| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Guest Access | Sign In via User Name/Password
|
|
|
|||||||
Guest Access | Sign In via User Name/Password |
|||||||||
Dr. Carlon is the Director of Medical Affairs, Keane Healthcare Solutions Division, and Professor of Clinical Anesthesiology, Cornell University Medical College. Dr. Combs is President and Chief Executive Officer at FutureTech Strategies, Inc.
Correspondence to: Graziano C. Carlon, MD, FCCP, 425 East 58th St, Apt 39A, New York, NY 10022; e-mail: gccarlon{at}aol.com
Chronic ventilatory failure (CVF) is a devastating condition that recognizes multiple etiologies and is associated with many different pathophysiologic findings. From the point of view of the patient, it is a frightening, relentlessly progressive disease that affects one of the most fundamental needs, the ability to breathe without discomfort.
The list of individual illnesses that may precipitate CVF is considerably long, and it is always worthwhile remembering that the primary organ or system affected by the disease is not necessarily the lung. Degenerative neurologic processes, such as multiple sclerosis or amyotrophic lateral sclerosis, or muscoloskeletal defects, such as severe kyphoscoliosis, may be as likely causes of CVF as COPD, cystic fibrosis, or usual interstitial pneumonitis. Accordingly, though some of the most important clinical endpoints, such as hypercapnia and hypoxemia, are usually shared by all manifestations of the disease, the road leading to them may be quite different. Thoracopulmonary compliance, airway resistance, bronchoalveolar time constant, and static and dynamic pressure-volume curves may exhibit very different, even opposite, characteristics, depending on the conditions of the lungs and chest cavity.
These considerations help explain why CVF shares a major management limitation with acute lung failure. In both cases, the need may arise to empirically apply a mechanical device to maintain life-supporting gas exchange, but the clinician usually lacks objective data to match the characteristics of the instrumentation used with specific, measurable aspects of the disease treated. As a consequence, even though ventilators have become very sophisticated, processor-driven mechanisms, much of the decision-making process is based on trial and error, or on the clinical intuition and predilections of the providers.
During the last 35 years, a few fundamental improvements have been added to mechanical ventilators, including the ability to deliver positive end-expiratory pressure, intermittent mechanical ventilation, and pressure support. These advances were directly aimed at improving two aspects of respiratory function—augmenting lung volume and supporting spontaneous ventilation. In both cases, readily measurable variables—PaO2, PaCO2, and respiratory rate—can be used to estimate the physiologic response to changes in ventilator settings. Newer ventilators, however, also incorporate technology to provide a far wider range of options, including airway pressure-release ventilation, proportional pressure assist, and inverse inspiratory/expiratory ratio, to name just a few of the available features. Despite this additional functionality, many patients with acute lung failure are still managed using therapeutic protocols that have changed very little in > 3 decades.1 2 The main reason is that clinicians are usually unable to identify and monitor lung function variables that will reliably predict that an individual patient will respond positively to a specific type of mechanical support. As a consequence, new techniques are applied without a knowledge-based decision-making process, often when the patient’s condition has severely deteriorated, and when reversal of lung injury has become impossible.
Other modalities of support, such as high-frequency ventilation, high-frequency oscillation, negative pressure ventilation, and independent lung ventilation (whether or not combined with partial liquid ventilation or nitric oxide inhalation), have failed to gain any popularity for the same reason: there are no reliable tests or measurements that allow providers to determine, in advance, which patients would benefit from them. Even more important from the patients’ point of view, there is no study showing a reduction in morbidity or mortality in any cohort of adult patients. Though investigations actively continue, it is hard to deny that "promising results" have now been "imminent" for > 20 years.3 4
The article by Vitacca et al, published in this issue of CHEST (see page 2105), suggests that the inability to predict which mechanical device will be most effective in a specific type of lung disease extends to noninvasive ventilators. The authors themselves state as much in the "Discussion" section. Even though they collected very sophisticated measurements of the function of the thoracopulmonary system, including the respiratory muscles, they were unable to predict from those findings the response to individual mechanical ventilators. Their conclusion is that patients should be treated with multiple devices to determine which is best tolerated. "Best tolerated," however, is not synonymous with "comfortable." By definition, patients with CVF are dyspneic and would clearly welcome breathing comfortably. Only 50% of patients with CVF continue to use their nasal intermittent positive pressure ventilation (NPPV) device long term.5 One imagines that, if true comfort were rendered by these devices, compliance would be near 100%.
The present study further confounds the mystery of patient comfort by failing to find any correlation between patients’ perception of comfort and any measured variable, including physiologic responses and respiratory system mechanics. Physiologically, adaptation to perturbations in respiratory system compliance and airway resistance are characterized by adjustment of both respiratory rate and tidal volume in order to accomplish appropriate minute ventilation at the least possible work of breathing. This is true in both health and disease and presumably operates on a continuous basis. Thus, what patients find comfortable may also change both in time and in relation to their body position, proximity to meals and medications, psychological or emotional state, status of their chronic condition, and myriad other variables not addressed by the NPPV device or its settings.
Comfort, in the present study, while evaluated objectively using the visual acuity scale, is not truly defined (system or ventilator?), as all patients used the same mask. The authors themselves point out that from 20 to >50% of patients’ lack of tolerance of NPPV is based on the mask alone; indeed, 2 of their 31 enrollees dropped out because of the nasal mask. One also wonders whether patients are offered the full range of interface options to any greater extent than they are offered selections among ventilators.
A limitation of the present study is the small number of devices—and ventilatory modalities—that were tested. Newer devices offer more sophisticated control variables such as inspiratory pressure rise time, or exhalation triggering options; some offer advanced modes such as proportional assist ventilation,6 which may well address both comfort and its device attribute correlates. Another limitation is the use of a single interface for all patients and devices. Different interfaces with the same device may also have produced different perceptions of comfort, especially since subjective reports of comfort, using their single interface, were significantly different only between two of the five devices tested.
Ironically, the real issues pointed out by the current study have nothing to do with the NPPV device characteristics per se. Rather, they are as follows:
The current study offers us useful information: patient comfort (and, presumably, long-term compliance and therefore benefit) does vary among devices, and the perception of comfort is not correlated with either their physiologic effects or their impact on respiratory mechanics. The current study also concludes that long-term home NPPV should be tailored to the individual patient. In essence, this is another plea for patient-centered care and participation by the patient in relevant care decisions. If only we practiced in a system where that were even remotely possible ....
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
