Special Feature
PAV+: A New Mode Coming To A Ventilator Near You
By Uday Nanavaty, MD, Assistant Director, AICU, St. Agnes Hospital, Baltimore, is Associate Editor for Critical Care Alert.
Dr. Nanavaty reports no financial relationship to this field of study.
Mechanical ventilation has been evolving ever since the introduction and large scale use of iron lungs. Advances in the fields of medicine and technology have resulted in nearly an alphabet soup of modes of mechanical ventilation. We have come a long way from CMV (controlled mechanical ventilation) to AC (assist control) to IMV (intermittent mandatory ventilation) to SIMV (synchronized intermittent mandatory ventilation) modes. We have run through PCV pressure control ventilation), PCIRV (pressure control with inverse-ratio ventilation), and pressure support, and we continue to struggle with HFJV (high-frequency jet ventilation), HFOV (high-frequency oscillatory ventilation), and APRV (airway pressure release ventilation) or "Bi-Level." PRVC (pressure-regulated volume control) and other so-called dual-control modes have come (and, for my practice, may have even gone).
Now we are being introduced to PAV+, a version of proportional-assist ventilation (PAV). As this "software upgrade" comes through the "market," at some point, you will hear about this new mode of spontaneous breathing that will become available on the ventilator (if your institution uses the ventilator manufactured by this particular device maker). Here I briefly review the theory behind PAV, review the data (or lack thereof) regarding its effectiveness, and suggest some of the uses of this mode (before the abuses become prevalent).
The Need for Synchrony: Two Controllers of One System
Mechanical ventilation, invasive or non-invasive, is initiated when a patient's own respiratory system is unable to support oxygenation or ventilation adequately. Often as acute respiratory failure develops, a patient's effort to breathe becomes out of proportion to the perceived need, resulting in the sensation of dyspnea. Mechanical ventilators traditionally perceive this patient effort based on a change in flow or pressure in the respiratory system (triggering mechanism), and initiate a ventilator breath that terminates typically when a predetermined volume or pressure or flow parameter is reached (cycling mechanism). Volume-targeted ventilation, as with CMV or AC, provides a predetermined tidal volume and then turns off the flow of gas toward the patient in anticipation of exhalation. Thus the ventilator partially controls the respiratory system and the patient is in partial control. Whenever the patient is tachypneic (that is, wishes to breathe more rapidly than the set respiratory rate), in AC the ventilator delivers a predetermined tidal volume at that frequency.
Often, this dual control of the respiratory system is not synchronous and we have patient-ventilator dys-synchrony, for which patients often require sedatives. If patients are spontaneously breathing, pressure support is often applied to "help patients breathe easier." Unfortunately, this pressure support is a fixed pressure support and if the patient's condition changes, especially on low levels of pressure support, the patient may be doing substantial work of breathing, raising the possibility of muscle fatigue and inadequate ventilation. To improve this patient-ventilator dys-synchrony and to provide ventilator support in proportion to patient effort, PAV was described by Magdy Younes in 1992.1,2 Now, 15 years later, Puritan Bennett Ventilators of the 840 series offer a "PAV+" option that allows physicians to use PAV by adjusting the support setting from 5% to 95% of the patient's own inspiratory effort.
The Theory Behind PAV+
For any patient to breathe while intubated and connected to a ventilator, without any support from the ventilator, the respiratory muscles have to expand the chest and lungs and overcome the tendency of the total respiratory system to collapse to its resting state (functional residual capacity). This tendency of the lungs and thorax to collapse is reflected in the static compliance that we measure during inspiration and with the end-inspiratory pause maneuver. The concept of compliance is familiar to clinicians, and elastance is the inverse of compliance. The respiratory muscles also have to overcome the resistive properties of natural and artificial airways.
The resistive properties of an artificial airway can be reasonably predicted based on the internal diameter of the airway. Based on the patient's body weight and the internal diameter of the airway, the PAV+ software further estimates the resistive properties. When patients are placed on PAV+ mode, for the first 4 breaths, the elastance and resistance of the system are assessed to determine the level of pressure support necessary to meet patient demands. Then, based on these calculations and the clinician-set level of assistance, the ventilator performs that portion of the patient's work of breathing. The patient remains in control of the breathing, and as soon as patient effort stops, the ventilator turns off the positive pressure.
Once the patient is on PAV+ mode, the ventilator continues to measure the elastance and resistance every 4 to 10 breaths, randomly. These new values of elastance and resistance are then reviewed by the software and averaged over previous values. Extreme values are rechecked to avoid excessive support or too little support. If the compliance changes, the machine will detect the change and make appropriate adjustments in the degree of ventilatory augmentation. Since there is no set minute ventilation or rate or tidal volume, the patient must have spontaneous breathing and an intact neural drive, and not require large amounts of sedatives or narcotics.
Does the Theory Work in Practice?
Data on the use of PAV+ in clinical practice are limited. For example, there is no large randomized trial to support its routine use in acute lung injury or the acute respiratory distress syndrome.
In 1996, Ranieri et al3 used artificially increased dead space to induce acute hypercapnia and studied its effect on ventilation and patient comfort with 80% and 40% PAV vs 20 and 10 cm H2O of pressure support (PSV). The minute ventilation increased in PSV mode upon hypercapnia by means of an increased frequency, whereas in PAV mode it primarily changed the tidal volume, leaving frequency unchanged. In this study, patient comfort was greater in PAV mode as compared to PSV mode. In 2000, Ranieri et al4 reported that if during weaning, the load on the chest and abdominal wall was increased by applying binders to the chest and abdominal wall, patients in PSV mode developed tachypnea and reduced tidal volumes. The degree of drop in tidal volume and increase in respiratory rate were again reduced in PAV mode. Although similar clinical situations may exist, both these studies were performed in patients who were otherwise stable and weaning from mechanical ventilation.
In a trial of 8 patients with COPD and difficulty to wean, Rossi et al5 reported in 1999 that PAV mode, in addition to continuous positive airway pressure adjusted to the patients' intrinsic positive end-expiratory pressure, was successful in reducing the work of breathing. Weaning success was not part of their study to suggest that PAV would be a better weaning mode.
Non-invasive application of PAV has been tried, and small studies have shown it to be as good as PSV in improving tidal volumes and reducing respiratory frequency in patients with chronic respiratory failure. In a small study by Hill et al6, nasal application of PAV was better tolerated than PSV. Small studies in patients with COPD, cystic fibrosis and restrictive chest wall disease suggest that non-invasive PAV is tolerated by such patients.7 However, the recently-introduced PAV+ is approved to be used only with an endotracheal or tracheostomy tube, and not with non-invasive mask ventilation. The principal reason for not using PAV+ in non-invasive ventilation is the issue of air leak: currently, there is no way to compensate for such leaks. This may result in frequent alarming and can result in under- or over-ventilation; hence PAV+ is not FDA-approved for non-invasive application. At present, the suggestion is also to avoid using PAV+ in COPD patients. I believe this comes from the issue of intrinsic PEEP. Since patients with COPD may have variable amounts of dynamic hyperinflation and intrinsic PEEP, and since the measurement of patient effort occurs at the level of the endotracheal tube, which does not reflect intrinsic PEEP, it would be difficult to know whether patients were receiving appropriate augmentation and support.
Summary
At present, it seems to me personally that PAV+ is a mode that is in search of the patient population where it will be ideal or best mode. Honestly speaking, spontaneous breathing modes are so rare in my practice except for weaning and a rare patient with severe COPD who is completely awake and in absolute dys-synchrony in spite of adjustment of set PEEP and flow patterns, that even though it may be the best spontaneous breathing mode so far, I believe it will have limited utility in everyday practice. If non-invasive application is allowed (which in many instances consists of all spontaneous breathing), I believe it will help many more patients avoid getting to the ventilator, as PAV is based on sound physiological principles.
References
- Younes M. Proportional assist ventilation, a new approach to ventilatory support. Theory. Am Rev Respir Dis.1992;145(1):114-120.
- Younes M, et al. Proportional assist ventilation. Results of an initial clinical trial. Am Rev Respir Dis.1992;145(1):121-129.
- Ranieri VM, et al. Patient-ventilator interaction during acute hypercapnia: pressure-support vs. proportional-assist ventilation. J Appl Physiol. 1996;81(1):426-436.
- Grasso S, et al. Compensation for increase in respiratory workload during mechanical ventilation. Pressure-support versus proportional-assist ventilation. Am J Respir Crit Care Med. 2000;161(3 Pt 1):819-826.
- Appendini L, et al. Physiologic response of ventilator-dependent patients with chronic obstructive pulmonary disease to proportional assist ventilation and continuous positive airway pressure. Am J Respir Crit Care Med. 1999;159(5 Pt 1):1510-1517.
- Gay PC, et al. Am J Respir Crit Care Med. 2001;164(9):1606-1611.
- Ambrosino N, Rossi A. Proportional assist ventilation (PAV): a significant advance or a futile struggle between logic and practice? Thorax. 2002;57(3):272-276.
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