A comparison between two different alveolar recruitment maneuvers in patients with acute respiratory distress syndrome

Abstract

Background:

Alveolar recruitment is a physiological process that denotes the reopening of previously gasless lung units exposed to positive pressure ventilation. The current study was aimed to compare two recruitment maneuvers, a high continuous positive airway pressure (CPAP), and an extended sigh in patients with ARDS.

Materials and Methods:

Forty patients with acute respiratory distress syndrome were randomly divided into two groups, 20 patients each. Group I received a CPAP of 40 cm H₂O for 40 seconds and group II received extended sigh (providing a sufficient recruiting pressure × time). In our study, we assessed the effects of both recruitment maneuvers on respiratory mechanics, gas exchange, and hemodynamics. These data were analyzed using two-way analysis of variance (ANOVA) followed by a Student--Newman--Keuls post hoc comparison test. P < 0.05 was considered statistically significant.

Results:

Both methods improved the compliance, increased arterial oxygenation (PaO₂), increased the PaO₂/FiO₂ ratio, and reduced the pulmonary shunt fraction (Q_s/Q_t). However, the extended sigh improved both PaO₂ and PaO₂/FiO₂ ratios more than continuous positive airway pressure. Also the hemodynamic parameters were better maintained during the extended sigh.

Conclusion:

Alveolar recruitment maneuvers are effective in management of mechanically ventilated ARDS patients. We conclude that extended sigh is more effective than continuous positive airway pressure as a recruitment maneuver.

INTRODUCTION

Acute respiratory distress syndrome (ARDS) is still considered a great challenge encountered in critical care medicine. The Consensus Conference of the American College of Chest Physicians recommended the following strategy to ventilate ARDS patients (low tidal volume “V_t” to keep the plateau pressure less than 35 cm H₂O, adequate positive end-expiratory pressure “PEEP” to improve oxygenation, and FiO₂ below 60%).[1] The ventilation strategy of using small V_t alone has shown disappointing outcomes.[2,3] The use of small V_t + PEEP set at 2 cm H₂O above the lower inflection point of the pressure-volume curve as a lung protective strategy provided a great improvement in survival.[4]

Several studies have shown that anesthetized patients with normal lungs ventilated with small V_t, developed progressive alveolar collapse and hypoxemia which were prevented by using large tidal volumes delivered either continuously or intermittently.[4,5] The same results were observed in patients with ARDS.[6,7]

Recruitment is a physiological process that denotes the reopening of previously gasless lung units exposed to positive pressure ventilation through increasing the alveolar pressure to a level sufficient to aerate stiff lung units filled with edema fluid and inflammatory cells.[8] The policy of alveolar recruitment was applied recently as a form of a lung protective ventilatory strategy in ARDS patients although it was described a long time ago in 1952.[9]

Up till now, the data comparing the safety and efficacy of different recruitment maneuvers (RM) are not enough.[10–14] The aim of this study was to compare the effects of two recruitment maneuvers, a high continuous positive airway pressure (CPAP) and an extended sigh as an adjunct of mechanical ventilation in patients with ARDS.

MATERIALS AND METHODS

Forty patients were enrolled in the intensive care unit between April 2008 and May 2010. Approval for this study was obtained from the Institutional Review Board and informed consent was obtained from the patients or their next of kin. All patients fulfilled the ARDS criteria of the American European Consensus Conference on ARDS[1] which include the following: identifiable associated condition, acute onset, bilateral infiltrates on chest radiography, pulmonary artery wedge pressure ≤18 mmHg or absence of signs suggesting left atrial hypertension, and PaO₂/FiO₂ ratio ≤200. Patients with hypotension (systolic blood pressure <100 mmHg), significant arrhythmia, or gross baro-trauma in any form (subcutaneous emphysema, pneumo-mediastinum, or pneumo-thorax) were excluded from the study. Also, patients who were having ARDS of more than 3 days duration were excluded from the study. All the patients were intubated with a cuffed tracheal tube, ventilated with a Hamilton Gallileo ventilator which is equipped with a display screen and software capable of plotting and analyzing the pressure-volume curves. All patients had an arterial line and pulmonary artery thermodilution catheter inserted. The patients were sedated with fentanyl and midazolam and paralyzed with cisatracurium bromide.

Tidal volume was set at 6 ml/kg, and respiratory rate at 12 per minute in volume control mode. PEEP and FiO₂were set to obtain an arterial oxygen saturation (SaO₂) value of 90-95% or an arterial oxygen partial pressure (PaO₂) of 60-80 mmHg (baseline). Patients were randomly divided into two groups. Each group contained 20 patients. Randomization was done centrally by an independent statistician using a random number table generated by Microsoft Excel to ensure proper concealment of the study management until the release of the final statistical results. An investigator who was responsible for the collection of the data was blinded in respect to the study protocol during whole process of data evaluation.

Group I underwent the following recruitment maneuver

The ventilator was set on the CPAP mode and a pressure of 40 cmH₂O was applied for 40 seconds. The ventilator was then set back to its baseline values.

Group II underwent the following recruitment maneuver

The V_t-PEEP values were changed to 6-15, 4-20, and 2-25, each step being 30 seconds (inflation phase). After V_t-PEEP 2-25, the mode was switched to CPAP of 30 cmH₂O for a duration of 30 seconds (pause), after which the baseline setting was resumed following the reverse sequence of inflation (deflation phase). This maneuver, which is called extended sigh, was performed twice with one minute of baseline ventilation in between.

After application of recruitment maneuvers in both groups, derecruitment was prevented by applying a PEEP equal to the lower inflection point (LIP) determined in P-V curve + 2 cmH₂O. If the LIP couldn’t be determined, the PEEP was set at 10 cmH₂O.

The primary outcome measures were changes in the respiratory mechanics and gas exchange data whereas the secondary outcome measures were changes in the hemodynamic data as follows:

Respiratory mechanics

Static and dynamic compliance of the respiratory system (C_st and C_dyn) were also recorded from the display on the ventilator digital monitor at baseline, 3, and 20 minutes after application of the recruitment maneuver. Airway pressures (peak, mean, plateau) were read from the display on the ventilator digital monitor at baseline, 3, and 20 minutes after application of the recruitment maneuver. Pressure volume curves were obtained by the quasistatic method[15] at baseline, 3, and 20 minutes after recruitment; analysis of the pressure-volume (P-V) curve of the respiratory system was done as follows: the P-V curve was displayed on the screen of the ventilator, the obtained P-V curve was frozen on the screen, and controlled mechanical ventilation was resumed. Two cursors present on the screen were used to determine the lower and the upper inflection points of the P-V. The parameters were calculated automatically by the ventilator after positioning the cursors. The whole maneuver took 2 minutes at the bedside without requiring any special equipment. Values of pressures at the upper and lower inflection points of the P-V curve were quantified.

Gas exchange and hemodynamic data

Gas tensions and pH were measured at baseline, 3, and 20 minutes after application of the recruitment maneuver. The PaO₂/FiO₂ ratio was calculated at baseline, 3, and 20 minutes after application of the recruitment maneuver. The pulmonary shunt (Q_s/Q_t) fraction was calculated using a standard formula: Q_s/Q_t=Cc’O₂-CaO₂/Cc’O₂-CvO₂ where Q_s is the shunt flow, Q_t is the cardiac output, and Cc’O₂, CaO₂, and CvO₂ represent the oxygen content of pulmonary end-capillary, arterial and mixed venous blood, respectively.

Heart rate, mean arterial (MAP), pulmonary artery (P_pa), pulmonary wedge (P_pw), and central venous (P_cv) pressures were measured with pressure transducers zeroed at the level of the midaxillary line. Cardiac output and cardiac index were measured by the thermo-dilution method. Pulmonary and systemic vascular resistance indexes (dyn s/cm⁵/m²) were calculated. All these hemodynamic parameters were measured at baseline, 3, and 20 minutes after application of the recruitment maneuver.

Two chest X-ray films were taken at baseline and one hour following recruitment. Changes in the chest radiographic patterns induced by the recruitment maneuvers were analyzed. The lung was divided into four quadrants and was evaluated for the presence and extent of intense parenchymal opacification (a homogeneous increase in density that obscures the vascular margins and airway walls) or ground-glass opacity (a hazy homogeneous density with preserved vascular margins). The extent of these radiographic densities was scored using a four-point scale: 0=none; 1=1 quadrant; 2=2 quadrants; 3=3 quadrants and 4=4 quadrants. Emphasis was based on the presence of pneumo-thorax, pneumo-mediastinum, or pneumatocoeles for assessment of the safety of the recruitment maneuvers.

Statistical methodology

With a two-sided type I error of 5% and study power at 80%, a mean sample size of 20 patients in each group was found sufficient to demonstrate a clinically relevant difference of 5 ml/cm H₂O in the respiratory system compliance.

Baseline patient characteristics and ventilator data in both groups were compared by using the Student t test for parametric data and the Mann-Whitney U test for nonparametric data. The Kolmogorov-Smirnov test was used to verify normal distribution of the quantitative data. Changes in respiratory mechanics, gas exchange, and hemodynamic data were analyzed using two-way analysis of variance (ANOVA) followed by a Student–Newman-Keuls post hoc comparison test. P < 0.05 was considered statistically significant.

RESULTS

Both groups were comparable in baseline characteristics and ventilator data (P >0.05) as shown in Table 1.

Table 1.

Baseline patient characteristics and ventilator data in both groups

Both C_st and C_dyn increased in both groups after recruitment maneuvers but there was no significant difference between both groups regarding the magnitude of increase. Peak inspiratory, mean airway, and plateau pressures decreased slightly in both groups but there was no significant difference between both groups [Table 2].

Table 2.

Respiratory mechanics in both groups

In group I, the upper inflection point (UIP) disappeared in eight patients following the recruitment maneuver; in the remaining 12 patients, there was an increase in the UIP following the recruitment maneuver. In group II, the UIP disappeared in six patients following the recruitment maneuver; in the remaining 14 patients, there was an increase in the UIP following the recruitment maneuver. There was no significant difference between both groups [Table 2].

In group I, the LIP could not be detected at the baseline in five patients; in the remaining 15 patients, the LIP did not show significant change. In group II, the LIP could not be detected at the baseline in four patients. In the remaining patients, the LIP did not show significant change. There was no significant difference between both groups before or after recruitment maneuvers [Table 2].

Both methods improved arterial oxygenation, increased the PaO₂/FiO₂ ratio, and decreased the pulmonary shunt fraction. The extended sigh improved both arterial oxygenation and PaO₂/FiO₂ ratio more than the CPAP method [Figures 1 and 2].

Figure 1.

PaO2 (mmHg), PaCO2 (mmHg), PaO2/FiO2, SaO2 (%), SvO2 (%) in both groups. †P <0.05 within the CPAP group, ‡ P <0.05 within the extended sigh group, *P <0.05 between both groups. CPAP=continuous positive airway pressure, PaO2=arterial oxygen tension, PaCO2=arterial carbon dioxide tension, FiO2=fraction of inspired oxygen tension, SaO2=arterial oxygen saturation, SvO2=mixed venous oxygen saturation.

Figure 2.

pH and Qs/Qt (%) in both groups. † P <0.05 within the CPAP group, ‡ P <0.05 within the extended sigh group, P >0.05 between both groups. CPAP=continuous positive airway pressure, Qs/Qt=pulmonary shunt fraction

MAP in the CPAP group was significantly lower than in the extended sigh group whereas HR was comparable in both groups. P_pa, P_pw, and P_cv were significantly higher in the CPAP group. CI showed no significant differences between both groups. PVRI decreased and SVRI increased significantly in both groups at 20 minutes after the recruitment maneuvers; however, there was no significant difference between both groups [Figures 3 and 4].

Figure 3.

HR (beat/min), MAP (mmHg), SVRI (dynes.cm^-5.m^-2), PVRI (dynes. cm^-5.m^-2) in both groups. † P <0.05 within the CPAP group, ‡ P <0.05 within the extended sigh group, * P <0.05 between both groups. CPAP=continuous positive airway pressure, HR=heart rate, MAP=mean arterial pressure, SVRI=systemic vascular resistance index, PVRI=pulmonary vascular resistance index

Figure 4.

Pcv (mmHg), Ppa (mmHg), Ppw (mmHg), CI (l.min^-1.m^-2) in both groups. † P <0.05 within the CPAP group, ‡ P <0.05 within the extended sigh group, * P <0.05 between both groups. CPAP=continuous positive airway pressure, Pcv=central venous pressure, Ppa=pulmonary arterial pressure, Ppw=pulmonary wedge pressure, CI=cardiac index

In group I, chest X-rays were done before applying the recruitment maneuver and 1 hour after applying it. The extent of the disease in lung parenchyma was detected by using a four-point scale. The average of the score was 3 before applying the recruitment maneuver and 3 after applying it. No baro-trauma, pneumo-thorax, or any other complications were found in the chest X-rays. In group II, the average of the score was also 3 before applying the recruitment maneuver and 3 after applying it. No baro-trauma, pneumo-thorax, or any other complications were found in the chest X-rays.

DISCUSSION

The results of the present study showed that both CPAP and extended sigh methods of recruitment maneuvers improved the compliance and arterial oxygenation, increased the PaO₂/FiO₂ ratio, and reduced the pulmonary shunt fraction. However, the extended sigh improved both arterial oxygenation and PaO₂/FiO₂ ratio more than CPAP. Also the hemodynamic parameters were better maintained by the extended sigh. No pneumo-thorax or baro-trauma were detected.

There is no gold standard in the literature for recruitment maneuvers. The ideal timing, duration, pressure, and mode of recruitment maneuvers have not yet been clearly identified.

Several methods have been employed to carry out recruitment maneuvers in the clinical setting as well as in experimental models. Rothen et al.[11] showed that a sustained inflation maneuver of 40 cmH₂O for 7-8 seconds could reexpand all collapsed lung units as detected on computed tomography scans of the chest and improve oxygenation. In another study,[12] three consecutive sighs of 45 cmH₂O plateau pressure were added every minute for an hour. This maneuver provided a significant increase in the end-expiratory lung volume, an increase of the PaO₂, and a reduction of intrapulmonary shunt.

Lapinsky et al.[13] applied a sustained inflation maneuver using a pressure of 45 cmH₂O or the peak pressure at a tidal volume of 12 ml/kg, whichever was lower. The maneuver was applied for a period of 20 seconds. Significant improvement in oxygenation occurred in most of the patients within 10 minutes. The oxygen saturation improved from 86.9 ± 5.5 to 94.3 ± 2.3%. No baro-trauma or significant complications were recorded; only few patients developed hypotension and mild oxygen desaturation during the 20-second inflation which were reversed immediately upon termination of the inflation. They concluded that sustained inflation is a safe maneuver for alveolar recruitment that improves oxygenation in selected patients and is clinically applicable in the ventilatory strategies for ARDS patients.

Lim et al.[14] used an extended sigh as a recruitment maneuver. This included gradually reducing tidal volumes from 8 to 2 ml/kg and increasing the PEEP from 10 to 25 cmH₂O in a stepwise manner, each step lasting 30 seconds (inflation phase). When a tidal volume of 2 ml/kg and a PEEP of 25 cmH₂O were reached, a CPAP level of 30 cmH₂O was applied for 30 seconds (pause), after which a reverse sequence was applied until the baseline settings were resumed (deflation phase). This maneuver provided a significant increase in PaO₂ and static respiratory compliance and was free of any major respiratory or hemodynamic complications.

Recruitment maneuvers may also be practicable in spontaneously breathing patients. Patroniti et al.[16] applied a CPAP of 20% higher than the peak pressure or at least 35 cm H₂O on pressure support ventilation (PSV) for 3-5 seconds every minute, for a minimum of 1 hour. This maneuver showed an improvement in gas exchange, lung volume, and compliance.

Constantin et al.[17] compared two different recruitment maneuvers: a CPAP of 40 cmH₂O for 40 seconds without tidal ventilation and an extended sigh (e-sigh) through maintaining a PEEP level at 10 cmH₂O above the lower inflexion point on the pressure-volume curve for 15 minutes on volume-controlled ventilation. Both maneuvers improved oxygenation at 5 and 60 minutes but improvement was significantly higher with e-sigh than CPAP. Only the e-sigh was associated with an increase of recruited volume whereas the CPAP failed to give this effect. The systolic blood pressure decreased below 70 mmHg during the CPAP maneuver on two occasions resulting in interruption of the recruitment maneuver while there was no significant decrease in blood pressure with the e-sigh.

Other studies[18,19] also suggested that recruitment maneuvers using a sustained high inflation pressure may have a deleterious effect on the hemodynamics and thereby may not be recommended.

Brower et al.[20] performed the ALVEOLI trial to compare higher versus lower PEEP in patients with ARDS. The application of higher PEEP levels improved oxygenation but did not affect survival rate, suggesting that higher PEEP does not provide good impact on mortality when used during low tidal volume ventilation.

It is well known that higher PEEP values may increase the risk of hyperinflation and baro-trauma, so a compromise must be done between PEEP induced lung recruitment and prevention of hyperinflation. Meade et al.[21] performed the “lung open ventilation” trial in 983 patients with ALI or ARDS and the experimental ventilation strategy included tidal volumes of 6 ml/kg, plateau pressures not exceeding 40 cmH₂O, recruitment maneuvers (40-second breath hold at 40 cmH₂O airway pressure) and a table-based PEEP using a scale of PEEP versus FiO₂. The 28-day mortality rate was 28% in the experimental group and 32% in the controls, but this difference was not statistically significant. Also both groups showed no significant difference related to baro-trauma. However, the authors concluded that this “open-lung” strategy provided improvement in the secondary end-points related to hypoxemia and use of rescue therapies.

Recently, Lowhagen et al.[22] performed the SLRM “slow moderate pressure recruitment maneuver” and compared it against a vital capacity recruitment maneuver (VICM). The two recruitment maneuvers were performed on patients with early ALI/ARDS followed by decremental PEEP titration. SLRM provided the best response in compliance, PaO₂/FiO₂ and venous admixture at significantly lower PEEP and plateau pressure.

Our study suffers from a number of limitations, as the duration of alveolar recruitment was progressive in group II, and for a longer time period, as opposed to group I. Group II was thus exposed to longer duration of maneuvers overall. This difference in the duration of recruitment could potentially have an impact on the outcome measures. Time and pressure stand in relation to the efficacy of a recruitment maneuver. A longer recruitment maneuver with a lower pressure results in fewer hemodynamic side effects and give the lung tissue time to reopen in a more gentle way. Another limitation of the current study may result from the fact that the patients were hypoventilated under baseline ventilator data, and accordingly we cannot answer the following question: if the patients were normoventilated at baseline would the effect of recruitment on gas-exchange variables still be there?

CONCLUSION

We found that alveolar recruitment maneuvers were effective in improving oxygenation of mechanically ventilated ARDS patients. The extended sigh was more effective than CPAP as a recruitment strategy. However, the difference between both recruitment maneuvers may not be clinically significant and the outcome seems of no apparent difference as the long-term effects were not provided but only short-term or transient improvement in oxygenation was provided whereas a potential influence on clinical outcome is lacking.