Abstract
A 1-d point-prevalence study was performed with the aim of describing the characteristics of conventional mechanical ventilation in intensive care units ICUs from North America, South America, Spain, and Portugal. The study involved 412 medical-surgical ICUs and 1,638 patients receiving mechanical ventilation at the moment of the study. The main outcome measures were characterization of the indications for initiation of mechanical ventilation, the artificial airways used to deliver mechanical ventilation, the ventilator modes and settings, and the methods of weaning. The median age of the study patients was 61 yr, and the median duration of mechanical ventilation at the time of the study was 7 d. Common indications for the initiation of mechanical ventilation included acute respiratory failure (66%), acute exacerbation of chronic obstructive pulmonary disease (13%), coma (10%), and neuromuscular disorders (10%). Mechanical ventilation was delivered via an endotracheal tube in 75% of patients, a tracheostomy in 24%, and a facial mask in 1%. Ventilator modes consisted of assist/control ventilation in 47% of patients and 46% were ventilated with synchronized intermittent mandatory ventilation, pressure support, or the combination of both. The median tidal volume setting was 9 ml/kg in patients receiving assist/control and the median setting of pressure support was 18 cm H2O. Positive end-expiratory pressure was not employed in 31% of patients. Method of weaning varied considerably from country to country, and even within a country several methods were in use. We conclude that the primary indications for mechanical ventilation and the ventilator settings were remarkably similar across countries, but the selection of modes of mechanical ventilation and methods of weaning varied considerably from country to country.
The basic understanding of mechanical ventilation has advanced to a stage that randomized controlled trials can be undertaken to evaluate different ventilator strategies. Such trials usually consist of the comparison of a new treatment against conventional therapy. Given the diversity of ventilator modes and settings, however, it is difficult to know what constitutes “conventional mechanical ventilation.” The problem of defining conventional mechanical ventilation in arbitrary terms is highlighted by recent studies evaluating the efficacy of strategies designed to minimize ventilator-induced injury in patients with the acute respiratory distress syndrome (ARDS). In comparison with conventional ventilation, Amato and colleagues (1) reported that a lung-protective ventilator strategy decreased mortality, whereas Stewart and colleagues (2) and Brochard and coworkers (3) did not observe a reduction in mortality with such a strategy. In these studies, patients in the conventional arm were ventilated with volume-cycled ventilation, but tidal volume was larger in the study of Amato and colleagues (1) (12 ml/kg of body weight) than in the studies of Stewart and colleagues (2) (10.1 to 10.8 ml/kg) and Brochard and coworkers (3) (9.9 to 10.7 ml/kg). Peak inspiratory pressure was specifically limited to 50 cm H2O in the conventional arm of the study of Stewart and colleagues (2) and to 60 cm H2O in the study of Brochard and coworkers (3). In contrast, peak inspiratory pressure was not limited in the study of Amato and colleagues; this lack of pressure limit in the conventional group combined with a larger tidal volume favor a beneficial outcome in the intervention arm. To be able to decide if some modification of ventilator management represents a true therapeutic advance, it is necessary to first define what is meant by conventional mechanical ventilation.
In 1992, the Spanish Lung Failure Collaborative Group conducted a survey of the use of mechanical ventilation, specifically the indications, modes employed, and methods of weaning (4). The study involved 290 patients in 47 intensive care units (ICUs). Of patients admitted to ICUs in the study, 46% were receiving mechanical ventilation at the moment of the survey. Most patients were ventilated with assist/control (A/C) ventilation (55%) or synchronized intermittent mandatory ventilation (SIMV) (26%), and the following techniques were used for weaning: T-tube trials, 24%; SIMV, 18%; pressure support (PS), 15%; SIMV plus PS, 9%; or a combination of two or more methods in succession, 33%. Because the study was confined to Spain, it is difficult to know if the data are representative of the general use of mechanical ventilation. To better define the characteristics of conventional mechanical ventilation today, we undertook a study of ventilator use in North America, South America, Spain, and Portugal—the first large international study of its kind. The aims of this study were to define the indications for mechanical ventilation in an ICU setting, the characteristics of patients receiving mechanical ventilation, the use of available ventilator modes, settings, and artificial airways, and the methods employed for discontinuation of mechanical ventilation.
A prospective study was conducted in Argentina, Brazil, Chile, Spain, and Uruguay on November 27, 1996, and in Canada, Portugal, and the United States of America on January 15, 1997. The study was performed at 11:00 a.m. on the stated date and involved 412 ICUs. Before the collection of data, the study protocol was reviewed and approved by Institutional Review Committees of each hospital. For an ICU to be included in the study, it had to possess six or more beds and at least 60% of the physicians had to have undergone ICU training and/or have more than 5 yr of experience in an ICU. Pediatric ICUs, postoperative recovery areas, and units that provide exclusive coronary care were excluded. To minimize a change in behavior as a result of being observed, only the investigator and research coordinator of a given ICU were aware that the study was about to be undertaken.
Each investigator and research coordinator was provided with a comprehensive manual describing data collection requirements and variable definitions. Each National Coordinator was able to answer questions regarding the process of data collection, and they collected questionnaires in their country after completion of the study. The forms were then sent to Spain, where data were entered in a computer program specifically designed for this study. Each questionnaire was checked by three study coordinators to identify omissions, and inconsistent data were corrected whenever possible. Information on each hospital, each study ICU, and each patient receiving ventilator support were collected on the data sheets. The information on the ICU and hospital included the number of beds in the hospital, number of beds in the ICU, number of patients in the ICU at the moment of the study, and number of patients receiving mechanical ventilation at that time. The following information was collected in each patient receiving mechanical ventilation at the moment of the study:
1. Demographic data. The sex, age and weight of each patient, date of admission to the ICU, APACHE II score at the time of admission to the ICU, date of initiating mechanical ventilation, and mode of access to the patient's airway, viz., orotracheal intubation, nasotracheal intubation, facial mask, or tracheostomy were recorded. If a patient had undergone a tracheostomy, the date of its performance was recorded.
2. Ventilator indications. The indication for the initiation of mechanical ventilation was selected from the following predefined list of categories. (1) Acute exacerbation of chronic respiratory failure, which described patients with obstructive and/or restrictive lung disease, requiring mechanical ventilation because of infection, bronchospasm, heart failure, or another acute episode. (2) Coma, which described patients requiring mechanical ventilation caused by loss of consciousness secondary to organic or metabolic conditions (hepatic encephalopathy, cerebral hemorrhage, etc.). (3) Neuromuscular disease, which described patients whose respiratory failure was due to impairment of the peripheral nerves, myoneural junction, muscle mass, etc. (4) Acute respiratory failure, which described patients without a preexisting obstructive or restrictive lung disease requiring mechanical ventilation because of respiratory failure. Whenever a patient had more than one indication for mechanical ventilation, the data collector recorded the reason judged dominant.
The patients who fell in the category of acute respiratory failure were separated into the following subgroups: (1) acute respiratory distress syndrome (ARDS), defined according to the criteria of the American-European consensus conference (5); (2) postoperative state, consisting of patients who required the continuation of mechanical ventilation after surgery because of a serious underlying medical problem, advanced age or the high risk of the operative procedure; (3) acute pulmonary edema/congestive heart failure, consisting of patients with dyspnea, bilateral alveolar infiltrates, hypoxemia and evidence of cardiac disease or patients in cardiogenic shock; (4) aspiration, defined by visualization of gastric contents in the airways or in a tracheal aspirate; (5) pneumonia, defined by the development of a new alveolar infiltrate or worsening of previous alveolar infiltrates, accompanied by fever/hypothermia and leukocytosis/leukopenia; (6) sepsis/septic shock, defined by preestablished criteria (6); and (7) trauma, consisting of patients requiring mechanical ventilation because of injury to the thorax, abdomen, or head.
3. Ventilator data. In each patient, the ventilator mode and settings at the moment of the study were recorded. The patient was considered to be in a weaning phase when so judged by the responsible physician, and the method being used for weaning was recorded. The study was confined to patients who were receiving mechanical ventilation at 11:30 a.m. on the study date, and, consequently, patients undergoing a spontaneous breathing trial at that moment were excluded. Patients listed as undergoing spontaneous breathing trials were those who underwent a least one such trial in the preceding 24 h.
4. Physician questionnaire. One week after the study date, the physician who took care of patients included in the study completed another data sheet to indicate his or her preferred ventilator mode and preferred method of weaning. It was completed in anonymity and delivered to the investigator of each ICU in the study.
Results are expressed as mean ± standard deviation, median with the first and third quartiles, or proportions with 95% confidence intervals. Continuous variables were compared by analysis of the variance (ANOVA) and qualitative variables were compared with the chi-square test. A difference was considered significant if the p value was less than 0.05.
In the 412 ICUs included into the study, 4,153 patients occupied beds at that time. The median ICU occupancy rate, calculated as the number of patients per number of beds in the ICU at the moment of the study, was 83% (25th and 75th percentiles: 63%, 100%). In the 1,638 patients receiving mechanical ventilation (39% of the total group), the median age was 61 yr (25th and 75th percentiles: 44, 71), and men predominated in every country (average, 60%) except Brazil (Table 1). The median APACHE II score on admission to the ICU was 19 (25th and 75th percentiles: 14, 25). At the time of the study, the median duration of ICU stay in the ventilator-supported patients was 8 d and the median duration of mechanical ventilation was 7 d; an exception was Portugal, where the median was 12 d.
| USA/Canada | Spain | Argentina | Brazil | Chile | Portugal | Uruguay | Total | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ICUs, n | 167 | 79 | 89 | 25 | 27 | 10 | 15 | 412 | ||||||||
| Patients admitted | 1,833 | 1,028 | 680 | 273 | 160 | 90 | 89 | 4,153 | ||||||||
| Patients ventilated | 747 | 443 | 154 | 122 | 60 | 68 | 44 | 1,638 | ||||||||
| Occupancy, %† | 83 (67, 100) | 92 (82, 100) | 73 (50, 89) | 83 (71, 94) | 79 (50, 100) | 100 (88, 100) | 50 (47, 83) | 83 (63, 100) | ||||||||
| Ventilation, %† | 40 (20, 60) | 39 (27, 60) | 17 (0, 33) | 47 (25, 50) | 40 (20, 50) | 75 (50, 76) | 45 (25, 86) | 36 (17, 58) | ||||||||
| Age† | 60 (45, 71) | 64 (46, 71) | 59 (40, 68) | 58 (38, 71) | 60 (39, 69) | 66 (50, 73) | 56 (43, 68) | 61 (44, 71) | ||||||||
| APACHE II† | 19 (15, 25) | 19 (14, 25) | 17 (12, 24) | 19 (14, 26) | 20 (12, 25) | 19 (15, 26) | 17 (12, 21) | 19 (14, 25) | ||||||||
| Women, %‡ | 43 (39, 46) | 35 (30, 39) | 35 (28, 43) | 58 (49, 67) | 38 (26, 52) | 37 (26, 49) | 32 (19, 48) | 40 (38, 43) | ||||||||
| Days in ICU | 8 (3, 19) | 8 (3, 16) | 6 (3, 12) | 8 (4, 18) | 8 (4, 17) | 13 (4, 46) | 7 (2, 15) | 8 (3, 18) | ||||||||
| Days of ventilator support | 7 (3, 18) | 7 (3, 15) | 6 (2, 10) | 7 (3, 18) | 8 (3, 16) | 12 (2, 47) | 5 (2, 12) | 7 (3, 17) | ||||||||
| Days from intubation to tracheostomy | 12 (6, 18) | 12 (7, 20) | 8 (1, 20) | 8 (1, 15) | 9 (0, 12) | 14 (4, 33) | 9 (8, 17) | 11 (5, 19) |
Acute respiratory failure was the most frequent reason for the initiation of mechanical ventilation (Table 2), being the precipitating cause in 66% of the total study group; this indication was more common in North America (73%) than in Spain, Uruguay, Portugal, and Argentina (p < 0.01). ARDS accounted for 12% of patients in acute respiratory failure and 8% of the total study population. An acute exacerbation of chronic respiratory failure was the precipitating cause in 13%; in Portugal it accounted for 23% of patients (p < 0.001). Coma, which was the indication in 15% of the total study group, was the condition with the greatest variation among countries. In most countries, a neuromuscular disorder was the indication in less than 10% of patients.
| USA/Canada | Spain | Argentina | Brazil | Chile | Portugal | Uruguay | Total | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Reason for MV | (n = 747 ) | (n = 443) | (n = 154 ) | (n = 122) | (n = 60) | (n = 68) | (n = 44 ) | (n = 1,638) | ||||||||
| COPD | 16 (13, 18) | 11 (8, 14) | 10 (6, 16) | 9 (5, 16) | 10 (4, 21) | 23 (14, 36) | 5 (1, 17) | 13 (11, 15) | ||||||||
| ARF | 74 (70, 77) | 64 (59, 68) | 50 (42, 58) | 66 (56, 74) | 62 (48, 74) | 47 (35, 59) | 50 (35, 65) | 66 (63, 68) | ||||||||
| Coma | 7 (6, 9) | 20 (17, 25) | 32 (25, 40) | 21 (15, 30) | 15 (7, 27) | 10 (5, 21) | 43 (29, 59) | 15 (13, 17) | ||||||||
| Neuromuscular | 3 (2, 5) | 4 (3, 7) | 8 (5, 14) | 4 (1, 10) | 13 (6, 25) | 19 (11, 31) | 2 (0, 13) | 5 (4, 6) | ||||||||
| Cause of ARF | (n = 547 ) | (n = 283) | (n = 77 ) | (n = 80) | (n = 37 ) | (n = 32) | (n = 22) | (n = 1,078) | ||||||||
| ARDS | 9 (7, 12) | 14 (10, 18) | 18 (11, 29) | 11 (6, 21) | 32 (18, 50) | 6 (1, 22) | 14 (4, 36) | 12 (10, 14) | ||||||||
| Postoperative | 17 (14, 21) | 13 (10, 18) | 10 (5, 20) | 5 (2, 13) | 5 (1, 19) | 28 (14, 47) | 23 (9, 46) | 15 (13, 17) | ||||||||
| Heart Failure | 13 (10, 16) | 14 (11, 19) | 16 (9, 26) | 5 (2, 13) | 5 (1, 19) | 16 (6, 33) | 4 (0, 25) | 12 (10, 14) | ||||||||
| Aspiration | 3 (1, 4) | 2 (1, 5) | 8 (3, 17) | 8 (3, 16) | 3 (0, 16) | 3 (0, 18) | 14 (4, 36) | 3 (2, 5) | ||||||||
| Pneumonia | 13 (10, 16) | 18 (13, 23) | 17 (10, 27) | 29 (20, 40) | 19 (9, 36) | 19 (8, 37) | 23 (9, 46) | 16 (14, 18) | ||||||||
| Sepsis | 17 (14, 20) | 12 (8, 16) | 19 (12, 30) | 16 (9, 27) | 24 (12, 42) | 12 (4, 30) | 18 (6, 41) | 16 (13, 18) | ||||||||
| Trauma | 13 (10, 16) | 15 (12, 20) | 3 (0, 10) | 9 (4, 18) | 8 (2, 23) | 12 (4, 30) | 4 (0, 25) | 12 (10, 14) | ||||||||
| Others | 16 (13, 19) | 11 (8, 15) | 9 (4, 18) | 16 (9, 27) | 3 (0, 16) | 3 (0, 18) | — | 13 (11, 15) |
The access to the airway for delivery of mechanical ventilation at the moment of the study consisted of an endotracheal tube in 75% of patients, a tracheostomy in 24%, and a facial mask in 1% (Table 3). Of the endotracheal tubes, 96% were passed through the mouth and 4% through the nose. In patients who had undergone a tracheostomy, it was performed at a median of 11 d (25th and 75th percentiles: 5, 19) after intubation, and this elapsed time did not differ among countries.
| USA/Canada | Spain | Argentina | Brazil | Chile | Portugal | Uruguay | Total | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Intubation | 546 (73%) | 351 (79%) | 115 (75%) | 85 (70%) | 52 (87%) | 44 (65%) | 36 (82%) | 1,229 (75%) | ||||||||
| Orotrach | 517 (95%) | 338 (96%) | 115 (100%) | 85 (100%) | 51 (98%) | 37 (84%) | 36 (100%) | 1,179 (96%) | ||||||||
| Nasotrach | 29 (5%) | 13 (4%) | — | — | 1 (2%) | 7 (16%) | — | 50 (4%) | ||||||||
| Tracheostomy | 192 (26%) | 87 (20%) | 34 (22%) | 33 (27%) | 8 (13%) | 24 (35%) | 8 (18%) | 386 (24%) | ||||||||
| Facial mask | 5 (1%) | 4 (1%) | 1 (1%) | 4 (3%) | — | — | — | 14 (1%) |
A tracheostomy was performed in 24% of patients. The frequency of tracheostomy varied significantly depending on the patient's underlying condition and the time from initiation of mechanical ventilation. Over the initial 3 wk, a tracheostomy was performed more frequently in patients with neuromuscular disease (31.3%) than in those with COPD (14.8%, p < 0.05) or acute respiratory failure (9.1%, p < 0.001). After the third week, the proportion of patients with a tracheostomy did not differ among the diagnostic categories (Table 4).
| Duration of Mechanical Ventilation | ||||||||
|---|---|---|---|---|---|---|---|---|
| Indication for MV | 1 to 7 d | 8 to 14 d | 15 to 21 d | > 21 d | ||||
| COPD | 5/82 (6.1) | 8/36 (22.2) | 8/24 (33.3) | 36/49 (73.5) | ||||
| ARF | 14/555 (2.5) | 29/203 (14.3) | 35/101 (34.6) | 122/188 (64.9) | ||||
| Neuromuscular | 4/25 (16.0) | 4/14 (28.6) | 8/12 (66.7) | 22/32 (68.7) | ||||
Of the total group of ventilator-supported patients, 47% received assist/control (A/C) ventilation (Table 5), and this was the most common mode of ventilation in Argentina, Chile, and Spain. An almost equal proportion (46%) of the overall group were ventilated with synchronized intermittent mandatory ventilation (SIMV), pressure support (PS), or the combination of both. The use of SIMV on its own was infrequent in all countries, with the exception of Uruguay (20%), and overall it was 6%. In no country was PS alone the most frequently used mode, and overall it was employed in 15% patients. The combination of SIMV and PS showed considerable variation among countries, ranging from the 7% (Argentina) to 52% (Uruguay); in North America, this combination was used with the same frequency as A/C (34% in both instances).
| USA/Canada | Spain | Argentina | Brazil | Chile | Portugal | Uruguay | Total | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ventilation | (n = 747 ) | (n = 443) | (n = 154) | (n = 122) | (n = 60) | (n = 68) | (n = 44 ) | (n = 1,638) | ||||||||
| A/C | 34 (31, 38) | 62 (57, 66) | 68 (60, 75) | 40 (31, 49) | 72 (58, 82) | 44 (32, 57) | 25 (14, 41) | 47 (44, 49) | ||||||||
| SIMV | 6 (4, 8) | 7 (1, 10) | 9 (5, 15) | 4 (1, 10) | 5 (1, 15) | — | 20 (10, 36) | 6 (5, 8) | ||||||||
| PS | 18 (15, 21) | 11 (9, 15) | 10 (6, 16) | 10 (5, 17) | 5 (1, 15) | 34 (23, 46) | 2 (0, 13) | 15 (13, 16) | ||||||||
| SIMV + PS | 34 (31, 38) | 13 (11, 17) | 7 (4, 13) | 31 (23, 40) | 17 (9, 29) | 13 (7, 24) | 52 (37, 67) | 25 (23, 27) | ||||||||
| Others† | 7 (6, 10) | 6 (4, 9) | 6 (3, 11) | 15 (9, 23) | 2 (1, 10) | 9 (4, 19) | — | 7 (6, 9) | ||||||||
| Weaning | (n = 274 ) | (n = 108) | (n = 39) | (n = 39) | (n = 9) | (n = 32) | (n = 19) | (n = 520) | ||||||||
| PS‡ | 45 (39, 51) | 23 (16, 32) | 28 (15, 45) | 28 (16, 45) | 33 (9, 69) | 31 (17, 50) | 21 (7, 46) | 36 (32, 40) | ||||||||
| SIMV | 5 (3, 9) | 1 (0, 6) | 13 (5, 28) | 3 (0, 15) | — | 6 (1, 22) | 16 (4, 40) | 5 (4, 8) | ||||||||
| SIMV + PS | 32 (26, 38) | 25 (17, 34) | 3 (0, 15) | 36 (22, 53) | 44 (15, 77) | 6 (1, 22) | 47 (25, 70) | 28 (24, 32) | ||||||||
| Intermittent SB trials§ | 6 (3, 9) | 39 (30, 49) | 33 (20, 50) | 8 (2, 22) | 22 (4, 60) | 37 (22, 56) | 10 (2, 34) | 17 (14, 21) | ||||||||
| Daily SB trial§ | 3 (1, 5) | 6 (3, 13) | 10 (3, 25) | 8 (2, 22) | — | — | — | 4 (3, 6) | ||||||||
| Others¶ | 9 (6, 14) | 5 (2, 11) | 13 (5, 28) | 18 (8, 34) | — | 19 (8, 37) | 5 (0, 28) | 9 (7, 12) |
The median tidal volume setting was 9 ml/kg (25th and 75th percentiles: 8, 10) in patients receiving A/C. The median PS setting was 18 cm H2O (25th and 75th percentiles: 13, 23). The setting of tidal volume was remarkably constant among countries (Table 6), with little interquartile variation. The median level of PS was also similar among countries, although interquartile variation was considerable. In patients receiving A/C, tidal volume did not differ between patients with ARDS and those with other causes of acute respiratory failure receiving A/C (8.7 ± 2.3 ml/kg versus 9.1 ± 2.2 ml/kg).
| USA/Canada | Spain | Argentina | Brazil | Chile | Portugal | Uruguay | Total | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Assist Control | (n = 256 ) | (n = 273) | (n = 105) | (n = 49) | (n = 42) | (n = 30) | (n = 11) | (n = 767 ) | ||||||||
| Tidal volume, ml/kg | 8 (7, 10) | 9 (8, 10) | 9 (8, 10) | 8 (7, 10) | 9 (8, 11) | 8 (7, 9) | 10 (9, 12) | 9 (8, 10) | ||||||||
| Respiratory rate, breaths/min | 16 (12, 20) | 16 (13, 18) | 14 (12, 17) | 18 (16, 24) | 18 (15, 20) | 20 (16, 20) | 14 (12, 18) | 16 (13, 20) | ||||||||
| Peak pressure, cm H2O | 32 (27, 38) | 32 (25, 40) | 27 (20, 30) | 26 (22, 30) | 28 (20, 32) | 22 (20, 30) | 29 (22, 42) | 30 (25, 37) | ||||||||
| PSV | (n = 134 ) | (n = 51) | (n =15) | (n = 12) | (n = 3) | (n = 23) | (n = 1) | (n = 239) | ||||||||
| Tidal volume, ml/kg | 6 (5, 8) | 9 (7, 9) | 8 (5, 10) | 7 (7, 10) | — | 7 (6, 9) | — | 7 (5, 9) | ||||||||
| Respiratory rate, breaths/min | 23 (20, 28) | 21 (17, 25) | 18 (16, 24) | 21 (17, 25) | — | 19 (16, 24) | — | 22 (18, 26) | ||||||||
| Peak pressure, cm H2O | 18 (13, 23) | 19 (15, 25) | 18 (15, 22) | 21 (15, 26) | — | 20 (15, 26) | — | 19 (15, 25) | ||||||||
| PEEP, cm H2O | 5 (5, 6) | 5 (5, 8) | 5 (5, 7) | 5 (4, 8) | 5 (4, 6) | 4 (4, 5) | 5 (4, 6) | 5 (5, 6) |
Positive end-expiratory pressure (PEEP) was not employed in 31% of patients (range: 16 to 50) (Figure 1). In those receiving PEEP, the median value was 5 cm H2O (25th and 75th percentiles: 5, 6), with no difference among countries (Table 6). A significantly higher level of PEEP was employed in patients with ARDS than in those with an acute exacerbation of chronic lung disease (8.0 ± 3.4 versus 5.5 ± 2.3 cm H2O, p < 0.05).
Physicians considered 32% of the patients to be in a weaning phase at the moment of the study or during the preceding 24 h. Because the study was confined to patients actually receiving mechanical ventilation at 11:00 a.m. on the study day, patients undergoing a trial of spontaneous breathing with a T-tube setup or flow-by at that point in time were not included in the study. The methods varied considerably among the countries. Overall, the most frequent method of weaning was PS, used in 36% of the patients. The combination of SIMV and PS was used in 28% of the patients (Table 5), but varied considerably among countries, ranging from 3% of patients in Argentina to 47% in Uruguay. Use of some form of spontaneous breathing trial also varied considerably among countries.
A total of 2,226 physicians completed the questionnaire. A/C was the preferred mode of 62%, but that percentage varied considerably among countries (Table 7). In general, the mode listed by physicians corresponded with the mode most frequently employed in a given country. The preferred methods for weaning were PS (34% of the respondents) and SIMV with or without PS (35% of the respondents).
| USA/Canada | Spain | Argentina | Brazil | Chile | Portugal | Uruguay | Total | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ventilator modes | ||||||||||||||||
| A/C | 128 (32.5%) | 481 (72.0%) | 540 (82.6%) | 76 (39.8%) | 77 (70.0%) | 13 (54.2%) | 18 (14.6%) | 1,333 (61.6%) | ||||||||
| SIMV | 55 (13.9%) | 43 (6.4%) | 41 (6.3%) | 10 (5.2%) | 8 (7.3%) | 1 (4.2%) | 14 (11.4%) | 172 (7.9%) | ||||||||
| PSV | 33 (8.4%) | 25 (3.7%) | 81 (12.4%) | 6 (3.1%) | 33 (30.0%) | 8 (33.3%) | — | 87 (4.0%) | ||||||||
| SIMV + PSV | 163 (41.4%) | 111 (16.6%) | 40 (6.1%) | 74 (39.3%) | 25 (22.7%) | 7 (29.2%) | 90 (73.2%) | 511 (23.6%) | ||||||||
| PCV | 14 (3.5%) | 3 (0.4%) | 11 (1.7%) | 24 (12.6%) | — | — | 1 (0.8%) | 53 (2.4%) | ||||||||
| Methods of weaning | ||||||||||||||||
| SIMV | 20 (5.1%) | 21 (3.1%) | 85 (12.8%) | 8 (4.2%) | 5 (4.6%) | — | 11 (8.9%) | 150 (6.8%) | ||||||||
| PSV | 163 (41.4%) | 163 (23.9%) | 70 (10.5%) | 57 (29.8%) | 14 (12.8%) | 11 (37.9%) | 7 (5.6%) | 485 (22.1%) | ||||||||
| SIMV + PSV | 125 (31.7%) | 176 (25.8%) | 95 (14.3%) | 96 (50.3%) | 54 (49.5%) | 2 (6.7%) | 80 (64.5%) | 628 (28.6%) | ||||||||
| Intermittent SB trials | 36 (9.1%) | 271 (39.7%) | 352 (53.0%) | 24 (12.6%) | 31 (28.4%) | 12 (41.4%) | 22 (17.7%) | 748 (34.1%) | ||||||||
| Daily SB trial | 27 (6.8%) | 50 (7.3%) | 62 (9.3%) | 6 (3.1%) | 5 (4.6%) | 4 (13.8%) | 4 (3.2%) | 158 (7.2%) |
The primary indications for mechanical ventilation were remarkably similar across countries. In contrast, the selection of modes of mechanical ventilation and methods of weaning varied considerably from country to country.
Of patients in the ICUs, an average of 39% were receiving mechanical ventilation, but the proportion varied considerably among countries. The variation may reflect differences in type of ICU, admission and discharge policies, and patient profiles. The ICUs were not selected randomly, and the identification of ICUs and also their voluntary participation may have produced a selection bias. It cannot be assumed that the studied ICUs are necessarily representative of a given country, and observed differences among countries must be interpreted with caution. The percentage of patients receiving mechanical ventilation would have been higher if patients receiving ventilation for a brief period of time had been enrolled in the study. When planning the study, we deliberately selected 11:00 a.m. to minimize the influence of patients receiving brief ventilator support after surgery on the overall study population. This aspect of study design may explain why postoperative patients constituted less than 10% of the total study population and that patients receiving short-term ventilator support (less than 24 h) accounted for only 18% of the study population.
The average age of our patients was high: 25% of the ventilated patients were older than 71 yr of age. This finding was not confined to countries where the average age of the population is high. In recent published series of ventilator-supported patients, the mean age has been around 60 yr (2, 4, 7-10). The data suggest that advanced age did not preclude patients from being admitted to the ICU; indeed, it has been demonstrated that the prognosis in elderly patients admitted to an ICU (11) or receiving mechanical ventilation (12) does not depend exclusively on age, but rather on the severity of the acute illness and the functional status. Men accounted for 60% of the patients receiving mechanical ventilation. Other investigators have also reported a higher proportion of men among patients receiving mechanical ventilation (2, 4, 7, 9, 10) or admitted to an ICU (13).
Acute respiratory failure was the most frequent indication for mechanical ventilation, accounting for two thirds of patients in four countries and for half the patients in the other three countries. Among the subgroups of acute respiratory failure, ARDS accounted for 12% of ventilated patients. Because of the complex pulmonary pathophysiology in patients with ARDS and the many new modes of mechanical ventilation developed to deal with this problem, attempts have been made to obtain more precise information on the incidence of acute lung injury and ARDS. The reported incidence of ARDS ranges from 1.5 to 8 cases per 100,000 inhabitants (14-16). Of all 4,153 patients in ICUs on the day of our study, only 3% had ARDS—similar to the rates of 2 to 3% in two recent retrospective studies (17, 18).
Patients with COPD accounted for 13% of all patients receiving mechanical ventilation, being the most common indication in North America and greater than any subcategory of acute respiratory failure in all countries. We cannot calculate the proportion of patients with COPD receiving mechanical ventilation since we do not know the total number admitted to ICUs at the time of the study. In control groups of recent prospective studies of noninvasive ventilation in patients with COPD, Brochard and colleagues (19) and Kramer and coworkers (20) reported intubation rates of 74 and 67%, respectively. These figures are much higher than the intubation rate of 35% reported by Connors and colleagues (21) in a study of 1,016 patients admitted to hospital with an acute exacerbation of COPD complicated by hypercapnia. The proportion of patients with COPD in our study is markedly higher than in the APACHE III data base (22), where only 1% of 17,440 unselected admissions to 42 ICUs received mechanical ventilation because of an acute exacerbation of COPD. The fact that medical ICUs accounted for only 10% of ICUs in the APACHE III data base (23) is probably responsible, at least in part, for the small number of patients with COPD in their study.
Neuromuscular disease was the indication for mechanical ventilation in less than 8% of patients in all but two of the countries. Although patients with neuromuscular disease constitute a small proportion of all ICU patients, they account for a disproportionate amount of costs because their stay is usually prolonged. From the initiation of mechanical ventilation until the day of the study, the median duration of ventilator support was 16 d (25th, 75th percentiles: 5, 48) in patients with neuromuscular disease, compared with 10 d (25th, 75th percentiles: 3, 22) in patients with COPD (p < 0.01) and 7 d (25th, 75th percentiles: 3, 16) in patients receiving ventilation because of acute respiratory failure (p < 0.01).
The incidence of tracheostomy varied with the underlying condition: 39% among patients with neuromuscular disease, 28% in patients with COPD, and 20% in patients with acute respiratory failure. These findings are quite similar to our previous report (4): of 290 patients receiving mechanical ventilation in Spanish hospitals, 47% of the patients with neuromuscular disease and 30% of the patients with COPD had a tracheostomy. In the present study, the proportion of patients with a tracheostomy varied according to elapsed time from initiation of mechanical ventilation. During the first 3 wk, tracheostomy was performed more frequently in patients with neuromuscular disease than in those with COPD or acute respiratory failure (Table 3). These data give credence to the notion that physicians base the decision to perform a tracheostomy on the anticipated duration of mechanical ventilation (24).
Of patients in the study, only 1% were receiving noninvasive mechanical ventilation. Since non-invasive ventilation is commonly delivered intermittently throughout the day, our number may underestimate the true use of this form of ventilator support.
The selection of ventilator settings was remarkably similar among countries. Of patients ventilated with A/C, the median selected tidal volume was 9 ml/kg. Interestingly, in patients ventilated with PS, which does not permit a direct selection of volume, the achieved median tidal volume was similar (7 ml/ kg); respiratory rate was lower with A/C. Peak airway pressure was higher in patients ventilated with A/C mode than with PS (30 versus 19 cm H2O). Given the different flow-delivery systems with A/C and PS, this difference in peak pressure is expected. The setting of tidal volume has been somewhat arbitrary in nature, although from the early 1970s a setting of 10 to 15 ml/kg had become conventional (24, 25). Some investigators, however, reported tidal volumes of 20 to 24 ml/kg (26, 27). Of the patients receiving A/C in the present study, 63% (486/767) had a tidal volume of less than 10 ml/kg (the percentage in patients with ARDS [52/73, 71%] was similar to the total group). For more than a decade, there has been growing awareness of the potential for ventilator-induced injury, so called volutrauma (28), which has culminated in the deliberate use of low tidal volumes—a strategy referred to a controlled hypoventilation or permissive hypercapnia (1-3, 29). Milberg and colleagues (30) reported a decrease in mortality in patients with ARDS from the early 1980s to the early 1990s, and opined that this might be due to the greater use of permissive hypercapnia. However, permissive hypercapnia was listed as being in use in only 2.3% of our entire cohort of patients and in only 7.8% of those with ARDS.
Probably no aspect of mechanical ventilation has been the subject of such a voluminous literature as PEEP. Stemming from the initial favorable experience of Petty and Ashbaugh (31) extensive research has been undertaken to understand the mechanisms of its salutary actions and decide on the optimal level (32). The findings of the present study suggest that physicians make little effort in attempting to define the optimal level of PEEP for an individual patient, but instead write a generic order of “5 of PEEP,” as indicated by the median level of 5 cm H2O, and the narrow 25th and 75th percentiles (5, 6 cm H2O). The level of PEEP was similar in all countries, almost a third of patients (31%) were ventilated without PEEP, and only three patients were receiving a PEEP above 15 cm H2O.
Despite the introduction of several new ventilator modes, A/C was used in 40 to 72% of patients in all countries, with the exceptions of North America and Uruguay. In a survey published by Venus and colleagues in 1987 (33), 72% of physicians in the United States listed SIMV as their preferred ventilator mode. Ten years later, SIMV on its own was used in only 6% of patients receiving mechanical ventilation in North American ICUs, and the frequency of use differed little among countries. In North America, the use of a combination of SIMV and PS tied with A/C as the most commonly used mode; in Uruguay, this combination was used twice as often as any other modality. The popularity of the SIMV and PS combination is surprising, in that this modality has undergone little scrutiny (34). The clinical use of ventilator modes recently introduced, such as pressure-control, inverse-ratio ventilation, or airway pressure release ventilation, seems very limited, to judge by the present data and those we collected in Spanish ICUs 6 yr ago (4).
The greatest diversity of practice among countries was in the method of weaning. For example, trials of spontaneous breathing were being used in 46% of the patients in Spain versus only 16% in North American patients, whereas the SIMV and PS combination was used in 66% of patients in Uruguay versus only 7% of patients in Portugal. The exclusion of patients undergoing a spontaneous breathing trial at 11:00 a.m. on the study date necessarily results in an underestimate of the frequency with which this weaning modality was employed. The SIMV plus PS combination was the second most frequently used method in the United States despite its being the only technique of weaning whose efficacy has not been adequately evaluated. Weaning is an aspect of mechanical ventilation that has been the subject of several randomized controlled trials. In preselected groups of difficult-to-wean patients, Brochard and colleagues (7) reported difference in the rate of successful weaning between patients weaned with PS and patients weaned with either SIMV or with a T-tube pooled together, whereas the Spanish Lung Failure Collaborative Group (8) found that a single daily trial of spontaneous breathing achieved a threefold and twofold increase in the rate of successful weaning compared with IMV and PS, respectively. More recently, Ely and colleagues (9) found that systematic measurement of predictive indices, including the frequency-to-tidal volume ratio (35), combined with spontaneous breathing trials (8) resulted in a greater than twofold increase in the rate of successful weaning than in a control group weaned with IMV or PS, and decreased the rate of reintubation and ICU costs. The demonstration of the superiority of spontaneous breathing trials by the Spanish Lung Failure Collaborative Group may account for the almost twofold increase in the use of this approach in Spain between 1992 (24%) (4) and 1997 (46%).
In summary, the primary indications for mechanical ventilation were quite consistent from country to country. Although greater variation was noted in the selection of ventilator modes, more than 90% of patients were ventilated with A/C or PS (alone or in conjunction with SIMV). Greater consistency was noted in the selection of tidal volumes, 7 to 10 ml/kg, and PEEP, which, if used at all, was usually 5 cm H2O. A striking feature was the infrequent use of certain modalities such as SIMV as a stand-alone mode, noninvasive ventilation, permissive hypercapnia, and other new modes of ventilation. In about one third of the patients, an attempt at weaning was being made at the time of the study. The techniques employed for weaning showed considerable variation from country to country, and even within a country several techniques were in use. The results suggest that findings from research on mechanical ventilation and weaning are incorporated into clinical practice at a very slow pace. In conclusion, the study provides a unique picture of the types of patients receiving mechanical ventilation in many different countries and the methods employed; the data should prove useful in the design of trials of newer ventilator modalities and in serving as a baseline against which future changes in ventilator techniques can be gauged.
The writers are indebted to the study coordinators, clinical research associates, medicine residents, and pulmonary/critical care fellows who participated in the data collection of this study.
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