiving pressure-targeted breaths as respiratory system mechanics change and/or patient effort fluctuates. Indeed, Kallet et al9Kallet RH Campbell AR Dicker RA Katz JA Mackersie RC Work of breathing during lung-protective ventilation in patients with acute lung injury and acute respiratory distress syndrome: a comparison between volume and pressure-regulated breathing modes.Respir Care. 2005; 50: 1623-1631PubMed Google Scholar found that with PACV Vts “markedly” exceeded the Vt target of 6 mL/kg IBW in 40% of patients with acute lung injury/ARDS—twice the rate observed with VACV. Interestingly, the volume-feedback mode, pressure-regulated volume control ventilation, yielded similar results to PACV, with 40% of patients still having low Vt violations. Taken together, these data argue that if Vt control is considered important, the flow/volume targeted breath makes considerable sense. At the end of the day, there are very few studies that directly compare flow/volume-targeted and pressure-targeted strategies for lung-protective ventilation in ARDS, and most have confounding issues. The most extensive study was performed by Meade et al,10Meade MO Cook DJ Guyatt GH Lung Open Ventilation Study Investigators et al.Ventilation strategy using low tidal volumes, recruitment maneuvers, and high positive end-expiratory pressure for acute lung injury and acute respiratory distress syndrome: a randomized controlled trial.JAMA. 2008; 299: 637-645Crossref PubMed Scopus (1066) Google Scholar who compared these approaches in a large international multicenter randomized controlled trial. In both arms of this trial, a Vt target of 6 mL/kg IBW was used, and outcomes were found to be similar (28-day mortality in VACV vs PACV was 32.3% vs 28.4%, respectively, [P = .2], and barotraumas were 9.1% vs 11.2%, [P = .33]). Unfortunately, different positive end-expiratory pressure/Fio2 strategies were used in the two arms, confounding the comparison. Advocates of flow/volume targeting point to another potential advantage in providing lung protection: There is considerable evidence that clinicians have not reliably applied low Vt ventilation.11Young MP Manning HL Wilson DL et al.Ventilation of patients with acute lung injury and acute respiratory distress syndrome: has new evidence changed clinical practice?.Crit Care Med. 2004; 32: 1260-1265Crossref PubMed Scopus (176) Google Scholar Indeed, Rubenfeld et al12Rubenfeld GD Cooper C Carter G Thompson BT Hudson LD Barriers to providing lung-protective ventilation to patients with acute lung injury.Crit Care Med. 2004; 32: 1289-1293Crossref PubMed Scopus (228) Google Scholar identified “unwillingness to relinquish ventilator control” as a primary barrier to initiating lung-protective ventilation in a survey of ICU respiratory therapists and nurses. When a written protocol focused on Vt settings was used, however, compliance with low Vt strategies was found by these investigators to be improved.13Umoh NJ Fan E Mendez-Tellez PA et al.Patient and intensive care unit organizational factors associated with low tidal volume ventilation in acute lung injury.Crit Care Med. 2008; 36: 1463-1468Crossref PubMed Scopus (88) Google Scholar This is important because successful implementation of new treatment algorithms requires simplicity and familiarity. Explicitly setting Vt, rather than setting airway pressure and repeatedly measuring Vt, is certainly simpler and, thus, seems intuitively advantageous in achieving widespread adoption. The other point that proponents of flow/volume targeting argue is that the pressure limiting and synchrony features of pressure targeting are overblown and unnecessary. With flow/volume targeting, pressure limits/alarms can always be set to assure that an end-inspiratory pressure limit is not exceeded. Regarding patient-ventilator synchrony, there is literature showing that the variable flow of pressure targeting might be easier to synchronize with patient efforts than fixed flow breaths.14MacIntyre NR McConnell R Cheng KC Sane A Patient-ventilator flow dyssynchrony: flow-limited versus pressure-limited breaths.Crit Care Med. 1997; 25: 1671-1677Crossref PubMed Scopus (89) Google Scholar Results in clinical trials, however, are not always supportive of this notion. Chiumello et al15Chiumello D Pelosi P Calvi E Bigatello LM Gattinoni L Different modes of assisted ventilation in patients with acute respiratory failure.Eur Respir J. 2002; 20: 925-933Crossref PubMed Scopus (53) Google Scholar showed in patients with acute respiratory failure that when the peak inspiratory flow of VACV was adjusted properly to support a given Vt, there were no differences in work of breathing and inspiratory pressure against a closed shutter after 100 milliseconds compared with PACV. Kallet et al9Kallet RH Campbell AR Dicker RA Katz JA Mackersie RC Work of breathing during lung-protective ventilation in patients with acute lung injury and acute respiratory distress syndrome: a comparison between volume and pressure-regulated breathing modes.Respir Care. 2005; 50: 1623-1631PubMed Google Scholar also showed in a group of patients with acute lung injury receiving small Vts that PACV had similar patient work reductions to those obtained with carefully titrated VACV. Taken together, these studies suggest that if pertinent mechanical parameters (eg, peak flow, Vt) are adjusted properly by skilled clinicians, flow/volume targeting can produce patient-ventilator synchrony similar to pressure targeting in most (if not all) patients. There are other potential advantages to flow/volume-targeted modes. For example, under circumstances in which patient drive is reduced/absent (eg, sleep, sedatives, neuromuscular blockers, and so forth) and/or respiratory system mechanics worsen, a fixed Vt assures continuation of a desired level of support. Indeed, flow/volume targeting would seem to be an ideal choice to assure adequate ventilation in patients with potential for respiratory mechanical variability, breathing pattern unreliability, or both to breathe spontaneously at an adequate level. Recently, another use for a fixed flow breath was introduced. Ranieri et al16Ranieri VM Zhang H Mascia L et al.Pressure-time curve predicts minimally injurious ventilatory strategy in an isolated rat lung model.Anesthesiology. 2000; 93: 1320-1328Crossref PubMed Scopus (150) Google Scholar described the concept of a “stress index” to assess respiratory system mechanics and thereby help guide the positive end-expiratory pressure and Vt setting. This stress index requires a clinician-controlled constant flow Vt. If there is no appreciable recruitment or overdistention occurring during this type of breath, the airway pressure profile should be a smooth, straight, diagonal line. In contrast, if recruitment occurs at the beginning of the breath, the airway pressure curve is convex downward; if overdistention occurs at the end of the breath, the airway pressure curve is concave upward. Whether this concept will translate into more lung-protective ventilator settings remains to be demonstrated.