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* From the Division of Pulmonary and Critical Care Medicine, University of Miami School of Medicine at Mount Sinai Medical Center, Miami Beach, FL.
Correspondence to: Bruce P. Krieger, MD, FCCP, Mount Sinai Medical Center, 4300 Alton Rd, Miami Beach, FL 33140; e-mail: BKriegermd{at}aol.com
Key Words: ARDS • COPD • mechanical ventilation • noninvasive ventilation • sedation • weaning
Lung Protective Ventilation
1. The ARDS Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the ARDS. N Engl J Med 2000; 342:1301–1308
Data from animal studies in the 1970s and 1980s showed that ventilation with high tidal volume (VT) levels was injurious to the lung. Approximately a decade ago, the concept of pressure-targeted, low VT ventilation for patients with ARDS was born. A consensus conference report1
on mechanical ventilation published in 1994 recommended this approach even though "permissive" hypercapnia often resulted from it.2
However, it was not until the ARDS Network study was published that this recommendation was buttressed by evidence-based medicine. In this study, 361 patients were randomized to receive either a standard VT (at 12 mL/kg predicted body weight) and a plateau pressure (measured after a 0.5-s pause at end inspiration) of 
50 cm H2O or a low VT (at 6 mL/kg predicted body weight) with a plateau pressure of 30 cm H2O. In the lower VT group, the 28-day mortality rate was significantly lower than in the standard VT group (31.0% vs 39.8%, respectively; p = 0.007), as was the number of days without the failure of nonpulmonary organs (p = 0.006) even though barotrauma was similar (10% vs 11%, respectively). This study therefore represents clinical confirmation that a lung protective ventilatory strategy for patients with ARDS is beneficial.
2. Richard NC, Maggiore SM, Jonson B, et al. Influence of tidal volume on alveolar recruitment: respective role of PEEP and a recruitment maneuver. Am J Respir Crit Care Med 2001; 163:1609–1613
Although the study by the ARDS Network showed a significant improvement in mortality when low VT levels were used for ventilating patients with ARDS, smaller studies had previously failed to show statistical significance. Richard et al hypothesized that this inconsistency may have been due to alveolar derecruitment associated with reducing VT from 10 to 6 mL/kg body weight. In their study of 15 patients with ARDS, volume differences between a pressure-volume curve that was recorded from 0 positive end-expiratory pressure (PEEP) vs a curve recorded from different PEEP levels were measured. The study demonstrated that the recruited volume and oxygenation were significantly lower when smaller VT levels were used to ventilate these patients. However, the recruited volume was reestablished either after an increase in PEEP (by 4 cm H2O) above the lower inflection point of the pressure-volume curve or by two sustained inflations at 45 cm of H2O. Their study demonstrated a potential hazard of the lung-protective ventilatory strategy and corroborated previous observations that either periodic sighs (with the risk of alveolar overdistention) or high levels of PEEP may be necessary to optimize the benefits of the ARDS Network strategy. The ARDS Network is presently conducting a randomized study to clarify the optimal approach.
3. Crotti S, Mascheroni D, Caironi P, et al. Recruitment and derecruitment during acute respiratory failure: a clinical study. Am J Respir Crit Care Med 2001; 164:131–140
A basic assumption of lung-protective ventilation is the concept of open lung ventilation, wherein the pulmonary damage induced by recurrent alveolar collapse and reopening is minimized by adjusting PEEP, inspiratory time, and VT. Proof of this tenet has required extrapolation from the physiologic data. Two pioneers in this field (Drs. Gattinoni and Marini) collaborated in this study, and in an accompanying study of oleic acid-induced lung injury in animals,3 to demonstrate recruitment utilizing quantitative analysis of CT images. Five patients with ARDS were studied at multiple combinations of inspiratory plateau pressures (10 to 45 cm H2O) and PEEP levels (5 to 20 cm H2O). Lung volume recruitment occurred over the entire spectrum of inspiration (independent of lower and upper inflection points), and progressively from nondependent to dependent lung regions. This study has provided direct evidence that end-expiratory collapse is dependent on the preceding inspiratory pressure and the superimposed pressure surrounding alveolar units during end-expiration. However, future studies with analyses of lung perfusion will be necessary to fully optimize lung-protective ventilatory techniques.
4. Gattinoni L, Tognoni G, Pesenti A, et al. Effect of prone positioning on the survival of patients with acute respiratory failure. N Engl J Med 2001; 345:568–573
Another lung-protective strategy that has been successfully utilized to improve oxygenation in mechanically ventilated patients with ARDS is accomplished by changing the patient from the supine position to the prone position. The mechanism for this improvement is hypothesized to be the result of improved and better distributed ventilation to the previously dependent areas of the lung. The Prone-Supine Study Group from Europe conducted a randomized, unblinded, controlled trial with a 6-month follow-up in 28 ICUs in Italy and 2 ICUs in Switzerland. One hundred fifty-two patients were allocated to prone positioning for 
6 h per day for 10 days, while 152 patients were allocated to conventional (supine) positioning throughout their treatment. There was no difference in the 10-day mortality rate between the prone and the supine groups (21% vs 25%, respectively), at ICU discharge (51% vs 48%, respectively), or at the 6-month follow-up (63% vs 59%, respectively). This lack of significant improvement occurred despite significant (p = 0.02) increases in oxygenation in the prone group. This study has been criticized for using only 6 h of prone positioning. Therefore, two multi-institutional studies in Europe are presently underway to determine whether a more prolonged use of the prone position may improve mortality rates in ARDS patients, as has been suggested in smaller, uncontrolled series.
Noninvasive Ventilation
5. Vitacca M, Ambrosino N, Clini E, et al. Physiologic response to pressure support ventilation delivered before and after extubation in patients not capable of totally spontaneous autonomous breathing. Am J Respir Crit Care Med 2001; 164:638–641
Over the past 6 years, there has been a plethora of clinical articles showing the benefits of noninvasive ventilation (NIV). These results were extensively reviewed in a recent state-of-the-art article by Mehta and Hill.4 NIV has been established as a preferred mode of ventilation during acute hypercapnic exacerbations of COPD.5 Vitacca et al performed a physiologic study in 12 patients who were intubated because of hypercapnic respiratory failure due to COPD. Measurements included diaphragm energy expenditure (using esophageal and gastric balloons to record the pressure time product of the diaphragm), lung resistance and elastance, breathing pattern, dyspnea (as measured by a visual analog scale), and arterial blood gas measurements. The study also compared invasive pressure support (PS) ventilation to noninvasive PS ventilation. None of the patients were ready to sustain total spontaneous breathing. Both invasive and noninvasive PS ventilation resulted in similar reductions in diaphragm energy expenditure and improvements in arterial blood gas levels. However, noninvasive PS ventilation was better tolerated by patients, as determined by their dyspnea analysis. This study showed subjective advantages of noninvasive PS ventilation in patients with COPD. In addition, the data documented improvement in the pressure time product of the diaphragm compared to ventilation through a T-piece when PS was delivered noninvasively or via an endotracheal tube. The physiologic data in this article support the clinical recommendation that NIV be used as a bridge between mechanical ventilatory support and total spontaneous breathing in patients with hypercapnic ventilatory failure.5
Physiology and Overview
6. Tobin MJ. Advances in mechanical ventilation. N Engl J Med 2001; 344:1986–1996
This article is an update of Dr. Tobin’s review of mechanical ventilation that was published in 1994 in the New England Journal of Medicine. It succinctly summarizes the significant changes that have occurred in the area of mechanical ventilation, including the interaction between the patient and the ventilator, lung-protective ventilation, and weaning. The review allows the clinician to appreciate the interplay among basic research, animal research, and the large, randomized, controlled clinical trials that have reshaped the intensivist’s approach to diseases such as ARDS. This manuscript will remain the standard reference review article on mechanical ventilation for the next few years.
Weaning
7. Kress JP, Pohlman AS, O’Connor MF, et al. Daily interruption of sedative infusions in critically ill patients undergoing mechanical ventilation. N Engl J Med 2000; 342:1471–1477
The transition from mechanical ventilator support to spontaneous breathing has traditionally been called weaning. Most efforts to expedite weaning have focused on manipulating and comparing different mechanical ventilatory modes. This sentinel article by Kress et al marks a conceptual change by linking a non-mechanical ventilation treatment that clinicians commonly use (sedation) to the weaning process. The authors conducted a randomized, controlled trial involving 128 adult patients who required infusions of sedative drugs (midazolam or propofol) because they were being mechanically ventilated. In 57 patients, the continuous infusion of sedative medication was interrupted on a daily basis to determine whether it was still required. If the researchers noted significant agitation after the withdrawal of sedation, the continuous IV infusions were restarted at half the previous dose. In a control group, the sedative infusions were interrupted at the discretion of the treating clinician. There was a significant decrease in the number of days of mechanical ventilation in the intervention group (4.9 vs 7.3 days, respectively; p = 0.004) as well as their length of stay in the ICUs (p = 0.02). There was no significant difference in complications, such as self-extubations. This article reminds clinicians that many "routine" aspects of critical care may affect the outcome of patients receiving mechanical ventilation.
8. MacIntyre NR, Cook DJ, Ely EW, et al. Evidence-based guidelines for weaning and discontinuing ventilatory support: a collective task force facilitated by the American College of Chest Physicians; the American Association for Respiratory Care; and the American College of Critical Care Medicine. Chest 2001; 120(suppl):375S–395S
The "Top 10 List in Mechanical Ventilation" would be incomplete without inclusion of this article from a CHEST supplement (December 2001), which was devoted to evidence-based guidelines for weaning and discontinuation of ventilatory support. This consensus is an exhaustive review of hundreds of studies involving weaning and highlights the heterogeneity, and often incompleteness, of many of these studies. The reference quoted is from section 1 ("Guidelines" section) and summarizes the consensus opinions of the task force that was facilitated by the American College of Chest Physicians, the American College of Critial Care Medicine, and the American Association for Respiratory Care. The article includes 12 recommendations, all of which are accompanied by the rationale for the recommendation and the scientific evidence supporting the recommendation. A reference list of 224 articles accompanies the article so that the reader may refer to the primary literature. This section is part of an exhaustive review of the literature that includes 11 other articles.
9. Vitacca M, Vianello A, Colombo D, et al. Comparison of two methods for weaning patients with COPD requiring mechanical ventilation for more than 15 days. Am J Respir Crit Care Med 2001; 164:225–230
Until approximately 8 years ago, weaning patients from mechanical ventilatory support was as much of an art as a science. Since then, large, randomized, multi-institutional studies have provided evidence that weaning time is prolonged when intermittent mandatory ventilation is used as the weaning mode6 7 and that a 30-min trial of spontaneous breathing is as effective as 2-h trial to determine whether weaning will be successful.8 However, these studies included patients with varying diagnoses. The article by Vitacca and coworkers focused on 52 patients who required mechanical ventilation because of an acute exacerbation of COPD. Half of these patients were weaned using continuous positive airway pressure (CPAP) from an initial PS setting of approximately 19 cm H2O in 2-cm increments twice daily until they tolerated CPAP with a PS of 8 cm for 8 h. The remaining 26 patients were weaned by spontaneous breathing trials via a T-piece performed twice daily. Both groups achieved equal rates of weaning success (73% vs 77%, respectively), duration of ventilatory support (181 vs 130 h, respectively), and ICU lengths of stay. Therefore, in this select group of COPD patients, neither method appeared to be superior or inferior. The authors also compared the study patients (combined data) to 55 patients who were being managed without a formal protocol. In this comparison, the study patients whose conditions had been managed via a protocol were successfully weaned more frequently (87% vs 70%, respectively), and experienced shorter durations of mechanical ventilation and shorter stays in the ICU (27 vs 38 days, respectively) than the group of patients whose conditions had been managed without a protocol. Therefore, although there was no difference in weaning patients with COPD using CPAP and PS vs T-piece trials, there were significant benefits when a set protocol was utilized.
10. Scheinhorn DJ, Chao DC, Stearn-Hassenpflug MS, et al. Outcomes in post-ICU mechanical ventilation: a therapist-implemented weaning protocol. Chest 2001; 119:236–242
Approximately 20% of patients fail to wean from mechanical ventilation in the ICU and are eventually transferred to alternative settings for continued care (ie, long-term acute care facilities). Strategies for weaning that are applicable to the acute care setting are not necessarily designed for long-term facilities. Therefore, the article by Scheinhorn and associates at the Barlow Respiratory Hospital is very pertinent to this group of patients. The investigators reported the results of treating 271 consecutive patients who had been admitted for weaning to their long-term facility during an 18-month period and compared them to a group of 238 patients who had been treated at the same facility by the same physicians for the previous 2 years. The patients were managed by a therapist-implemented, patient-specific weaning protocol that was detailed in the article. Compared to the control subjects, the protocol patients weaned significantly faster (17 vs 29 days, respectively; p < 0.001). There were no differences in other outcomes such as weaning success (55% vs 58%, respectively), mortality rate (27% vs 31%, respectively), or patients who remained ventilator-dependent (18% vs 11%, respectively). This article is important because it documented the outcomes of patients in long-term facilities and described a successful respiratory therapy-driven protocol that resulted in improved outcomes.
Acknowledgements
The author thanks Ms. Elizabeth Colón for her assistance in preparing this manuscript.
Footnotes
Abbreviations: CPAP = continuous positive airway pressure; NIV = noninvasive ventilation; PEEP = positive end-expiratory pressure; PS = pressure support; VT = tidal volume
Received for publication May 2, 2002. Accepted for publication May 7, 2002.
References
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