Introduction

Continuous rotational therapy, also known as kinetic or oscillation therapy, has been shown to exert beneficial effects on pulmonary function in critically ill patients. It was primarily developed for mobilization of stroke patients and prevention of pneumonia, and its effects have been assessed in both animal and human trials [1, 2, 3, 4, 5]. During continuous rotational therapy using a specially designed bed allowing rotation in the longitudinal axis from one lateral position to the other, beneficial effects on oxygenation in patients suffering from acute respiratory failure have also been reported [6]. In a previously published study we found comparable effects on oxygenation in patients suffering from adult respiratory distress syndrome (ARDS) with continuous rotation therapy and with prone positioning [7], although the beneficial effect of rotation seemed to take more time to develop. Compared with the body of literature dealing with the physiologic effects of prone positioning (summary in [8]), reports on the effects of continuous rotation therapy and lateral steep positioning are rare. An early report showed little influence of body position during rotation therapy on hemodynamics and respiratory mechanics, while changes in oxygenation occurred with high interindividual variability [9]. Bein et al. reported beneficial effects of continuous rotation therapy on ventilation–perfusion inequality in patient suffering from acute lung injury (ALI) [10], as well as a reduction in extravascular lung water [11]. The same group reported also on hemodynamically relevant increases of cardiac output related to left lateral steep positioning, while cardiac output decreased in right lateral position [12]. Since the physiologic mechanisms of the lateral steep position are not exactly known, no recommendations regarding the duration of pauses in the “extreme” steep position exist. We hypothesized that prolonged lateral steep position, i.e. maintenance of the “extreme” lateral position in a continuous rotation therapy bed, would improve gas exchange. We therefore performed a study to assess respiratory mechanics, gas exchange, and hemodynamics during a prolonged period of lateral steep position in patients with respiratory failure.

Materials and methods

The study was conducted according to the Helsinki Declaration of 1975, and the study protocol was approved by the institutional ethics review board. Patients were eligible for inclusion if (a) they suffered from acute respiratory failure requiring mechanical ventilation, (b) the diagnosis of ALI or ARDS had been made within 96 h prior to inclusion, (c) the decision to treat the patients with continuous lateral rotation therapy had been taken within 48 h prior to inclusion, (d) the patients had been hemodynamically stable during rotation over the maximal angle for at least 12 h prior to inclusion, and (e) they were between 18 and 85 years of age.

Admission APACHE II and SAPS II scores were calculated according to the original publications [13, 14] by using the most unfavorable values available during the first 24 h in the intensive care unit (ICU). Based on the guidelines of the American–European Consensus Conference [15], ARDS was defined as underlying disease known to be a risk factor for the development of ARDS plus (1) PaO2/FiO2 ratio < 200, (2) pulmonary arterial occlusion pressure < 18 mmHg, and (3) a chest radiograph showing bilateral patchy shadowing. ALI was defined by a PaO2/FiO2 ratio < 300 plus the same criteria as in ARDS. Lung injury scores were calculated at inclusion according to Murray et al. [16]. All patients received analgo-sedation using continuous infusion of midazolam and sufentanil and/or ketamine. For the study period, sedation was titrated to achieve a level of 5 on the Ramsay sedation scale and to suppress spontaneous breathing. None of the patients needed muscle relaxants during the study period to achieve this goal. Mechanical ventilation was performed by using a time-cycled pressure-controlled mode (Servoi, Maquet, Solna, Sweden). Positive end-expiratory pressure (PEEP) levels were adjusted in increments of 2 mbar to maintain FiO2 at 0.6 or less with arterial oxygen saturation of more than 91% between 5 and 20 mbar. Inspiration/expiration ratio was set to 1.0 in all patients. Breathing frequency was chosen to keep PaCO2 below 60 torr and to avoid dynamic hyperinflation. Gas trapping and dynamic hyperinflation was ruled out by interpreting expiratory flow curves and by routine measurement of intrinsic PEEP by the ventilator using the “expiratory hold” function [17]. In the case of air trapping, respiratory rate was reduced. In none of the patients was a change of inspiration/expiration ratio necessary due to persistent air trapping. Peak inspiratory pressure was kept to the lowest possible level to apply tidal volumes of about 8 ml/kg BW. FiO2 was chosen with the goal to achieve an arterial oxygen saturation of 92–96%. A heated respiratory humidifier was used in all patients (Fisher Paykel Healthcare, Auckland, New Zealand). All patients were monitored by continuous ECG, pulse oximetry and arterial blood pressure recording. Blood pressure was taken from an indwelling arterial line placed in a radial or femoral artery. All patients were equipped with a balloon-tipped triple-lumen thermodilution pulmonary artery catheter (Baxter Healthcare, Irvine, USA). Catheters were inserted into the internal jugular or subclavian vein, and catheter position was confirmed by a chest radiograph. Pressure transducers were zeroed to the midaxillary level. Hemodynamics were stabilized by adequate volume substitution to keep a pulmonary capillary occlusion pressure of 12–15 torr and vasopressors, if necessary.

Continuous lateral rotation therapy was performed by positioning the patients in a specially designed continuous rotation therapy bed (Roto Rest®, KCI Mediscus, Vienna, Austria) allowing continuous rotation of the whole body along its longitudinal axis from one lateral position to the other with a maximum angle of 124°. A rotation cycle from one “extreme” position to the other lasts 4 min. A rotation pause of 5 s was maintained bilaterally at maximum rotation angle. Prior to study entry, rotation was stopped only for specific therapeutic, diagnostic or other routine bedside procedures.

The study consisted of two phases of measurements: The first phase of the study was designed to stop the bed only briefly for measurements, thus we assume the results approximately resemble continuous turning. The second phase was intended to investigate the effect of prolonged pauses in steep position. In phase I, measurements were taken during a short pause from continuous turning of about 1 min in supine position (baseline, SUP in Table 3) as well as in both maximal lateral steep positions of 62° (RLP and LLP). Patients were randomized to stop either in left or right lateral steep position first (Table 1). Between the measurements, continuous rotation was reinstituted for a full turning period of about 8 min. A turning period of 1 h followed before initiation of phase II. In phase II, rotation was stopped in supine position and measurements were taken after 10 min (baseline, SUP 1). Patients were then randomized to stop either in right or left maximal steep position after a full cycle of turning (8 min), and measurements were taken after 10, 20, and 30 min (RLP or LLP 10, 20, 30). After a full cycle of turning, the bed was stopped in supine position again and measurements were taken after 10 min (SUP 2). After another full cycle of turning, the bed was stopped in the other maximal lateral steep position and measurements were taken (RLP or LLP 10, 20, 30). After a last full cycle of turning, the bed was stopped in supine position again and the final measurements were taken after 10 min (SUP 3). Thus, the duration of the whole study period was about 3.5 h. Ventilator settings were kept unchanged throughout the study period.

Table 1 Patients' characteristics
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The study was prematurely broken off if hemodynamic or respiratory instability occurred, defined as sustained decline in blood pressure necessitating vasopressor therapy or a dose increase in vasopressor therapy and/or a decline in arterial oxygen saturation measured by pulse oximetry < 88%.

At each time point of measurements, the following parameters were recorded: arterial and mixed venous blood gas analysis, arterial blood pressure, arterial oxygen saturation, cardiac index, pulmonary shunt fraction, respiratory mechanics (tidal volume, peak inspiratory pressure, PEEP, static compliance). Arterial blood pressure was measured by an indwelling arterial line with the pressure transducer system fixed to the movable part of the kinetic system close to the patient to guarantee the position of the tip always at the level of the left atrium. For blood gas analysis an automated blood gas analyzer (BGE Electrolytes, Instrumentation Laboratory, Vienna, Austria) was used. Cardiac output was obtained by averaging three measurements by thermodilution technique using a 10-ml bolus of sodium chloride injected at room temperature. Hemodynamic calculations were performed using standard formulas provided by the software of a patient data management system (Care Vue 9000, Hewlett-Packard, Vienna, Austria). Exhaled tidal volume and respiratory pressures were recorded as measured by the ventilator. Static compliance was calculated by using the formula Cstat = Vt/Pplat–PEEP, where Vt is tidal volume, Pplat is plateau pressure obtained after performing an inspiratory hold maneuver and PEEP is the total end-expiratory pressure after exclusion of intrinsic PEEP by performing an end-expiratory hold maneuver. A chest radiograph was performed on the day of inclusion.

To compare the changes in the different positions, a non-parametric one-way ANOVA for repeated measures (Friedman test) was used. Dunn's Multiple Comparison post-test was used to compare pairs of time points. A p value < 0.05 was regarded as statistically significant. Calculations were performed by statistics software packages (GraphPad Prism™, GraphPad Software, San Diego, USA).

Results

Twelve consecutive patients fulfilling the inclusion criteria were enrolled in the study. Patients' characteristics, baseline parameters and ventilator settings are shown in Tables 1 and  2. Eleven patients suffered from pneumonia as the cause of respiratory failure. One patient suffered from near-drowning as cause for ARDS. Ten patients were able to complete the study protocol; in two patients the study had to be terminated during the second phase of the study at time points LLP 10 and LLP 20, respectively, due to combined hemodynamic and respiratory instability. In each of these two patients the arterial oxygen saturation declined to values below 80%. Concomitantly, arterial blood pressure as well as tidal volumes and compliance decreased, while cardiac index and pulmonary shunt fraction did not change.

Table 2 Respiratory baseline characteristics, ventilator settings at inclusion, and baseline hemodynamics
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Phase I revealed no significant changes in PaO2/FiO2 ratio, mean arterial blood pressure, pulmonary shunt fraction, and cardiac index. Static compliance was significantly lower in supine position than in both lateral steep positions (p < 0.01). Concomitantly, PaCO2 was significantly lower – although the difference was not clinically relevant – in supine position than in left and right lateral steep position (p < 0.01).

Phase II led to comparable results. No significant changes in PaO2/FiO2 ratio, mean arterial pressure, pulmonary shunt fraction, and cardiac index were observed. Cstat was significantly higher at all three time points in supine position than in left and right lateral steep positions (p < 0.001). Accordingly, tidal volumes were highest in supine position, and PaCO2 was lower in supine position than in left and right lateral steep positions with a significant difference between SUP 1 and RLP 30 (p < 0.01). Changes in oxygenation related to body position markedly varied interindividually (Fig. 1), while changes in Cstat changed uniformly depending on body position (Fig. 2). None of the measured parameters changed significantly during the 30-min period of unilateral steep positioning. The results of all measurements are shown in Table 3.

Fig. 1
figure 1

PaO2/FiO2 ratio during phase II. SUP denotes supine position, RLP and LLP denote right and left lateral steep positions after 10, 20, and 30 min, respectively

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Fig. 2
figure 2

Static compliance during phase II. SUP denotes supine position, RLP and LLP denote right and left lateral steep positions after 10, 20, and 30 min, respectively

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Table 3 Measured values during phase 1 and phase 2 of the study
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Discussion

The main finding of our study was the lower static compliance in lateral steep position than in supine position. The results have to be regarded in light of the limitations of our study. The measurement of compliance by the ventilator is a major shortcoming of our study, as additional effects of time of occlusion and pressure transducers cannot be excluded. However, since measurements in different positions in the same patient were compared, conditions remained constant for each measurement. Thus, changes in compliance (rather than absolute values) can be regarded as reliable. Moreover, as we did not measure lung and chest wall compliance separately, we are only able to describe the net effect on the whole respiratory system. It can be hypothesized that a decrease in chest wall mechanics contributed substantially to the decrease of overall compliance, but as the measurement of respiratory system mechanics was not partitioned into its lung and chest wall components, the change in lung compliance remains unclear. A decrease in chest wall compliance during steep position could well be explained by the surface pressure of the bed supports on the gravity-dependent hemithorax. The position of one hemithorax above the other certainly will also lead to different influences of gravity on the alveoli. It can be speculated that ventilation is improved in the upper regions of the lungs, while perfusion may be diverted to the lower parts. Overdistension of the upper lungs could be the reason for the unchanged oxygenation in most patients, which could lead to detrimental effects over a longer period of time and should be avoided.

Although we used a low-volume ventilation strategy, the goals of the ARDS Network Trial were not reached in most patients [18]. Most patients received tidal volumes of about 8 ml/kg BW. Pressure-controlled ventilation lead to a marked decrease of static compliance in lateral steep position. This was solely the effect of a decrease in tidal volumes, since during pressure-controlled ventilation plateau pressures and PEEP remained constant (Table 3). In patients ventilated with tidal volumes as low as recommended by the ARDS Network guidelines in supine position, this could lead to substantial intermittent hypoventilation during continuous rotation therapy in lateral steep position. Thus, a volume-controlled ventilation mode seems preferable during continuous rotation therapy.

No statistically significant differences in oxygenation among positions could be found in the whole group of patients. However, in some patients, oxygenation changed markedly in different positions, including two patients who developed marked hypoxemia in lateral steep position. These changes correlated neither with static compliance nor with any other of the measured parameters, which made them unpredictable. In some patients, chest X-rays revealed bilateral shadowing with one side predominant. Oxygenation showed no predictable correlation between the position of the “better side” up or down. In a study on 44 patients with unilateral pneumonia or tuberculosis, Choe and co-workers found a relationship between the amount of ventilation going to the normal lung and improvement in oxygenation [19]. These findings conform with the data published by Nelson and Anderson [9], who also found high interindividual variability with oxygenation and attributed the phenomenon to asymmetric lung disease, a conclusion which we were not able to draw from our findings. In some of the patients in that study, changes in oxygenation associated with positional changes correlated with changes in pulmonary shunt fraction. We were not able to reproduce these findings in our patients and it may be doubted whether the 90° lateral decubitus position used by Choe and co-workers can be compared with the 62° lateral position used with continuous rotation therapy.

Hemodynamics remained unchanged in most of our patients, with the exception of the two in whom the study had to be terminated prematurely. In these two patients the arterial oxygen saturation declined to values below 80% after 10 and 20 min in the left lateral steep position during phase II, respectively. Concomitantly, arterial blood pressure as well as tidal volumes and compliance decreased, suggesting hypoventilation. In one of these patients, asymmetrical shadowing had been observed on the chest radiographs. This patient worsened during the “good side up” position. The patients had been stable in the left lateral steep position during phase I and quickly recovered after being restored to supine position. Cardiac index and pulmonary shunt fraction did not change significantly. Bein et al. described changes in cardiac index during lateral steep positioning, showing an increase in left lateral position and a decrease in right lateral position [12]. The authors attributed these changes to gravitational forces. In our patients, comparable changes of cardiac index could not be observed, although a small, yet not statistically significant, tendency towards higher values of cardiac index in LLP was seen.

To date, no recommendations regarding the duration of the pause in lateral steep position exist. Our data do not advocate a prolonged duration of the extreme position: none of the patients showed clinically relevant improvement regarding oxygenation during 30 min of lateral steep position, but two patients developed hypoxia. Therefore, no beneficial effects of continuous rotation therapy can be derived from our data. However, we do not believe our study proves the method worthless. Since we investigated the effect of keeping a patient in lateral steep position for a certain period of time, we can only conclude that doing this does not lead to beneficial effects and can even be detrimental. We cannot derive any conclusions about long-term effects of continuous rotation from our data.

In conclusion, our results suggest that lateral steep positioning impairs the compliance of the respiratory system. With respect to gas exchange, individual response to the different positions is unpredictable. While most patients remained stable with regard to cardiac output and blood pressure, prolonged steep positioning can lead to a severe impairment of vital parameters in some cases. The pauses in the “extreme” position should be as short as possible.