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CRITICAL CARE AND TRAUMA

Determinants of Tidal Volumes with Adaptive Support Ventilation: A Multicenter Observational Study

Dave A. Dongelmans^*, Denise P. Veelo^*, Alexander Bindels, Jan M. Binnekade^*, Kees Koppenol, Matty Koopmans, Joke C. Korevaar^||, Michael A. Kuiper^*¶, and Marcus J. Schultz^*¶#

From the *Department of Intensive Care Medicine, Department of Anesthesiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands; Department of Intensive Care Medicine, Catharina Hospital Eindhoven, Eindhoven, The Netherlands; Department of Intensive Care Medicine, Medical Centre Leeuwarden, Leeuwarden, The Netherlands; ||Department of Clinical Epidemiology and Biostatistics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands; ¶HERMES Critical Care Group, Amsterdam, The Netherlands; and #Laboratory for Experimental Intensive Care and Anesthesiology (L.E.I.C.A), Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.

Address correspondence and reprint requests to Dave A. Dongelmans, Department of Intensive Care Medicine, C3–415, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. Address e-mail to d.a.dongelmans{at}amc.uva.nl.

	Abstract

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INTRODUCTION: In the present study, we investigated the behavior of adaptive support ventilation (ASV) in patients after cardiothoracic surgery. We determined tidal volumes (Vt) and factors that influence Vt with this mode of microprocessor-controlled mechanical ventilation (MV).

METHODS: This was a prospective, multicenter, observational study in three Dutch intensive care units over a 5-mo period. MV data were collected during steady-state after arrival in the intensive care unit.

RESULTS: Data were collected for 346 consecutive patients after cardiothoracic surgery: 262 patients weaned with ASV, and 84 patients weaned with pressure-controlled/pressure-support MV. With ASV the mean (± sd) Vt expressed per kilogram actual body weight was 7.1 ± 1.6 mL. Expressed per kilogram ideal body weight (IBW), Vt was 8.3 ± 1.5 mL. In patients with a correctly set body weight (SBW) (i.e., the IBW), Vt was 8.1 ± 1.4 mL/kg. With pressure-controlled/pressure-support-MV Vt was 7.3 ± 1.4 mL/kg IBW (P < 0.001 vs ASV). Multivariate logistic regression analysis showed Vt with ASV to be dependent on only two parameters: respiratory rate and the correctness of SBW.

CONCLUSIONS: Vt with ASV seems to be dependent on two parameters: respiratory rate and the correctness of SBW. The first factor is not clinically important because respiratory rate is automatically chosen by the microprocessor. The second factor is clinically important because it is the only factor that can be influenced by the operator. Our data show the importance of setting the correct weight with ASV. With ASV, Vt are >8 mL/kg IBW in a substantial number of patients. Randomized clinical trials should be performed to compare ASV with other ventilation modes.

	Introduction

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Adaptive support ventilation (ASV) is a microprocessor-controlled mode of mechanical ventilation (MV) that maintains a preset minimum minute ventilation (MMV) based on a patient's body weight.1 With ASV, the ventilator generates pressure-controlled (PC) breaths for patients who are unable to trigger a breath (passive or paralyzed patients), automatically adjusting inspiratory pressure (Pinsp) and timing to achieve a target tidal volume (V_T) and target respiratory rate (RR). In patients who are able to trigger a breath (active patients), the ventilator generates pressure-support (PS) breaths and, if necessary, delivers additional PC breaths if the RR is less than the target RR. Individual target Vt and target RR are calculated from the Otis equation,2 striving for minimum work of breathing, which is based on the time constant of the respiratory system. The time constant is estimated on a breath-by-breath basis by the expiratory time constant (RCexp), as obtained from the expiratory flow-volume curve via the least-square fitting method described previously by Iotti et al.3 Of note, with ASV, RR and Vt cannot be set by the user.

Two clinical trials showing reduced morbidity and mortality with lung-protective MV have stimulated the use of lower Vt (i.e., 6 mL/kg ideal body weight [IBW]) in patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS).4,5 Results from a secondary analysis of the ARDS Network trial suggest that there is a beneficial effect of Vt reduction from 12 to 6 mL/kg IBW, regardless of the plateau pressure (Pplat).6 In cardiothoracic surgery patients, it is not clear whether lower Vt are beneficial. However, one study suggests that conventional Vt may cause a more profound pulmonary inflammatory response compared to lung-protective MV using lower Vt.7 Another study indicates that the use of lower Vt may help attenuate the postoperative pulmonary dysfunction in these patients.8

In the present study, we investigated the behavior of ASV in steady-state post cardiothoracic surgery patients in three Dutch intensive care units (ICU). We determined the Vt applied by the ventilator and identified factors that influence Vt with ASV. In addition, we compared ASV results with PC/PS MV in one center.

	METHODS

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Patients
From October 2005 until February 2006, consecutive patients scheduled for elective coronary artery bypass surgery, valve surgery or combined surgery under cardiopulmonary bypass in three Dutch ICUs were considered for enrollment: one ICU in a university-affiliated hospital (Academic Medical Center [AMC], Amsterdam, The Netherlands) and two ICUs in non university hospitals (Catharina Hospital Eindhoven, Eindhoven, The Netherlands, and Medical Center Leeuwarden, Leeuwarden, The Netherlands). Patients with a history of chronic obstructive pulmonary disease necessitating bronchodilator therapy and/or previous pulmonary surgery were excluded.

In one center (AMC), patients who had elective cardiothoracic surgery were connected to either a Galileo ventilator (Hamilton Medical AG, Rhäzüns, Switzerland) or an Evita-4 ventilator (Dräger Medical, Zoetermeer, The Netherlands) based on ventilator availability. Galileo ventilators allowed the use of the ASV mode, which was the standard mode chosen when post cardiothoracic surgery patients were connected to this type of ventilator in this center. When post cardiothoracic surgery patients were connected to an Evita-4 ventilator, PC/PS MV was the standard mode of weaning. In the other two centers, all post cardiothoracic surgery patients were connected to Galileo or Raphael ventilators (Hamilton Medical AG, Rhäzüns, Switzerland) and weaned with ASV.

The ICU physicians and nurses caring for the patients were unaware of the purpose of data registration. The local medical ethical committee decided that there was no need to obtain informed consent since the objective of this study was to evaluate normal patient care.

Ventilator Settings
The researchers were not involved in choosing or adjusting the ventilator settings for the patients studied. All patients were treated according to the local postoperative MV guidelines.

ASV patients were connected to Galileo ventilators (software version 03.4) or Raphael ventilators (software version 3.0c). Ventilators were set according to the local postoperative MV guidelines, which included the guidelines for ASV provided by Hamilton. After a patient arrived in the ICU, the ventilator was set as follows: weight was entered in the ventilator (set body weight, SBW) and the percentage MMV was chosen (typically 100%). The protocol specifically stated to use the IBW, based on the patient's height. Positive end-expiratory pressure (PEEP) was set at 10 cm H₂O, and fraction of inspired oxygen (FiO₂) was set at 40%.

PC/PS patients (only in AMC) were connected to the Evita-4 ventilator. Ventilators were set according to the local postoperative MV guidelines, which included a guideline on postoperative PC/PS MV settings. Of note, this protocol stated that ideal Vt was 6 to 8 mL/kg IBW and ideal Paco₂ was 4.5 to 5.5 k Pa. PEEP was set at 10 cm H₂O and FiO₂ was set at 40%.

Other Protocols
The local guidelines included standards for the restrictive use of blood products, infusion of colloids and crystalloids, the use of stimulants such as dobutamine, and the use of sedation in the postoperative ICU period.

Data Collection
The following data were collected for all patients: age, gender, height, actual body weight (ABW), and IBW. Ventilatory variables were recorded when hemodynamics and ventilation were stable (1 h after arrival in the ICU): SBW, Vt, and Pinsp with ASV, or maximum airway pressure with PC/PS MV, spontaneous respiratory rate (RRspont), total RR (RRtot), and PEEP. Arterial blood gas analysis was only performed when deemed necessary, at the discretion of the professionals taking care of the patient. Arterial blood gas values were recorded at the end of the first hour in the ICU or, if no arterial blood gas analysis was performed at this time, for the first recorded period.

Definitions
ABW was defined as the body weight of the patient measured before admission to the hospital or ICU. Patients' weights were routinely measured on the day of admission to the hospital and were taken from the preoperative screening list. IBW was calculated by the following formula: in men, IBW (kg) = 50 + 0.91 x (centimeters of height – 152.4); in women, IBW (kg) = 45.5 + 0.91 x (centimeters of height – 152.4).9 Applied Vt in milliliters was defined as expiratory volume measured by the ventilator. Vt expressed as mL/kg IBW was calculated by dividing applied Vt by IBW for each patient. SBW was defined as the weight used to set the ventilator. Minute ventilation in the ASV group was defined by the ventilator as 100 mL/kg SBW per min. Percentage minute ventilation could be adjusted by the operator (typically 100%, but ranging from 25% to 350%, according to arterial pH and PAco₂). RRspont was defined as the number of spontaneous breaths per minute. RRtot was defined as the total number of breaths (spontaneous and mandatory) per minute. The pressure more than PEEP, Pinsp, was defined as the maximum driving pressure generated by the ventilator resulting in a Vt.

Statistical Analysis
All data are presented as mean ± (SD) unless stated otherwise. Data were analyzed using SPSS for Windows (Version 14, SPSS, Chicago, IL). Based on the literature and our knowledge of pathophysiology, we selected a number of factors that could potentially influence Vt.

In the first step, univariate logistic regression analysis was applied to identify possible determinants of Vt. All determinants with a P value <0.20 were used in the multivariate logistic regression analyses. In the next step, a backward selection procedure was used to select determinants significantly associated with Vt. A P value of 0.05 was taken as the threshold value.

The same statistical procedure was repeated for the comparison of ASV patients and PC/PS patients in one center. First, data of the two groups were compared using an independent T test. Data of the PC/PS patients were then univariately analyzed for potential determinants of Vt.

	RESULTS

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Patients
Data of 346 consecutive postcardiothoracic surgery patients (262 ASV patients and 84 PC/PS patients) are presented in Table 1. There were no differences in these data among the three centers. Also, no differences were found in baseline characteristics of ASV and PC/PS patients.

V_T at Steady-State with ASV
V_T in the whole group was 566 ± 124 mL. Expressed per kilogram ABW, Vt was 7.1 ± 1.6 (range, 2.9–15.3) mL; expressed per kilogram IBW, Vt was 8.3 ± 1.5 (3.0–16.2) mL (Fig. 1). In patients with a correct SBW (i.e., SBW = IBW), Vt was 8.1 ± 1.4 mL/kg IBW. In 55.3% of all patients, Vt was >8 mL/kg IBW, and in 11.9% of the patients Vt was >10 mL/kg IBW. Vt was not statistically different among the three centers.

View larger version (20K): [in this window] [in a new window]   Figure 1. Tidal volume (VT) as a function of the difference between the set body weight (SBW) and the ideal body weight (IBW). The dotted lines represent target Vt (8 mL/kg IBW) and accurately set body weight in the ventilator (0 kg).

Collected Respiratory Variables with ASV
Respiratory variables collected at steady-state are shown in Table 2. In 157 patients, an arterial blood gas analysis was performed. The only difference among the three centers was in PEEP: the AMC and Medical Center Leeuwarden used a PEEP of 10 cm H₂O for the majority of patients, whereas the Catharina Hospital Eindhoven used a PEEP of 5 cm H₂O.

Determinants of Vt with ASV
Variables analyzed for their influence on Vt are given in Table 3. Determinants with a P value <0.20 were used in a multivariate analysis. Because PEEP levels and institute were correlated, it was decided to use PEEP levels in the multivariate analysis rather than institute. Only SBW minus IBW and RRtot remained significant in the model. The odds for SBW minus IBW were 1.2 (95% confidence interval 1.1–1.3, P < 0.001), and the odds for RRtot were 0.8 (95% confidence interval 0.77–0.93, P < 0.001).

Comparison of ASV and PC/PS MV Within One Center
The results of the comparison of 88 ASV patients and 84 PC/PS patients are shown in Table 4. Vt was significantly larger in ASV patients than in PC/PS patients (7.9 ± 1.0 mL/kg IBW vs 7.3 ± 1.4 mL/kg IBW, P < 0.001).

V_T was >8 mL/kg IBW in 19% of PC/PS patients, whereas in 2.4% of PC/PS patients Vt was >10 mL/kg IBW. In contrast, Vt was >8 mL/kg IBW in 49% of ASV patients but >10 mL/kg in PC/PS patients. Paco₂ and the RRtot were both lower in ASV patients.

Determinants of Vt in PC/PS MV
Since none of the parameters reached a P value <0.20, we concluded that none of the factors in Table 4 influenced Vt with PC/PS MV.

	DISCUSSION

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
With ASV, the only variables set by the physician are MMV, PEEP and F_iO₂. Indeed, with ASV, Vt is based on an algorithm and cannot be influenced by the operator. The present study shows that within the predefined ASV protocol more than half of patients are ventilated with large Vt. The only determinants of Vt were SBW and RR. Since RR is chosen by the ventilator based on the algorithm and cannot be influenced by the operator, the only determinant of Vt that can be influenced is SBW.

Our data are, at least in part, in line with other studies that evaluated Vt with ASV. In a group of 155 patients, Cassina et al.10 showed Vt to be 8.7 ± 1.4 mL/kg IBW. In another study by Arnal, Vt was 8.3 ± 1.3 mL/kg IBW in patients with noninjured lungs.11 In these studies, it was not mentioned whether SBW was set correctly (i.e., whether SBW was IBW). However, as Vt in our study was 8.3 ± 1.5 mL/kg IBW, and because it was 8.1 ± 1.4 mL/kg IBW when SBW was IBW, we conclude that Vt with ASV are larger than normal in a substantial number of patients. Of note, in other specific patient groups, such as patients with chronic obstructive pulmonary disease, ASV may apply Vt that are significantly different from those in patients with normal lung compliances.11In addition, postoperative patients could, due to intraoperative opioids, anesthetics and other drugs, behave differently with respect to their breathing pattern than other patients.12 However, 1 h after admission to the ICU, spontaneous respiratory activity was low, and even absent in most patients in our study.

Whether we can conclude that Vt are indeed too large with ASV is disputable. Indeed, whereas the concept of ventilator-associated lung injury has been proven to be an important clinical entity in patients with ALI/ARDS, studies on ventilator-associated lung injury in patients without ALI/ARDS demonstrate inconsistent results.13 The mean Pinsp in our study was 13.1 ± 4.1. One could argue that these pressures are acceptable and that large Vt with low Pinsp are not potentially dangerous. The commonly held view that ventilation with large Vt may be tolerated as long as the Pplat is <30 H₂O has been questioned in a secondary analysis of the prospective ARDS Network trial.4 To assess for independent effects of Vt reduction and Pplat on mortality, Hager et al.6 constructed a multivariable logistic regression model. The results suggested that there was a beneficial effect of Vt reduction from 12 to 6 mL/kg IBW, regardless of the Pplat. We assume this may also apply for patients at risk of ALI/ARDS.14

With ASV, Vt were significantly larger than with PC/PS MV. Although this was not a true randomized, controlled study (i.e., patients were not randomized to ASV or PC/PS MV, but were connected to a certain type of ventilator on the basis of availability), we consider the comparison between ASV and PC/PS MV valid. Indeed, patients were comparable with respect to baseline characteristics and were weaned in the same period. Unfortunately, the comparison was only possible in one center due to the fact that in the other two centers ASV was the standard form of MV and postoperative cardiopulmonary surgery patients were never weaned with PC/PS MV. It could be that the strict MV guideline in the AMC, with the statement that Vt ideally would be 6 to 8 mL/kg IBW, influenced our findings. Comparisons should be performed in other centers before we can provide evidence that ASV really "chooses" larger Vt than ICU physicians, respiratory therapists or ICU nurses would.

Due to the design of the study, we only collected data during steady-state and not at later or earlier time points. It might have been better to collect data over the whole ventilation period, since compliance of the lungs and the respiratory system changes over time in post cardiac surgery patients. Indeed, Ranieri et al.15 found elastance of the respiratory system and lung to be lower 7 h after cardiopulmonary bypass compared to elastance immediately after induction of anesthesia and early after surgery.

It is important to realize that in the present study MMV was typically set at 100%. Although this resulted in adequate gas exchange variables, Vt may vary if MMV is adjusted. If we had applied more variable settings of MMV, this parameter influenced by the operator might have proved to be another determinant of Vt.

Another matter is the estimation of the RCexp. This is estimated on a breath-by-breath basis as obtained from the expiratory flow-volume curve. Measuring the RCexp in non paralyzed patients is difficult. The least square fitting method for this purpose has been described by Iotti et al.3 Although there has been some discussion about the accuracy of this method,16 it is simple and accurate enough for clinical purposes.17

The most important finding of our study was that setting the correct weight (IBW, not ABW) influences Vt with ASV. If the operator uses ABW instead of IBW (at many times a weight higher than the IBW), the Vt applied by the ventilator will be (too) large. Although it was explicitly written in the guidelines to use IBW, ABW was still used in a substantial number of patients in the present study. This finding is in line with previous data from our group.18 Indeed, feedback and education on the use of the correct weight to titrate Vt led to more frequent prescription of lower Vt. In our ICU, the finding that ABW was frequently used to set the ventilator resulted in a repetition of feedback and education on the importance of using the IBW with ASV as well.

In conclusion, our data show the importance of setting the correct weight with ASV. With ASV, Vt are >8 mL/kg IBW in a substantial number of patients. Randomized clinical trials should be performed to compare ASV with other ventilation modes.

	Footnotes

 
Accepted for publication April 28, 2008.

	REFERENCES

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Dongelmans, D. A. Articles by Schultz, M. J. Search for Related Content PubMed PubMed Citation Articles by Dongelmans, D. A. Articles by Schultz, M. J. Related Collections Critical Care Ventilation