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(Chest. 2004;126:1281-1291.)
© 2004 American College of Chest Physicians

Changing Pattern of Ventilator Settings in Patients Without Acute Lung Injury

Changes Over 11 Years in a Single Institution*

Phunsup Wongsurakiat, MD, FCCP; David J. Pierson, MD, FCCP and Gordon D. Rubenfeld, MD, MSc, FCCP

Correspondence to: Phunsup Wongsurakiat, MD, FCCP, Division of Respiratory Disease and Tuberculosis, Department of Medicine, Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand; e-mail: sipwo{at}mahidol.ac.th


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
Study objectives: To determine whether the widely accepted concept of using lower tidal volume (VT) values in patients with ARDS or obstructive lung disease has affected the pattern of ventilator settings in mechanically ventilated patients who do not have one of these conditions.

Design and patients: We performed a retrospective chart review of all patients who had experienced out-of-hospital cardiac arrest and had received ventilatory support for ≥ 1 day at a university-affiliated county hospital during the years 1990, 1991, 1992, 1995, 1998, 1999, and 2000.

Results: In 139 such patients, the mean final VT values used on the first day of mechanical ventilation were 11.7, 12.4, 11.3, 9.6, 9.7, 9.2, and 9.8 mL/kg in those years, respectively. Multivariate analysis revealed that increasing year (ß-coefficient = –0.24; p = 0.001) and the presence of pulmonary edema (ß-coefficient = –1.2; p = 0.001) were independent predictors of the use of lower VT values. Patients managed with a low VT (ie, < 10 mL/kg; mean [± SD] VT, 8.4 ± 1.3 mL/kg) had a significantly higher incidence of atelectasis than the patients who were managed with traditional, larger VT values (ie, ≥ 10 mL/kg; mean VT, 11.8 ± 1.5 mL/kg) [61.1% vs 36.7%, respectively; p = 0.02]. Multivariate analysis revealed that the mean VT used on days 1, 2, and 3 (<10 mL/kg or ≥ 10 mL/kg) was the only predictor of the development of atelectasis during the first 3 days of mechanical ventilation (odds ratio, 0.33; p = 0.015). There was no difference in the incidence of pneumonia, the number of days spent receiving mechanical ventilation, PaO2/fraction of inspired oxygen ratio, or respiratory system compliance between the low VT group and the traditional VT group.

Conclusion: Currently, physicians at our hospital use lower VT values than they have in the past. This is associated with the increase in the incidence of atelectasis in the patients who received ventilation using low VT values.

Key Words: atelectasis • cardiac arrest • oxygenation • respiratory system compliance • tidal volume


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
In the past, most authorities recommended tidal volume (VT) values of 10 to 15 mL/kg for mechanically ventilated patients.1234 In the late 1980s and early 1990s, however, the concept of using lower VT values and permissive hypercapnia was introduced to prevent dynamic hyperinflation in patients with obstructive lung disease, and to prevent ventilator-associated lung injury in patients with acute lung injury and ARDS.567 We wondered whether the practice of using low VT values in patients with ARDS and obstructive lung disease also might have affected the VT values used in managing mechanically ventilated patients who did not have these conditions. Although current studies support the use of lower VT values in patients requiring mechanical ventilation for ARDS and obstructive disease, similar data do not exist in the ventilator management of patients with other causes of acute respiratory failure, and considerable variation in practice exists.8 In fact, some studies9101112 have reported a progressive decrease in respiratory system compliance (Ct) and impaired oxygenation in surgical patients during general anesthesia that can be attributed to partial lung collapse, which can be prevented with larger VT values. Relatively few data exist on how patients with acute respiratory failure have been ventilated over time. We hypothesized that the clinicians’ approach to the mechanical ventilation of patients with acute respiratory failure in general has been influenced by the data supporting lower VT ventilation for patients with ARDS and COPD, and that decisions to use lower VT values in patients with other causes of acute respiratory failure would be associated with adverse pulmonary sequelae due to atelectasis. To address this question, we examined ventilator settings and other clinical data from a retrospective cohort of patients without ARDS or obstructive lung disease who were seen over 11 years at a single institution.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusion
 References
 
Patient Population
Patients who had experienced out-of-hospital cardiac arrest were used as a representative sampling of the general population of mechanically ventilated patients. We selected this patient population because most of them had required mechanical ventilation at the time of hospital admission. Also, they were a homogeneous population of patients who had been managed by the medical ICU teams at our hospital, which made them suitable for comparing the change in practice over time. In addition, because their underlying cause of respiratory failure was either neurologic or cardiac, these patients were less likely to have received a diagnosis of acute lung injury or ARDS and to have been managed as having those conditions. Seattle Medic One, which is responsible for prehospital emergency care in our area, maintains a registry of all patients who have experienced out-of-hospital cardiac arrest who are brought to Harborview Medical Center, a county hospital and level I trauma center. All such patients who had been admitted to the ICU at Harborview Medical Center for ≥ 1 day during the study years of 1990, 1991, 1992, 1995, 1998, 1999, and 2000 were identified through a computer search of the Medic One registry. We included all patients who had received ventilation in a volume-targeted mode (ie, assist-control or intermittent mandatory ventilation, with or without added pressure support) for ≥ 1 day. Patients were excluded from the study if they met any of the following predefined exclusion criteria: (1) obstructive lung diseases, defined as patients who were listed in the hospital discharge summaries as having had cardiac arrest related to COPD or asthma, or patients who had an intrinsic positive end-expiratory pressure (PEEP) of > 5 cm H2O at any time during the first day of ventilatory support; (2) patients with a history of pneumonectomy; (3) patients who were documented in the hospital discharge summaries as having a neuromuscular disease such as spinal cord injury or myasthenia gravis; (4) patients who had cardiac arrest related to trauma or near-drowning; (5) patients who were documented in the hospital discharge summaries as having acute lung injury or ARDS; or (6) patients who were listed in the Harborview Medical Center ARDS registry, which includes all patients with ARDS seen in the ICU of this hospital since 1983.13 Patients in the ARDS registry were identified by daily ICU surveillance, and met the following diagnostic criteria: (1) PaO2/fraction of inspired oxygen (FIO2) ratio of ≤ 150 mm Hg, or ≤ 200 mm Hg PEEP of ≥ 5 cm H2O; (2) diffuse parenchymal infiltrates seen on a chest roentgenogram (ie, alveolar opacities in three or four lung quadrants); (3) pulmonary arterial wedge pressure (when available) of ≤ 18 mm Hg, or no evidence of congestive heart failure; and (4) no other obvious explanation for these findings. The study protocol was approved by the institutional ethics committee of the University of Washington. No informed consent was obtained, given that this retrospective data analysis epidemiologic study did not modify existing diagnostic or therapeutic strategies.

Data Collection and Definitions
The following information for each patient was collected by medical record review using a chart abstraction protocol. Chest radiograph reports were reviewed using a protocol. Actual chest radiographs were not reviewed, and the abstractor was not blinded to the year of hospitalization.

  1. Ventilator settings and lung mechanics data, including VT, PEEP, intrinsic PEEP, plateau pressure (PP), and Ct on days 1, 2, and 3 of mechanical ventilation, were obtained from respiratory therapy flow sheets. In general, ventilator checks at our hospital were performed at least once per shift for every patient as part of routine clinical care. Respiratory care department policy and procedures with respect to ventilator checks and data recording were not changed during the observation period. All of the VT values in this study were corrected for compressible volume. This was done consistently over the study period. Two sets of these measurements were recorded for day 1, the first values obtained following hospital admission and the final values, which were defined as constant values for VT, PEEP, and respiratory rate for at least 6 h. On day 2 and day 3, the values of these parameters at the time closest to 5:00 AM, the approximate time that routine daily chest radiographs were obtained. The VT values that were used for the longest duration on days 1, 2, and 3 also were collected.
  2. Oxygenation data were derived from the first value of the PaO2/FIO2 ratio (P/FO2) on day 1 (the first arterial blood gas measurement obtained while using the first VT values on day 1 after hospital admission), from arterial blood gas measurements while using final day 1 VT values (VTf), and from daily arterial blood gas measurements made at the time closest to 5:00 AM on days 2 and 3 while using VT values for days 2 and 3 (ie, P/FO2 for day 2 and P/FO2 for day 3).
  3. Information on atelectasis seen on the chest radiograph obtained on day 1 (ie, the first chest radiograph taken) and on days 2, 3, and 4 (ie, the routine daily morning chest radiograph) was obtained from the radiologist’s reports. The criteria for atelectasis were as follows: (1) no atelectasis, if there was no mention of atelectasis; or (2) atelectasis, if one specific term from among linear atelectasis, plate-like atelectasis, basilar atelectasis, atelectasis or pneumonia, atelectasis or aspiration, segmental atelectasis, lobar atelectasis, or lung atelectasis with no other differential diagnosis were used in a report. The incidence of new atelectasis was defined as no report of atelectasis on the first chest radiograph, but evidence of atelectasis on day 2, 3, or 4.
  4. The patients were counted as having developed pulmonary edema if the findings of their chest radiograph obtained on day 1, 2, 3, or 4 were reported as being compatible with pulmonary edema, pulmonary congestion, or heart failure for ≥ 2 days.
  5. The patients were counted as having had pneumonia if it was noted or concluded in the hospital discharge summaries that they had had pneumonia during their ICU stay.
  6. Outcome at ICU discharge including mortality and duration of mechanical ventilation.
  7. Age, gender, and body weight on day 1 of mechanical ventilation used. The patient’s weight was derived from the actual body weight measured on the first day of ICU admission.

Days 1, 2, and 3 were based on calendar days, and if patients were admitted to the hospital after 6:00 pm, the next calendar day was counted as day 1. If the patient was extubated or died, or if the mode of ventilatory support was changed to something other than a volume-targeted mode, the data collection and analysis were stopped at that point.

Statistical Analysis
Statistical analyses were carried out with a statistical software package (SPSS; SPSS; Chicago, IL). {chi}2 tests and t tests were used to compare groups on discrete and continuous variables, respectively. Linear regression was used to determine the relationship between increasing time and the VT used. Logistic regression was used to determine whether the VT used was independently associated with atelectasis. In addition, a Kaplan-Meier estimation was used to determine the probability of developing atelectasis over time. A p value of ≤ 0.05 was considered to be statistically significant. All p values were two-sided.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusion
 References
 
During the study years, there were 382 patients with out-of-hospital cardiac arrest who were admitted to the hospital for > 1 day. There were 72 patients whose medical records were not available for analysis, and 171 patients were excluded from the study by our a priori exclusion criteria. A total of 139 patients were included in the study. Table 1 shows the number and demographic characteristics of these patients for each study year.


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  		Table 1. Demographic Characteristics of the 139 Patients Included in the Study*

 
Mechanical Ventilation Settings and Atelectasis Over Time
Lower VT values tended to be used over time, as is shown in Figure 1 . The mean final VT (VTf) values per body weight (ie, VTf/kg) used on the first day of mechanical ventilation were 11.7, 12.4, 11.3, 9.6, 9.7, 9.2, and 9.8 mL/kg, respectively, for the years 1990, 1991, 1992, 1995, 1998, 1999, and 2000. This was accompanied by a trend to increase the percentage of patients who use PEEP over time. In 1990 and 1991, none of these patients received treatment with PEEP on the first day of mechanical ventilation. In contrast, all of the patients from 1998 to 2000 received PEEP, as is shown in Figure 2 . Univariate analysis demonstrated that increasing values for year (p < 0.001), the presence of pulmonary edema (p = 0.026), and PEEP used on the first day of mechanical ventilation (p < 0.001) were associated with lower VT values. In a multivariate analysis, only year and the presence of pulmonary edema were statistically significantly associated with VT after adjustment for PEEP (ie, whether it was used or not), age, sex, outcome (ie, dead or alive), initial PP, initial Ct, initial P/FO2, and initial arterial PCO2 (Table 2 ). In a multivariate analysis, year was the only independent predictor of the use of PEEP on the first day of mechanical ventilation (adjusted odds ratio, 1.8; 95% confidence interval, 1.4 to 2.2), after adjustment for the presence of pulmonary edema, VT, the day 1 P/FO2, age, sex, and outcome (ie, dead or alive).



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  		Figure 1. Mean final VT used on the first day of mechanical ventilation (VTf/kg) in each study year. Each dot represents the mean VT (mL/kg). Error bars represent the 95% confidence interval of the mean.

 


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  		Figure 2. Changing in the level of the final PEEP used on the first day of mechanical ventilaton in each study year. open bars = percentage of patients who received no PEEP (of the number of patients in each study year); closed bars = percentage of patients who received PEEP of ≥ 5 cm H2O (of the number of patients in each study year).

 

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  		Table 2. Independent Predictors of Using Lower VTf Over the Study Years by Multivariate Analysis*

 
The incidence of atelectasis, estimated by the percentage of patients who developed new atelectasis on a chest radiograph during the first 3 days of mechanical ventilation, increased over time, from 33.3% in 1990 to 63.2% in 2000, as is shown in Figure 3 .



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  		Figure 3. Incidence of new atelectasis during the first 3 days of mechanical ventilation. Each bar represents the percentage of atelectasis of the number of patients in each study year.

 
Mechanical Ventilation Settings and Outcome
The incidence of new atelectasis during the first 3 days of mechanical ventilation also appeared to be higher when the VT used was lower. The incidence of atelectasis was 43% when the mean VT used during the first 3 days of mechanical ventilation was > 11 mL/kg, and it increased to 58% when the VT used was < 9 mL/kg. However, this correlation was not made in a dose-response manner, as is shown in Figure 4 . We stratified the patients according to their VT values used during the first 3 days of receiving mechanical ventilation to be in the low-VT group when the VT used was < 10 mL/kg and to be in the traditional VT group when the VT used was > 10 mL/kg (the median VT in this patient population). The mean (± SD) VT values used were 8.4 ± 1.3 mL/kg in the low-VT group and 11.8 ± 1.5 mL/kg in the traditional VT group. The incidence of atelectasis was significantly higher in the low-VT group (Table 3 ). The median time to the development of atelectasis, estimated by Kaplan-Meier survival analysis, was significantly shorter in the low-VT group (low-VT group, 2 days; traditional VT group, > 3 days [p = 0.01 by log rank test]). After 3 days of mechanical ventilation, the probability of developing atelectasis was 0.75 in the low-VT group and 0.44 in the traditional VT group, as is shown in Table 3 and Figure 5 , top, A. When looking at the extent of atelectasis, the incidence of lobar atelectasis also tended to be higher in the low-VT group, but the difference was not statistically significant.



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  		Figure 4. The incidence of new atelectasis during the first 3 days of mechanical ventilation according to the VT used (ie, the mean of VT values that were used for the longest time on days 1, 2, and 3 of mechanical ventilation). Each bar represents the percentage of patients with atelectasis of the number of patients in each VT group.

 

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  		Table 3. Demographic Characteristics and Outcomes of Patients Receiving Low VT and Traditional VT*

 


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  		Figure 5. Top, A: probability of not developing new atelectasis over the first 3 days of mechanical ventilation in all patients. There was a significant difference (p = 0.01 [by log rank test]) between the low-VT group and the traditional VT group. Bottom, B: probability of not developing new atelectasis over the first 3 days of mechanical ventilation in the subgroup of patients who never used PEEP (low-VT group, 10 patients; traditional VT group, 28 patients). There was a significant difference (p = 0.02 [by log rank test]) between the low-VT group and the traditional VT group.

 
Because PEEP may prevent the development of atelectasis, an analysis of the subgroup of patients who had never received PEEP treatment during the entire 3 days of mechanical ventilation was performed, and it revealed that the incidence of atelectasis was higher and the time to develop atelectasis was shorter in the low-VT group, as is shown in Table 3 and Figure 5, bottom, B. Univariate analysis demonstrated that the mean VT value used (ie, < 10 mL/kg or ≥ 10 mL/kg) was the only predictor of developing new atelectasis (odds ratio, 0.37; 95% confidence interval, 0.17 to 0.79; p = 0.01). The mean VT used was still the only independent predictor of developing new atelectasis after adjustment for study year, outcome, age, sex, the presence of pulmonary edema, PEEP used, the number of days receiving mechanical ventilation, and the mean FIO2 value used during the study period (ie, mean of FIO2 values for day 1 [both the initial measurement and the final measurement], day 2, and day 3), as is shown in Table 4 . There was no difference in the incidence of pneumonia and the number of days spent receiving mechanical ventilation in patients who survived.


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  		Table 4. Independent Predictors of Developing New Atelectasis During the First 3 Days of Mechanical Ventilation by Multivariate Analysis*

 
Oxygenation and Ct
The initial PaO2/FIO2 ratio and FIO2 values were not different in patients with a VT of < 10 mL/kg and those with a VT of > 10 mL/kg. However, the final FIO2 value used on the first day was significantly higher in the low-VT group, with a significantly higher proportion of patients with an FIO2 of ≥ 0.6. There was also a trend toward a lower PaO2/FIO2 ratio and a higher FIO2 in the low-VT group on day 2. In addition, more PEEP was used in patients who were managed with smaller VT values, as is shown in Table 5 . The PP on day 2 was significantly higher in the low-VT group, but there was no difference in the proportion of patients who had PPs of ≥ 30 cm H2O, as is shown in Table 6 .


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  		Table 5. Comparison of Oxygenation Between Patients Receiving Low VT and Traditional VT*

 

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  		Table 6. Comparison of Ct and PP Values Between Patients Receiving Low VT and Traditional VT*

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusion
 References
 
During the past 15 years, there have been remarkable changes in the concept and practice of mechanical ventilatory support. Based on evidence from many studies, it has been widely accepted that patients with acute lung injury, and also those with obstructive lung diseases, should receive ventilation with a lower VT than that used in the past to prevent ventilator-induced lung injury and dynamic hyperinflation, respectively. Most critical care practitioners, or at least those who have been trained in the last decade, are familiar and feel comfortable with the use of low VT values.1415 However, there is no evidence from clinical trials to establish the optimal VT for the large proportion of patients with acute respiratory failure who do not have either acute lung injury or obstructive lung disease. In this study, we explored how clinicians have changed their approach to mechanical ventilation in this large group of patients, and it was shown that, currently, physicians at our hospital use lower VT values than those used in the past. This was associated with an increase in the incidence of atelectasis. The use of PEEP also changed over time. Currently, a PEEP of 5 cm H2O is applied to almost all patients receiving mechanical ventilation, regardless of diagnosis, from the first day of ventilatory support. This appears to be a true behavioral change, as in the multivariate analysis only changing the year was an independent predictor of using PEEP. Because both VT values and the use of PEEP changed over time during the period of our study, our observed outcomes are in fact associated with both a low VT (mean, 8.4 mL/kg) and PEEP, compared with the traditional VT (mean, 11.8 mL/kg) used without PEEP.

Several studies have explored the practice of mechanical ventilation across large patient populations, but none have had the opportunity to explore it over a long period of time. Esteban and coworkers14 reported an international utilization review of how mechanical ventilation was employed in a sampling of ICUs in North America, South America, Spain, and Portugal, in which the median VT used was 9 mL/kg and was similar in the different countries. In that study, the VT did not differ between patients with ARDS and those with other causes of acute respiratory failure. Likewise, Luhr and coworkers15 reported in a study designed to determine the incidence of acute respiratory failure and ARDS, and the mortality rate of patients with those conditions, in Sweden, Denmark, and Iceland that the mean VT used in patients with acute respiratory failure was 8.3 mL/kg, with no difference between patients with ARDS and those not fulfilling ARDS diagnostic criteria. However, these two surveys were single-point prevalence studies, without comparison made to the previous practice. Our study is the first to demonstrate that in the same institution, physicians currently use lower VT values than those used in the past in non-ARDS and non-obstructive lung disease patients, and that demonstrable adverse effects are associated with these changes.

Effect of Low VT on the Incidence of Atelectasis
The incidence of atelectasis appeared to increase over time. Also, the incidence of atelectasis seemed to increase when the VT used was lower, although the correlation between the incidence of atelectasis and the level of VT used was not made in a dose-response manner. Because there were several potential confounding factors for developing new atelectasis, such as the level of PEEP used,161718 we used multivariate analysis to demonstrate that the level of VT used was an independent predictor of developing new atelectasis after adjusting from other potential confounding factors. We chose a VT of 10 mL/kg to be the cutoff value, because this level of VT is generally recommended to be used in most mechanically ventilated patients and, coincidentally, was the median VT of this study population. The incidence of atelectasis during the first 3 days of treatment with mechanical ventilation in the patients who used the traditional VT (ie, ≥ 10 mL/kg) was 36.7%. The reported incidence of atelectasis in mechanically ventilated patients varies from 4.5 to 85%.19202122 This wide variation in the incidence of atelectasis was due to the difference in the patient population, the definition of atelectasis, and the duration of observation. Therefore, it is difficult to compare the incidence of atelectasis in our study with those in these studies. However, we used the radiologist’s reports at one institution as the criterion for diagnosing atelectasis, and the patients in our study were a homogeneous population. The follow-up time was similar in both groups. Also, respiratory therapy management and respiratory therapy staffing patterns that may have affected the incidence of atelectasis did not change during the study period. Therefore, the difference in the incidence of atelectasis between the traditional VT group and the low-VT group was most likely a real difference. There might be some changes in the radiographic techniques and in how the radiologists interpreted these chest radiographs over the long study period. However, from the multivariate analysis, the study year was not an independent predictor of developing atelectasis, which made the changes in the radiographic techniques used and their interpretation less likely to confound our results.

Interobserver variability in the assessment of atelectasis is well documented.23 Nevertheless, we diagnosed atelectasis retrospectively based on radiograph reports of which radiologists were blinded to ventilator settings. There should be no biased interpretation of atelectasis regarding the VT used. Another possible explanation for the difference in the incidence of atelectasis as reported by the radiologists was the effect of lung volume change on the interpretation of airspace disease seen on the chest radiograph. Ely and coworkers24 studied the relationships between the parameters of mechanical ventilation and the interpretation of the severity of airspace diseases seen on portable chest radiographs. Airspace disease was considered to be more severe with pressure support ventilation breaths than with intermittent mandatory ventilation breaths, and increasing VT was associated with an increase in lung length. Therefore, using a low VT may increase the chance of the patient’s condition being interpreted as airspace disease, including atelectasis. Nevertheless, if this were the case, we would also expect the incidence of pulmonary edema, which also was diagnosed by radiologists’ reports, to be significantly higher in the low-VT group. In addition, the incidence of atelectasis increased over time from day 1 to day 3 in both groups, and increased more rapidly in the low-VT group. These findings do not support the hypothesis that different VT values altered the interpretation of the chest radiographs.

Although the incidence of lobar atelectasis was not significantly different between the low-VT group and the traditional VT group, there was an obvious trend for a higher incidence of lobar atelectasis in the patients who used low VT values. Because of the limitations of a retrospective study, we do not know how often a therapeutic intervention such as bronchoscopy or chest physical therapy was required to manage these patients with lobar atelectasis. The duration of observation of the incidence of atelectasis in this study was only 3 days. It remains to be established whether the low VT value used may be associated with an increase in the incidence of lobar atelectasis and the requirement of therapeutic intervention in other populations of patients with lower mortality and a longer requirement of ventilatory support than the out-of-hospital cardiac arrest patients in this study.

Effect of VT Used on Incidence of Pneumonia, Duration of Mechanical Ventilation, Oxygenation, and Ct
Several early studies9101112 demonstrated that mechanical ventilation with a normal to low VT, without periodic deep breaths, was associated with a progressively increased alveolar-arterial oxygen difference, hypoxemia, and decreased pulmonary compliance. These changes were attributed to progressive atelectasis. All of these studies were performed in anesthetized patients who also received muscular relaxation drugs. So far, however, no study on the effects of low-VT ventilation in general critically ill patients has been reported. Such patients are different from anesthetized surgical patients in many aspects. Our patients were not anesthetized or paralyzed, and received occasional suctioning together with manual bag ventilation, which may have made them less likely to develop atelectasis. Nevertheless, these patients were more critically ill and required longer mechanical ventilatory support. Therefore, atelectasis may affect these patients more than would be the case with healthy surgical patients. Atelectasis compromises lung mechanics, gas exchange, and host defenses.252627 Drinkwater and coworkers28 showed that the clearance of Streptococcus pneumoniae was decreased in the presence of atelectasis, which may therefore predispose the patient to the development of pneumonia.27 We did not find an increase in the incidence of pneumonia in patients ventilated with a low VT. However, there are some limitations in interpreting this finding. We diagnosed pneumonia retrospectively by hospital discharge summaries, and we are unable to know exactly when the pneumonic process first occurred. The mean number of days of mechanical ventilation among the patients in this study was around 4.5 days. Therefore, most of these pneumonias were early-onset and most likely represented aspiration,29 the pathogenesis of which may not relate to atelectasis.

Likewise, we did not find a difference in the number of ventilator days between the low and traditional VT groups. Peterson and coworkers30 retrospectively reviewed the medical records of patients with C3-C4 tetraplegia, comparing the outcomes of patients who used large VT values (ie, > 20 mL/kg) and lower VT values (ie, < 20 mL/kg). These investigators found that patients in the large-VT group had significantly less atelectasis and successfully weaned from mechanical ventilation faster than did patients in the lower VT group (37.6 vs 58.7 days, respectively; p = 0.02). The post-cardiac arrest patients in this study had a much higher mortality rate and a shorter time spent receiving mechanical ventilation than did the tetraplegic patients. This may obscure the impact of the low VT used and atelectasis on the duration of mechanical ventilation.

The effect of low VT on oxygenation and Ct was not clinically significant. This was much less than the changes reported in anesthetized patients.9101112 Besides the difference between the patients in this study and the anesthetized patients, as stated above, another possible explanation for this discord, which we believe to be the most important aspect, is the fact that most of the low-VT patients in this study were receiving PEEP treatment. It has been demonstrated in several reports161718 that PEEP might prevent atelectasis and hypoxemia. This was supported by the finding that the patients who had never received PEEP treatment had a higher incidence of atelectasis and a shorter time to the development of atelectasis. In this study, the patients who used low VT values also used significantly higher PEEP levels than did the patients who were managed with traditional VT values on every day of observation. We cannot tell exactly whether this was due to the behavioral change of routinely using PEEP or whether PEEP was used as a measure with which to correct hypoxemia associated with low VT by the physicians managing these patients.

There are a number of limitations to our study. We could not obtain the medical records of all the patients on our list. We chose the study years based on the knowledge that the concept of using lower VT values was introduced in the late 1980s. Also, we found in a previous study of VT used in ARDS patients at our hospital31 that physicians started using lower VT values in 1990, and that they decreased further in 1995. Using the prospectively collected Harborview ARDS database, we analyzed initial VT settings on the day that ARDS diagnostic criteria were met for the time periods 1983 to 1985 (150 patients), 1990 (110 patients), and 1994 to 1995 (210 patients). After adjusting for age, gender, and race, there was a significant 13% decline in VT from 835 to 810 to 725 mL (p < 0.0001 for trend). We included the years 1991, 1992, 1998, 1999, and 2000 to confirm that this was a real change over time and not a secular trend of change among different years, and also to increase the power to detect adverse effects associated with these changes. The availability of the medical records from 1990 was low at only 40%. However, there is no reason to think that the patients whose medical records were not available were managed with different VT values than those whose medical records were available. This is confirmed by the fact that in the years 1991 and 1992, from which medical records were much more readily available, the VT values used were not different from those used in 1990. Also, we used multivariate analysis to control for other potential confounding factors to assure that changing years was an independent predictor of using lower VT values. Because of the retrospective nature of this study, we could not explain why the death rate of the study population was obviously higher during the years 1998 to 2000. However, from the multivariate analysis, the outcome (ie, dead or alive) was not an independent predictor of using lower VT values or developing atelectasis. Therefore, the difference in death rate among different study years would not confound the main results (ie, the changing of the VT used and atelectasis) of this study.


   Conclusion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
References
 
Despite the lack of evidence of benefit in this patient population, the trend at our hospital is to use lower VT values and more PEEP in the treatment of patients who have been intubated for cardiac arrest. Physicians appear to be extrapolating the lessons learned from the treatment of ARDS and COPD patients to the treatment of this patient population. This practice has been associated with an increase in the incidence of atelectasis but does not appear to be associated with pneumonia or duration of mechanical ventilation. Although PaO2/FIO2 ratio and Ct were also not different between these two VT groups, the patients in the low-VT group received significantly higher levels of PEEP on all 3 days of observation. It remains to be established whether the increase in atelectasis may be associated with an increase in the incidence of ventilator-associated pneumonia and the number of ventilator days in other populations of patients with lower mortality and a longer requirement of ventilatory support than the out-of-hospital cardiac arrest patients investigated in this study.


   Acknowledgements
 
The authors would like to express their appreciation to Leonard A. Cobb, MD, and Carol E. Fahrenbruch, of Seattle Medic One, Harborview Medical Center, University of Washington, for providing the list of the patients in this study.


   Footnotes
 
Abbreviations: Ct = respiratory system compliance; FIO2 = fraction of inspired oxygen; PEEP = positive end-expiratory pressure; P/FO2 = PaO2/fraction of inspired oxygen ratio; PP = plateau pressure; VT = tidal volume; VTf = final day 1 tidal volume value

Pulmonary and Critical Care Medicine Division, Harborview Medical Center, University of Washington, Seattle, WA.

Received for publication August 14, 2003. Accepted for publication April 26, 2004.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 

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