Introduction

Mechanical ventilation (MV) is routinely applied in critically ill patients. However, life-saving MV frequently induces injury to the lungs, known as ventilator-induced lung injury. Alveolar overdistension and repetitive opening and closing of atelectatic lung units are thought to play a causal role of ventilator-induced lung injury in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) [1]. To test this hypothesis investigators have used lung-protective MV strategies in patients with ALI/ARDS, applying lower tidal volumes [2] and/or sufficient levels of positive end expiratory pressure (PEEP) [3]. The ARDS Network trial confirmed that MV with lower tidal volumes significantly decreases mortality and increases the number of ventilator-free days. Recently published data also suggest that non-ALI/ARDS patients benefit from lower tidal volumes [4].

Many medical advances enter clinical practice only slowly. Although clinicians in teaching hospitals are often early supporters of new medical advances, several reports show reluctant use of lung-protective MV [5, 6, 7]. One reason for not using lower tidal volumes in non-ALI/ARDS patients is poor physician recognition of patients with ALI/ARDS [8, 9]. Other barriers to the use of lung-protective MV include concerns over patient discomfort and tachypnea as well as hypercapnia, acidosis, and hypoxemia [8]. In addition, the importance of using predicted body weight (PBW) instead of actual body weight (ABW) may have been neglected [10].

The aim of this observational study, the preliminary findings of which we have presented previously [11] was to analyze tidal volume settings in MV patients in three “closed format” intensive care units in The Netherlands. To determine the effect of feedback and education concerning use of lung-protective MV we studied tidal volume settings after 6 months in the same centers. In addition, to determine the long-term effect of our intervention we studied tidal volumes in an additional set of patients 1 year later in one center.

Methods

Study centers

We studied MV settings in one academic ICU [Academic Medical Center (AMC) in Amsterdam, The Netherlands] and two nonacademic ICUs [Gelre Hospital (GH) location Lukas in Apeldoorn, The Netherlands; Medical Center Leeuwarden (MCL) in Leeuwarden, The Netherlands]. The AMC is a large teaching hospital where the ICU is a 28-bed “closed format” department in which medical/surgical patients (including cardiothoracic and neurosurgical patients) are under the direct care of the ICU team. As part of the team nurses can make ventilator therapy recommendations, but unit policy mandates that all changes in MV settings be ordered by physicians. The ICU team comprises 8 full-time intensivists, 8 subspecialty fellows, 12 residents, and occasionally one intern. The GH is an affiliated teaching hospital where the ICU is a 10-bed “closed format” department. In this hospital nurses can make ventilator therapy recommendations and changes. Here, in contrast to the ICU of the AMC, physicians are only present at night on request. The ICU team consists of 2 full-time intensivists, 5 part-time intensivists who participate in duty shifts, and one resident. The MCL is a large general teaching hospital in the north of The Netherlands, with a 13-bed “closed format” mixed ICU. Here nurses can make ventilator therapy recommendations, although unit policy mandates that changes in MV settings be ordered by physicians. The ICU team comprises 6 full-time intensivists, 2 residents, and occasionally interns.

MV protocol

In the AMC a written MV protocol is available for all ICU members, both on the intranet and in printed form. Briefly, this protocol advises pressure-controlled (PC) MV or pressure-support (PS) MV in all patients. Levels of positive end-expiratory pressure (PEEP) are adjusted to PaO2 levels. Use of prone positioning is recommended in patients requiring injurious FIO2 levels (>0.6). There is a clear recommendation on tidal volume settings that tidal volumes be between 6 and 8 ml/kg PBW in all patients, irrespective of the presence of ALI/ARDS (the upper limit of 8 ml/kg PBW was chosen based on the ARDS Network protocol [2], in which tidal volumes were allowed the be as high as 8 ml/kg PBW in some circumstances). The protocol states that “mild hypercapnia” is accepted in patients with ALI/ARDS (but no limits for PaCO2 are given). In the case of PS MV the level of support is set to reach a respiratory rate between 10 and 20 breaths/min or a level at which the patient is comfortable breathing, according to patient and/or physician vision. However, use of lower tidal volumes takes precedence over respiratory rate, allowing higher respiratory rates. No written MV protocol is available in the other two hospitals.

Patients and data collection

The study included 114 consecutive, intubated MV subjects aged 18 years or over, divided into two groups: 50 before intervention (31 men, 19 women) and 64 after (37 men, 27 women; (Table 1). In a third data collection period 12 months after intervention 103 patients were examined at the AMC. MV settings were collected over a period of 2 weeks at three different times during the day (8 a.m., 2 p.m., 8 p.m.) and were recorded by one intensivist in each hospital and one of the authors (E.W.); other intensivists, fellows, and residents were unaware of the collection of data. Measurements were performed directly before and approximately 6 months after an educational program; data were collected at the AMC in June 2003 and January 2004, at the GH in October 2003 and April 2004, and at the MCL in November 2003 and May 2004. The third data-collection period was in September 2004 at the AMC; tidal volume settings were analyzed to determine long-term effects of the intervention.

Table 1 Patient characteristics in an academic ICU (Academic Medical Center) and two nonacademic ICUs (Gelre Hospital, Medical Center Leeuwarden), before and after education (APACHE II Acute Physiology And Chronic Health Evaluation II, SAPS II Simplified Acute Physiology Score II, IQR, interquartile range; ABW actual body weight, PBW predicted body weight)
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The following MV data were recorded: MV mode, FIO2, applied tidal volume, PEEP level, maximum airway pressure (Pmax) in PC-MV or plateau pressure in volume-controlled ventilation, and respiratory rate (breaths/min). Tidal volumes were not corrected for ventilator tubes or ventilator types, and only exhaled volumes were recorded. Patient’s height was measured, and weight was taken from the medical record; if no weight was recorded, weight used by the nursing team was taken as ABW. Arterial blood gases were recorded; the sample closest to the observation was taken for each set. ALI/ARDS (at any time during the MV episode) was diagnosed by one intensivist in every hospital and one of the authors (E.W.) using consensus criteria for ALI/ARDS: acute onset of respiratory failure, ratio of partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2/FIO2) of 300 mmHg or less, and bilateral infiltrates on chest radiography [12].

Definitions

Actual body weight was defined as the body weight of patient measured before admission to the hospital or ICU. Predicted body weight (PBW) was calculated by the following formula: in men, PBW (kg)=50+0.91 (centimeters of height–−152.4); in women, PBW (kg)=is 45.5+0.91 (centimeters of height–−152.4) [13]. Applied tidal volume was defined as expiratory volume measured by the ventilator. The tidal volume, expressed as milliliters/kilogram PBW, was calculated by dividing applied tidal volume by PBW for each patient.

Feedback and education

At the AMC the intervention consisted of: (a) a presentation of the results of the first data collection to all ICU-physicians (“feedback” on current practice); (b) a discussion on studies using lower tidal volumes and on potential reasons for not using lower tidal volumes, including the importance of using PBW instead of ABW to set tidal volumes (“education”); and (c) a demonstration of the results of the first data collection to all ICU nurses. At the GH a similar intervention took place, and a MV protocol was introduced stating that tidal volumes should be between 6 and 8 ml/kg PBW. At the MCL only feedback to the physicians of the first data collection took place, and the importance of using PBW to set tidal volumes was emphasized, but no written protocol was introduced.

Statistical analysis

After we determined that MV settings on day 1 did not differ from average data for all respiratory variables we chose to use the mean of all respiratory variables per patient for analysis and presentation of data. Statistical analysis of continuous data used the Kruskal-Wallis test and post-hoc analysis with the Mann-Whitney U test; for categorical data the χ2 test was used. Data are presented as mean ±SD or as median (with interquartile range). Differences with a p value less than 0.05 were considered significant.

Results

Tidal volumes

Patients without ALI/ARDS showed a decline in tidal volumes from 9.7±1.9 ml/kg PBW before the intervention to 8.0±1.7 ml/kg PBW thereafter (p<0.0001; Fig. 1). The proportion of non-ALI/ARDS patients with tidal volumes above 8 ml/kg PBW declined from 88% to 49% at the AMC (p=0.007) and from 78% to 40% at the GH and the MCL (p=0.011). The proportion of patients with tidal volumes above 10 ml/kg PBW declined from 41% only 9% at the AMC (p=0.008) and from 33% to 10% at the GH and the MCL (p=0.023). Patients with ALI/ARDS showed a decline from 9.9±2.2 ml/kg PBW before the intervention to 8.2±1.8 ml/kg PBW thereafter (p=0.013; Fig. 2). The number of observations in which tidal volumes were above 8 ml/kg decreased from 85% to 42% at the AMC (p=0.03) and from 82% to 67% at the GH and the MCL (p=0.45). The percentage of patients with tidal volumes above 10 ml/kg PBW declined from 39% to 8% at the AMC (p=0.08) and from 73% to 22% at the GH and the MCL (p=0.028).

Fig. 1
figure 1

Tidal volumes declined in non-ALI/ARDS patients after intervention. Tidal volumes measured before (closed symbols) and after intervention on the use of lung-protective mechanical ventilation (open symbols) in one academic and two nonacademic centers. Each dot mean tidal volume of an individual patient; tidal volumes were collected three times per day for 2 weeks; horizontal lines median tidal volumes; area between dotted lines target tidal volume (as described in the mechanical ventilation protocol)

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Fig. 2
figure 2

Tidal volumes declined in ALI/ARDS patients after intervention. Each dot mean tidal volume of an individual patient; tidal volumes were collected three times per day for 2 weeks; horizontal lines median tidal volumes

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Tidal volume based on predicted and actual body weight (before intervention)

Mean PBW (63.6±10.6 kg) was 15% less than mean ABW (76.4±14.2 kg). When tidal volumes were adjusted to ABW instead of PBW, tidal volumes were above 8 ml/kg in 43% of cases at the AMC and in 81% at the GH and the MCL, and above 10 ml/kg in 17% at the AMC and in 38% at the GH and the MCL.

Ventilator modes, PEEP and P-max

PC or PS modes of MV were most often used in all centers; adaptive support ventilation (as provided by Hamilton Galileo) was used in a significant number of patients at MCL (Table 2). Levels of PEEP, Pmax, respiratory rate, and ventilator mode are presented in Table 2. Pmax was higher than 30 cmH2O in 19% of observations before education, with no difference between ALI/ARDS and non-ALI/ARDS patients, and in 8% after the intervention (p=0.1).

Table 2 Mechanical ventilation data in an academic ICU (Academic Medical Center) and two nonacademic ICUs (Gelre Hospital, Medical Center Leeuwarden) before and after education (ASV adaptive-support ventilation, VCV volume-controlled ventilation, PSV pressure-support ventilation, PCV pressure-controlled ventilation, Pmax maximum airway pressure, Pplat plateau airway pressure)
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pH and PaCO2 levels

Before the intervention pH and PaCO2 levels in ALI/ARDS and non-ALI/ARDS patients did not differ; hypercapnia (PaCO2 ≥50 mmHg) occurred in only 2% of patients. After the intervention hypercapnia occurred in 13% of patients (p=0.04), with no difference between ALI/ARDS patients and non-ALI/ARDS patients

Relationship between mode of ventilation and tidal volume

To determine whether tidal volumes vary with the MV mode used in the ICU we differentiated those were receiving MV with PS and those receiving it with PC. No differences in tidal volumes were seen between the two groups, either before or after the intervention (Fig. 3).

Fig. 3
figure 3

Tidal volumes during spontaneous mechanical ventilation (pressure support, PS) or mandatory mechanical ventilation (pressure control, PC) were similar after intervention. Each dot mean tidal volume of an individual patient; horizontal line median tidal volume per group; NS not significant

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Long-term results

In the data collected at the AMC 1 year after intervention tidal volumes were 7.8±1.3, compared to 8.1±1.7 ml/kg PBW 6 months previously (p=0.27). These tidal volumes 6 and 12 months after the intervention were significantly lower than those before intervention (p<0.0001). Tidal volumes were 7.5±1.1 and 7.9±1.3 ml/kg PBW in those with and without ALI/ARDS patients, respectively (Fig. 4).

Fig. 4
figure 4

Long-term results. Tidal volumes 12 months after intervention in the academic center. Open symbols non-ALI/ARDS patients; closed symbols ALI/ARDS patients; each dot mean tidal volume of an individual patient; horizontal lines median tidal volumes per patient group

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Discussion

A valuable advance in the field of MV has been the clear demonstration that use of lower tidal volumes, rather than conventional MV using higher tidal volumes, significantly reduces mortality in patients with ALI/ARDS [2]. Other so-called lung-protective MV approaches such as use of PC MV, optimal PEEP, recruitment maneuvers, prone positioning, high-frequency oscillation, and permissive hypercapnia have not proven effective. Although guidelines support the use of lower tidal volumes in ALI/ARDS patients [14], physicians have been slow to adopt lung-protective MV using lower tidal volumes. The present study shows that an educational program showing actual daily practice to all ICU team members, emphasizing the use of PBW instead of ABW to set tidal volumes, can significantly reduce tidal volumes.

The results of our study are in accord with those of other investigators. Several published and unpublished studies have shown the reluctance of physicians and respiratory therapists in applying lung-protective MV using lower tidal volumes [6, 7, 15]. Reported reasons for not using lower tidal volumes are diagnostic uncertainty [9] and unwillingness to use lower tidal volumes [8]. Importantly, many ICU physicians believe that normal tidal volumes are greater than they actually are [16]. In addition, use of ABW instead of PBW to calculate tidal volumes may result in excessively high tidal volumes [10]. Our results highlight the importance of using PBW instead of ABW to calculate tidal volumes. Indeed, we found a substantial difference between ABW and PBW values. Tidal volumes were significantly lower when applied tidal volumes were expressed as milliliters/kilogram ABW. Others have reported various strategies to influence MV practices [17, 18]. Roche and coworkers [18] successfully used an intensive educational program encouraging an active role for ICU nurses and simple protocols written to support increased involvement of nursing staff in the set-up and adjustment of mechanical ventilators. It must be mentioned, however, that the report of these findings did not state whether tidal volumes were adjusted to PBW (i.e., if ABW was used for calculation of tidal volumes, these volumes may still have been too high).

Providing feedback on previous practice has been shown to be more effective than simple education [19]. Recently Cook and coworkers [20] have emphasized the importance in improving daily practice of using an environment scan (to establish the extent to which a prescribed strategy is being used) and understanding current behavior (to determine why a strategy is not used). We performed an environment scan by analyzing current MV settings in our ICUs (showing underutilization of lung-protective MV, using lower tidal volumes) and tried to understand why this happened (discussion on potential reasons for not using lower tidal volumes, illuminating that ABW instead of PBW was used to calculate tidal volumes). We then, successfully, targeted behavior by emphasizing the significance of use of lower tidal volumes, as well as the importance of using PBW to set the ventilator. Using this approach, within 6 months tidal volumes declined significantly. Importantly, after 12 months lower tidal volumes were still being used.

Although tidal volumes declined after intervention, in a number of observations levels were still above 8 ml/kg PBW. In addition, after intervention there was still residual variation. Several factors may explain these findings. First, in our experience, a number of patients have tidal volumes above 8 ml/kg PBW with the lowest level of pressure support (i.e., <5 cmH2O). Second, there may have been reasons for applying larger tidal volumes (e.g., severe acidosis). A wide variation in tidal volumes has also been noted by others [4, 7]. Results of scientific trials show that unwanted variation remains common in clinical care, and in our study it may also have been caused by unwillingness of the ICU team members to strictly apply to the MV protocol. However, our current analysis does not provide sufficient data to confirm this. Nevertheless, we do think that improving MV settings is a continuous process: new (probably similar) interventions are needed to bring about a further decline in tidal volumes.

It may seem surprising that we advocate the use of lower tidal volumes in all MV patients irrespective of the presence of ALI/ARDS. A recent study, however, shows that patients admitted for other reasons than ALI/ARDS and ventilated with higher tidal volumes have an increased incidence of late-onset ALI/ARDS [4]. In addition, animal studies have shown that lung overdistension induces lung injury [21]. Indeed, MV with relatively high tidal volumes can induce ventilator-induced lung injury in animals with healthy lungs [22, 23]. Although additional studies must be performed to determine whether lower tidal volumes truly benefit patients without ALI/ARDS, at present we also use lower tidal volumes in these patients.

The only two clinical trials reporting mortality benefit from lung protective ventilation in ALI targeted either tidal volumes less than 6 ml/kg PBW [2], with the allowance of using tidal volumes up to 8 ml/kg PBW in some circumstances, or tidal volumes less than 6 ml/kg ABW [3]. However, our protocol prescribed tidal volumes between 6 and 8 ml/kg PBW, and therefore, at least in patients with ALI/ARDS, our intervention may not be sufficient to improve lung-protective MV.

From the present findings it cannot be concluded which part of the intervention had the greatest impact. Although we consider feedback on current practice and discussions on new insights on MV and potential barriers to introduce results from trials of importance, it may be that simply a discussion on MV would have caused a change in practice. Further, one must bear in mind that any change observed may have been due to other factors than the intervention. It can be suggested that enough time had passed with further debate in the literature, and locally that tidal volumes would have been declined in any case.

In conclusion, after feedback and education ICU physicians prescribed lower tidal volumes more frequently. Our findings underscore the use of PBW instead of ABW to calculate tidal volumes.