Editorial Commentary
Ventilator Modes Old and New
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.
A few weeks ago a technical representative of one of the leading ICU ventilator manufacturers gave an "in-service" presentation to the leaders of the respiratory care department at my institution. It was about neurally adjusted ventilatory assist (NAVA), a ventilation mode soon to be released as an "add-on" feature for that company's ventilators. Using a sensor in a catheter placed in the lower esophagus, NAVA adjusts the level of ventilatory support in response to the electrical activity of the diaphragm.1,2 The presentation concluded with a slide listing the following advantages of NAVA over existing modes of ventilatory support: improved triggering in patients with COPD and others with intrinsic PEEP; reduced requirements for sedation; improved sleep patterns; satisfactory ventilation even in the presence of large air leaks; and decreased ventilator-induced lung injury.
NAVA thus joins proportional assist ventilation (PAV), as discussed by Dr. Nanavaty in the preceding article, as an add-on feature in an increasingly competitive market for high-end critical-care ventilators. PAV has been described as "Power Steering for the Respiratory System." When parallel parking, a lot of effort is required to turn a car's front wheels, and the power steering mechanism provides considerable assistance; when driving down the freeway at the speed limit, very little effort is needed to steer the car, and little assistance is added. By a similar concept, PAV provides more airway pressure to support inspiration when a patient is working hard to breathe, and less when there is less inspiratory effort. Approaching the problem of excessive patient respiratory effort and its relief in a different way, NAVA relies on diaphragmatic electrical activity as a better indicator than pressure or flow changes measured at the airway opening of how much ventilation the patient "wants," and changes the amount of ventilatory augmentation provided according to that signal.
The challenges to mechanical ventilation have increased as patients with increasingly deranged mechanics and gas exchange (who would not have been supportable a generation or two ago) are kept alive for longer and longer periods in our ICUs. These challenges, for manufacturers and critical care clinicians alike, have mainly been in 4 general areas: critical hypoxemia, dangerously high airway pressures, patient distress and ventilator dys-synchrony, and the problem of discontinuing ventilatory support once the patient has improved.
Critical Hypoxemia. Relatively few patients with acute respiratory failure die of unsupportable arterial oxygenation, but severe hypoxemia occurs often enough to fuel a continued search for more effective methods to counteract it. Thirty years ago some clinicians applied higher and higher levels of PEEP in an attempt to improve oxygenation. Arterial PO2 did increase in many patients, but PEEP levels of 25-30 cm H2O or more tended to be accompanied by hypotension (requiring aggressive fluid administration, vasoactive infusions, and more invasive monitoring), as well as by a high incidence of pneumothorax. Extracorporeal membrane oxygenation was proposed, tried, studied, found not to improve survival, and abandoned in the management of adults (at least in most centers). No consistent advantage of any of the "standard" ventilatory modes—volume- vs pressure-targeted ventilation, and controlled vs assist-control vs intermittent mandatory ventilation—has been demonstrated with respect to oxygenation.
High-frequency ventilation, first as high-frequency jet ventilation in the 1980s and more recently as high-frequency oscillation, may increase oxygenation in some patients and has enjoyed a resurgence of interest in the last several years; a randomized controlled trial comparing high-frequency oscillation with conventional ventilatory support in patients with hypoxemic acute respiratory failure is awaited. The roles of prone positioning, and, more recently, of recruitment maneuvers to optimally reverse atelectasis, both of which improve oxygenation in many patients, are still being defined in the face of uncertainty about whether they affect outcomes other than short-term mechanics and gas exchange. Inhaled nitric oxide also increases oxygenation in the majority of patients, but its use in adults with acute respiratory failure is off-label (and non-reimbursed) in the absence of evidence for a survival benefit in studies to date.
Dangerously High Airway Pressures. It took many years for clinicians (and some investigators) to understand that peak inspiratory pressure during volume-targeted ventilation was a poor indicator of the risk for what has become known as ventilator-induced lung injury. We know now that distending pressure at the alveolar level, best assessed at the bedside by end-inspiratory plateau pressure, rather than peak pressure during inspiration, is the most important pressure with respect to injuring the lung. Peak inspiratory pressure measured outside the patient varies with resistance and flow, and may be very high in settings that pose little risk for lung injury. However, the avoidance of high peak airway pressures was a major driver for the introduction of new modes and approaches during the 1970s and 1980s. Although there were other intended advantages, pressure support and pressure-control ventilation gained popularity for their ability to support patients at lower peak airway pressures.
Because of recognition of the importance of maintaining sufficient mean airway pressure in patients with hypoxemic respiratory failure, pressure-control inverse-ratio ventilation (a variant of pressure control) became popular in the late 1980s. Like high-level PEEP (which it tended to apply in the form of intrinsic PEEP), inverse-ratio ventilation faded from widespread use because of frequent hemodynamic compromise and clinical barotrauma. More recently, the avoidance of high peak pressures has been combined with augmentation of mean airway pressure and the ability of patients to breathe spontaneously if desired by means of airway pressure release ventilation (APRV, also called Bi-Level or BiPAP in some settings), which is essentially 2 alternating levels of continuous positive airway pressure. While data from clinical trials in patients with acute lung injury and the acute respiratory distress syndrome demonstrate the importance of minimizing end-inspiratory plateau pressure and distending volume, the relevance of the findings to the management of patients without those conditions (who constitute the majority of patients requiring ventilatory support) is yet to be established.
Patient Distress and Ventilator Dys-Synchrony. Although they have also been studied in other contexts, in the ICU the main advantage touted for both NAVA and PAV is better matching of ventilatory support to the patient's efforts, and thus more effective coordination between patient and machine. Decreased patient discomfort is also one benefit claimed for pressure support. The importance of patient-ventilator synchrony and the processes that can interfere with it have received increasing attention in recent years: they are the topic of next month's special feature by Dean Hess.
Discontinuing Ventilatory Support Once the Patient Has Improved. After decades of studies attempting unsuccessfully to identify the most accurate predictor of a patient's readiness to resume spontaneous breathing after a period of mechanical ventilation, attention during the present decade has shifted from "prediction" to "checking." That is, rather than supporting the patient fully until some threshold measurement indicates that weaning can begin, current guidelines3 recommend that, each day once they have improved, patients should simply be given the opportunity to demonstrate whether they are ready to have the ventilator taken away. Tom Robertson, a colleague of mine for many years, has long maintained that, "When the patient gets well, the patient will get off the ventilator." Current weaning recommendations are more consistent with this concept than with the notion that a particular method of ventilatory support will decrease the length of time that it is needed. Still, there is considerable interest in both PAV and NAVA for their possible roles in hastening liberation from ventilatory support.
Improved triggering, reduced requirements for sedation, better sleep, compensation for air leaks, and decreased ventilator-induced lung injury (as claimed for NAVA in the presentation I recently attended) are all extremely desirable goals. Whether all (or any) of these things will be shown to be the achievable with NAVA remains to be seen. The same is true for the claims (and hopes) made for PAV and other new approaches to ventilatory support.
In the meantime, clinicians would be well advised to master the use of the modes we have had all along. To return to the automobile analogy I used for PAV, managing a critically ill patient on a ventilator is in some respects like driving a car on a busy freeway. Whether the car is a Honda or a BMW, has a stick shift or an automatic transmission, or is equipped with the latest electronic navigation system, are probably less important than whether the driver knows the rules of the road, completely understands the operation of the vehicle, and exercises good common sense behind the wheel. The new modes may prove to offer definite advantages, but they also add cost and complexity to ventilator management, and I for one am not ready to entrust my patients to them until I have a lot more information about their safety and clinically-relevant benefits.
References
- Sinderby C, et al. Neural control of mechanical ventilation in respiratory failure. Nature Medicine. 1999;5:1433-1436.
- Navalesi P, Costa R. New modes of mechanical ventilation: Proportional assist ventilation, neurally adjusted ventilatory assist, and fractal ventilation. Curr Opin Crit Care. 2003;9:51-58.
- MacIntyre NR, et al. Evidence-based guidelines for weaning and discontinuation of ventilatory support. Chest. 2001;120(6 suppl):375s-396s.
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