Special Feature: What is the Standard of Care for Mechanical Ventilation?
By David J. Pierson MD
- A 72-year-old woman with severe COPD and 3 previous hospitalizations for exacerbations in the last year presents with a 2-day history of increasing dyspnea and sputum production. She is alert but in respiratory distress, with a respiratory rate of 36/min and an oxyhemoglobin saturation of 76% by pulse oximetry while breathing nasal oxygen at 2 L/min. Arterial blood studies show pH 7.22, PCO2 76 mm Hg, and PO2 48 mm Hg. A chest X-ray shows only severe hyperinflation. She is admitted to the ICU, where she is sedated, intubated, and placed on volume assist-control ventilation. Five days later the patient’s gas exchange and pulmonary mechanics have improved, but she develops fever and leukocytosis and is noted to have a new left lower lobe infiltrate. Methacillin-resistant Staphylococcus aureus is recovered on bronchoalveolar lavage.
- An obese 30-year-old man (BMI 30) develops hypoxemia and diffuse pulmonary infiltrates 36 hours following a motorcycle accident in which he sustained pelvic and femur fractures and a ruptured spleen. There no clinical evidence of fluid overload, and his PaO2/FIO2 ratio is 176 mm Hg. He is switched from volume-control to pressure-regulated volume control ventilation, with target tidal volume 950 mL (9 mL/kg), FIO2 0.6, and PEEP 12 cm H2O. End-inspiratory pressure (pressure control plus PEEP) to maintain this tidal volume averages 36 cm H2O. The patient remains on the ventilator and heavily sedated 2 weeks later, with PaO2/FIO2 ratio 250 mm Hg and requirement for 45% to 55% oxygen. He does not move spontaneously in response to painful stimuli, his serum transaminases are rising, and his serum creatinine, initially normal, is now 2.4 mEq/L.
- A 66 year-old man has been intubated and ventilated since undergoing mitral valve replacement and triple coronary artery bypass grafting 5 days ago. He is responsive to verbal stimuli, is hemodynamically stable, has no evidence of complicating infection or fluid overload, and has acceptable oxygenation and ventilation on 40% oxygen with a minute ventilation of 10 L/min. However, for the last 3 mornings, when assessed for possible ventilator weaning, his rapid shallow breathing index (f/VT) during momentary discontinuation of ventilatory support has been 120-130 and his maximal inspiratory pressure has been 18-22 cm H2O. A chest X-ray reveals a moderate right pleural effusion and a raised left hemidiaphragm.
Are these hypothetical patients being managed according to current standards of care with respect to mechanical ventilation? My answer is no, and in this short essay I will explain why.
What Are Standards of Care?
A standard is defined by the Merriam Webster On-Line Dictionary as "something established by authority, custom, or general consent as a model or example; something set up and established by authority as a rule for the measure of quantity, weight, extent, value, or quality." In clinical medicine in 2004, standards are defined by the precepts of evidence-based medicine (EBM), which refers to the conscientious, explicit, and judicious use of current best evidence.1
The gold standard in the era of EBM is the randomized controlled trial (RCT). A properly designed and carried out RCT, in patients similar to those the practitioner manages and using end points relevant to both practitioner and patient, is more likely to be free from bias and to produce results that will stand up over time than other types of investigation such as retrospective analyses, case-control studies, and unsystematic clinical observations. Meta-analysis, which combines the results of several clinical trials according to rigidly defined criteria, when properly and rigorously carried out and conservatively interpreted, can further increase the clinician’s confidence in the reported results.
Numerous schemes and rating scales have been propounded to assess the level of evidence presented by a particular study or available in managing a particular disorder. One that is broadly consistent with most of those published by EBM’s leaders places results from multiple RCTs (with consistent results, in relevant patient populations and in settings similar to those of the clinician) at the top, followed by single RCTs. After this in the hierarchy of evidence come uncontrolled prospective studies, case-control studies and other retrospective analyses, and unsystematic clinical observations. Although they may have prestigious origins and carry great weight politically, expert panel recommendations and clinical practice guidelines fall at the bottom of the evidence ladder if they are not rigorously evidence-based according to the criteria described. Clearly, practice according to one’s own preference and experience without reference to outside input of any kind would fall at the very bottom in this scheme.
For more than 3 decades, the evidence mustered to support new ventilator modes and approaches had often consisted of physiological or surrogate end points, such as effects of the new intervention on peak airway pressure or venous admixture. While logic says that improved function should translate to better outcomes, the fact is that most studies of mechanical ventilation since the 1970s have failed to produce data substantiating this assumption. Improved oxygenation as a result of some ventilator manipulation or other short-term intervention ought to improve the overall outlook for the patient, but in most cases outcomes such as complications, length of stay, survival, and other patient-centered results have either not been included in the studies or not been found. In fact, numerous interventions subjected to RCTs, that have been documented to improve short-term physiologic variables such as oxygenation, have failed to demonstrate differences from standard therapy with respect to such patient-centered outcomes.
Are There Standards of Care for Mechanical Ventilation?
Restating the question, are there aspects of ventilator management for which there is compelling evidence supporting the use of one approach over others? I think there definitely are, in the following areas:
- Noninvasive ventilation in exacerbations of severe chronic obstructive pulmonary disease (COPD);
- Lung-protective ventilation in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS);
- Weaning from ventilatory support;
- Managing and monitoring the patient-ventilator system;
- General ICU care of the ventilated patient.
Noninvasive Ventilation in COPD
Numerous RCTs have clearly and consistently demonstrated the clinical benefits of noninvasive ventilation in acute respiratory failure due to exacerbations of severe COPD.2 In such patients, noninvasive ventilation improves gas exchange, decreases tachypnea and respiratory distress, reduces the need for endotracheal intubation (by about 80%), and decreases both ICU length-of-stay and overall mortality. Because ventilator-associated pneumonia and several other complications are really endotracheal tube-associated complications, noninvasive ventilation also decreases the incidence of these things.
It is less clear whether noninvasive ventilation should be used in acute respiratory failure in circumstances other than COPD exacerbation. The number of RCTs in acute hypoxemic respiratory failure and cardiogenic pulmonary edema is increasing, but at the time of this writing the data supporting the routine use of this modality in these settings are unconvincing. Patients with COPD exacerbations are a relatively homogeneous group, whereas acute hypoxemic respiratory failure occurs in many clinical circumstances (such as severe bacterial pneumonia and trauma) in which noninvasive ventilation may be less appropriate for non-respiratory reasons. However, studies have shown that patients with acute respiratory failure complicating severe immunosuppression or solid organ transplants may be able to avoid intubation and have at least equivalent outcomes with the use of noninvasive ventilation.
Lung-Protective Ventilation in Acute Lung Injury and ARDS
Although there has been considerable controversy about the specifics of the management, it seems clear that a lung-protective strategy that includes lower tidal volumes and trans-alveolar distending pressures than have been employed in the past improves survival in ALI/ARDS.3 Lung-protective ventilation (LPV) has thus become a standard for mechanical ventilation in these conditions. The most widely used approach to LPV is that used by the ARDS Network in its study reported in 2000,4 which includes tidal volumes of 6 mL per kg of predicted (ideal) body weight and seeks to maintain end-inspiratory static pressure (Pplat) below 30 cm H2O.5 Whether volume- or pressure-targeted ventilation should be used has not been settled, so that at present ventilator mode is a matter of clinician preference. However, subsequent analysis of the ARDSNet data has shown that Pplat is a continuous variable with respect to outcome: the lower the Pplat (-22 vs 26 vs 30 cm H2O, for example) the lower the mortality. Thus, from available evidence, there appears to be no threshold Pplat that is "OK," and that would justify using tidal volumes greater than 6 mL/kg in patients with ALI/ARDS.
Weaning from Ventilatory Support
An exhaustive review of all published studies on weaning6 showed that no "weaning parameter" or other physiologic measurement could predict readiness for liberation from ventilatory support with acceptable accuracy. That is, if weaning attempts are delayed until some predetermined physiologic criteria are met, many patients will remain on ventilatory support who no longer need it. Further, well-designed RCTs have failed to demonstrate that any particular weaning strategy is best in terms of duration of mechanical ventilation or the need for reintubation. However, although pressure support and daily trials of spontaneous breathing appear to be equivalent, gradually reducing mechanical support using synchronized intermittent mandatory ventilation actually delays weaning, according to the best available evidence.6
In light of these findings, the evidence-based weaning recommendations of the American College of Chest Physicians, the American Association for Respiratory Care, and the Society of Critical Care Medicine7 are that all ventilated patients who meet minimal criteria be given a daily spontaneous breathing trial. Patients who look good clinically and have acceptable arterial blood gases after a spontaneous breathing trial of 30-120 minutes should be considered for prompt extubation, assuming that no additional reason for an endotracheal tube exists.
Respiratory therapist- or nurse-driven protocols for ventilator weaning have been shown to be safe, and in all studies published to date have been associated with equivalent or shorter weaning times, as compared with physician-directed, "usual care" weaning.8 All hospitals should thus develop and implement such protocols, which reduce practice variability and put moment-to-moment assessment and management in the hands of those who spend the most time at the bedside.
Managing and Monitoring the Patient-Ventilator System
The hour-to-hour management of intubated, ventilated patients has standards, a number of them evidence-based.9 It is clear that the likelihood of a patient developing ventilator-associated pneumonia is related to the number of times the ventilator circuit is broken, for suctioning, measurements, or routine circuit changes. Several large observational studies, and 4 RCTs, have examined the effect of the frequency of ventilator circuit changes on the incidence of ventilator-associated pneumonia. No study has shown that more frequent circuit changes are better, and a recent meta-analysis of the RCTs statistically favored less frequent changes. This practice is both cheaper and safer for the patient.
Adequate warming and humidification of inspired gas is important to prevent inspissation of secretions, bleeding, and other airway complications. Six RCTs have compared heated humidifiers to heat-and-moisture exchangers with respect to the incidence of ventilator-associated pneumonia. Only one of these studies has been individually unequivocally positive, but meta-analysis of all 6 shows that passive humidification with a heat-and-moisture exchanger is clearly superior.9
General ICU Care of the Ventilated Patient
The mortality of ARDS had already decreased substantially, at least in some centers, before the introduction of LPV. Most likely, this improved survival was due to improvements in the general care of critically ill patients. Prompt initiation of antimicrobial therapy for ICU-acquired infections and better nutritional support were likely factors. Other advances in management, buttressed with data from RCTs, include routine prophylaxis against deep venous thrombosis, prophylaxis against upper gastrointestinal bleeding and other enteric complications, avoidance of hyperglycemia through judicious insulin administration, and constant elevation of the head of the bed to prevent reflux and aspiration and thus prevent ventilator-associated pneumonia. Two interventions that have been shown to reduce ventilator days and overall ICU time are daily interruption of sedation ("sedation vacations") and daily spontaneous breathing trials, as mentioned previously.
Several ventilator-related interventions remain "on the bubble" with respect to evidence-supported outcome benefits, and thus have not yet become standards. These include several measures with undoubted physiologic rationales that improve oxygenation in many patients with ALI/ARDS, but have not been shown to have a measurable effect on survival. In this category are recruitment maneuvers to "open the lung," prone positioning, high-frequency oscillatory ventilation, and inhaled nitric oxide. Noninvasive ventilation as a bridge to weaning is also currently on this list. As with several of the other interventions, refinements in patient selection criteria may some day move it into the "standard" category. Finally, bedside percutaneous tracheotomy using progressive dilation and bronchoscopic guidance may prove to have patient-relevant advantages over its surgical counterpart, but this remains to be demonstrated.
If There Are Practice Standards for Mechanical Ventilation, Are We Using Them?
Not much information is available about how clinicians across the country and around the world actually manage patients on ventilators. What evidence is at hand suggests that many if not most patients are not receiving the potential benefits of management according to the current standards of care. At Harborview Medical Center in Seattle, one of the ARDS Network study sites, very few patients were actually receiving LPV according to the Network protocol either 4 months before or 4 months after publication of the study in the New England Journal of Medicine.10 In terms of tidal volume, it used to be that all patients (including those with ALI/ARDS) were ventilated with supranormal volumes (eg, 10-12 mL/kg), because of studies 40 years ago on the prevention of atelectasis in otherwise healthy people undergoing general anesthesia; now, it seems that most patients (including those with normal lungs) are being ventilated with tidal volumes in the 8 mL/kg range, which may be neither large enough to prevent atelectasis nor small enough to prevent ventilator-induced lung injury.
In a study of respiratory therapy documentation practice in managing patients on ventilators, a recent study by Akhtar et al11 found that 17 adult hospitals in one county had widely divergent practices. Using the American Association for Respiratory Care’s Clinical Practice Guideline for patient-ventilator system checks as a standard, the investigators found that only 15 of the 52 items in the guideline appeared on every hospital’s blank charting sheets, and that important measures such as Pplat and total minute ventilation did not appear anywhere in ALI/ARDS patients’ charts in 7 of the 17 hospitals. Even when respiratory therapy department managers stated that specific items were routinely measured in their facilities, review of actual patient charts showed this not to be the case in an alarming number of instances.
In mid-2004, there definitely are standards for mechanical ventilation. Those with the strongest evidence base include noninvasive ventilation for COPD, LPV for ALI/ARDS, daily spontaneous breathing trials for weaning, less frequent ventilator circuit changes, the use of heat-and-moisture exchangers rather than heated humidifiers, and several aspects of general ICU care for patients on ventilators. Thus, although there is room for discussion on some of the details (such as what kind of noninvasive ventilator interface to use, or which ventilator mode is best for ARDS), the onus today is on any clinician who elects not to manage patients in accordance with these standards to justify this practice.
In the hypothetical patient examples with which I began this essay, the standards I have discussed are clearly not being followed. The elderly woman with a COPD exacerbation, although presenting no contraindications to noninvasive ventilation, was intubated without giving the latter a try, and now 5 days later she has developed ventilator-associated pneumonia. The young man with ARDS is not receiving lung-protective ventilation, which could perhaps have increased his likelihood of survival by at least 20%. He has been ventilated from the beginning with tidal volumes substantially larger than those in the ARDSNet low-VT group—9 mL/kg actual body weight in a patient with a BMI of 30 is in excess of 12 mL/kg predicted body weight as used in that trial—and as a result his alveolar distending pressures have been excessive. He is now developing signs of multiple organ failure, which along with sepsis accounts for most deaths in patients with ARDS. And the patient who remains on the ventilator nearly a week following cardiac surgery may well no longer need it; he should have had daily spontaneous breathing trials from the beginning, in order to liberate him from the ventilator at the earliest possible moment and avoid the complications associated with intubation and prolonged ventilatory support.
Ventilating patients according to current standards is not just a matter of preference: it can save lives, reduce complications, shorten ICU stays, and save health care costs. For the clinical settings of the 3 patient examples I have used, the situation is pretty clear. The evidence is not yet convincing that lung recruitment maneuvers, prone positioning, high-frequency oscillation, and other innovations in ventilator management offer true advances in terms of patient-relevant outcomes. However, for some or all of these, or others, sufficient evidence may be brought to bear for them to become standards of practice in the future.
1. Hess DR. Respir Care. 2004;49(7). in press.
2. International Consensus Conferences in Intensive Care Medicine: noninvasive positive pressure ventilation in acute Respiratory failure. Am J Respir Crit Care Med. 2001;163(1):283-291.
3. Meade MO, Herridge MS. Respir Care. 2001;46(12):1368-1376.
4. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342:1301-1308.
5. Kallet RH, et al. Respir Care. 2001;46(10):1024-1037.
6. Meade MO, et al. Respir Care. 2001;46(12):1408-1415.
7. ACCP-AARC-SCCM Evidence-Based Guidelines for Weaning and Discontinuation of Ventilatory Support. Chest. 2001;120(6 suppl):375s-484s.
8. Ely EW, et al. Chest. 2001;120(6 Suppl):454S-463S.
9. Hess DR, et al. Respir Care. 2003;48(9):869-879.
10. Rubenfeld GD, et al. Am J Respir Crit Care Med. 2001;163A
11. Akhtar SR, et al. Chest. 2003;124(6):2275-2282.
David J. Pierson, MD, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, is Editor of Critical Care Alert.