Sigh Improves Gas Exchange and Lung Volume in Patients with Acute Respiratory Distress Syndrome Undergoing Pressure Support Ventilation

We enrolled 13 patients (table 1) who met the criteria for ARDS defined by the American European Consensus Conference 19and had no history of chronic obstructive pulmonary disease. All patients were studied within 7 days after the diagnosis of ARDS. At the time of enrollment, all subjects were intubated and had an arterial catheter in place. The level of sedation, obtained with benzodiazepines, propofol, and fentanyl, allowed patient arousal and coordinated motor responses to verbal commands. Subjects were studied during clinical and hemodynamic stability in a supine or semirecumbent position. On the day of the study, patients were already ventilated with PSV set by the attending physician according to empirical criteria. 16 

The protocol was approved by the institutional ethics committee, and informed consent was obtained from the patient's next of kin. The investigation was conducted according to the Helsinki Declaration. After enrollment, all patients were connected to an Evita-4 ventilator (Dräger, Lübeck, Germany), set in PSV with fraction of inspired oxygen (Fio²), inspiratory pressure, and PEEP levels previously selected by the attending physician. The time of inspiratory pressure rise was set at 0.2 s, and PSV parameters were kept unchanged throughout the study. The protocol consisted of three consecutive study periods: baseline period 1 (Base 1), sigh period (Sigh), and baseline period 2 (Base 2), each one lasting a minimum of 1 h.

During Base 1 and Base 2, patients were ventilated with PSV alone. During Sigh the ventilator was set in biphasic positive airway pressure (BIPAP) + pressure support ventilation (BIPAP+ASB on a Dräger Evita-4 ventilator), and sighs were performed by means of the higher BIPAP level. During BIPAP, 20ventilation is assisted by alternating between a PEEP high level and a PEEP low level. Spontaneous breaths are possible but not supported during the PEEP high level (CPAP mode), but they are supported by the set level of PSV during the PEEP low level. BIPAP parameters were initially set as follows: PEEP high level (Plat^S), 20% higher than the peak airway pressure during PSV or 35 cm H²O, whichever was higher; respiratory rate, 1 breath/min; and length of the sigh (Ti^S), individually set between 3 and 5 s, according to the patient's tolerability (mainly limited by coughing reflex and hemodynamics;fig. 1).

Because of the higher inspiratory pressure and the longer Ti^Sobtained by design during Sigh, the Paw^mcould have increased. To avoid the possible influence of changes in Paw^mon oxygenation, Paw^m(averaged over a 1-min period) was always kept constant by adjusting the PEEP level.

We determined all measurements at the end of each study period. We collected arterial blood samples to measure Pao², Paco², pH, and hemoglobin concentration and saturation (HbO²) (ABL 330; Radiometer, Copenhagen, Denmark). Blood pressures were measured with pressure transducers zeroed at the mid-axillary line. During Sigh, all hemodynamic parameters were measured between two sighs.

Airway pressure and flow were measured with a CP100 Pulmonary Monitor (Bicore Monitoring Systems, Irvine, CA), through a disposable flowmeter (VarFlex flow transducer; Bear Medical Systems, Bilthoven, The Netherlands) inserted between the Y-piece of the ventilator circuit and the proximal end of the endotracheal tube. The analog output of the pulmonary monitor was connected to a personal computer and processed via  an analog-to-digital converter (Colligo, Elekton, Italy) at a sample rate of 100 Hz.

A 5-min-long sample was recorded at the end of each study period and stored for subsequent computer analysis. After the first 3 min of recording, we performed a minimum of three end-expiratory and three end-inspiratory airway occlusions, by pressing for at least 3 s the appropriate button on the ventilator panel. During Sigh, the occlusion maneuvers were performed between two sighs. The airway pressure drop in the first 100 ms of an occluded inspiration (P^0.1) was measured as an index of the neuromuscular drive. 21We performed the end-expiratory occlusion first, to avoid interference with the measurement of P^0.1. Data were then analyzed with dedicated software (Computo; Elekton, Agliano Terme, Italy). Tidal volume was obtained by integration of the flow signal.

From the analysis of traces with nonoccluded breaths we obtained (1) respiratory rate, by counting the number of breaths in 1 min; (2) inspiratory time (Ti^PSV) and expiratory time (Te^PSV) and inspiratory to expiratory time ratio (I:E^PSV) by averaging at least 10 PSV breaths; (3) mean tidal volume in 1 min (V^T,mean); (4) V^E, calculated as V^T,mean× respiratory rate; (5) Paw^m, measured as the average of the Paw^msignal during a 60 s period; and (6) V^Tof PSV breaths (V^T,PSV), determined by averaging at least 10 breaths. From sigh breaths we determined (1) tidal volume (V^T,S); (2) inspiratory time (T^I,S); (3) inspiratory plateau pressure (Plat^S), corresponding to the higher BIPAP level applied to perform sighs; (4) and fraction of minute ventilation supplied by the sigh breaths (V^E,S), calculated as V^T,S/V^E.

From the analysis of occluded breaths we computed (1) end-inspiratory elastic recoil pressure (Pel,rsi), measured as the plateau airway pressure (Paw) at relaxation of respiratory muscles, identified by visual inspection during an inspiratory occlusion; (2) end-expiratory elastic recoil pressure (Pel,rse) measured as the plateau Paw at relaxation of the respiratory muscles, identified by visual inspection during an end-expiratory occlusion (all the occlusions that did not reach an identifiable relaxation were discarded); and (3) P^0.1, measured during the first end-expiratory pause as the deflection in Paw in the first 100 ms after the start of patient inspiratory effort. We discarded by inspection the initial 5 ms to avoid uncertainties in recognizing the actual start of the inspiratory effort; the Paw drop in the following 100 ms was taken as the P^0.1value. 22We computed the Crs as Crs = V^T,PSV/(Pel,rsi − Pel,rse).

To assess the effects of sighs on lung volumes, we measured the end-expiratory lung volume (EELV), which represents the total gas volume at end expiration when PEEP is applied. A 5-cm clampable tube was inserted between the endotracheal tube and the Y-piece of the ventilator. Immediately after an end-expiratory occlusion maneuver, performed as described above, we clamped the endotracheal tube. We then disconnected the patient from the ventilator and measured the lung volume by a simplified closed-circuit helium dilution technique. 23Measurements were performed in duplicate. During Sigh, the EELV was measured one or two breaths immediately before a new sigh.

Values are expressed as mean ± SD. We used one-way analysis of variance for repeated measurements to compare the three study steps. Individual comparisons were performed with use of paired t  tests, with application of the Bonferroni correction for multiple comparisons. We specifically planned to compare Sigh with Base 1 and Base 2 with Base 1. For all the tests used, P < 0.05 was considered significant.

Relevant characteristics of the patients at the time of enrollment are shown in table 1. The main study results are shown in table 2. Time elapsed from the diagnosis of ARDS was 3.7 ± 1.8 days. Hemodynamic parameters did not show any significant change throughout the study.

Plat^Swas 38 ± 3.2 cm H²O, corresponding to a V^T,Sof 1,148 ± 301 ml; Ti^Swas 3.6 ± 0.7 s.

The introduction of one sigh per minute was associated with an increase in Pao²(41.6 ± 33.9 mmHg [29.8 ± 16.2%];P < 0.001;fig. 2A), accompanied by an increase in HbO²(1.5 ± 1.5%[1.5 ± 1.5%];P < 0.01). Returning to Base 2 led to a reversal of the gas exchange improvement, and Pao²and Hbo²were not significantly different during Base 1 and Base 2. Paco²and pH did not show any significant change throughout the protocol.

PEEP was significantly lower during Sigh with respect to Base 1 (average decrease, 1.1 ± 0.7 cm H²O). V^Edid not show any significant difference among the three steps. EELV increased significantly from Base 1 to Sigh (average increase, 136 ± 140 ml [12.3 ± 10.8%]; range, −82–392 ml;P < 0.01;fig. 2B) and returned to baseline value after discontinuation of the sigh. Crs increased significantly during Sigh with respect to Base 1 (4.9 ± 4.9 ml × cm H²O⁻¹[10.4 ± 9.4%];P < 0.01), whereas Crs Base 2 was not significantly different from Crs Base 1 (fig. 2C).

During Sigh, there was a significant decrease of V^T,PSV(P < 0.01;fig. 3A), Ti^PSV(P < 0.01;fig. 3B), and P^0.1(P < 0.01;fig. 3C). All three parameters returned to baseline after discontinuation of sighs, and no significant differences were found between Base 1 and Base 2. There was no significant change in I:E^PSVduring the three study periods.

The main result of this study was that the addition of one sigh per minute during PSV in patients with early ARDS improved arterial oxygenation and likely promoted alveolar recruitment.

The use of sigh with Plat^Shigher than 35 cm H²O improved arterial oxygenation in all studied patients, in spite of a slightly lower PEEP and similar Paw^m(table 2). This positive effect on gas exchange was associated with an increase in EELV and Crs, suggesting that sighs possibly promoted alveolar recruitment. These results are in agreement with the reported beneficial effect of various types of RMs that have been applied in fully ventilated and paralyzed patients. These RMs included the administration of a sustained inflation lasting several seconds, 10,24a cyclical higher inflation pressure, 11and a cyclical higher PEEP. 12 

We computed Crs by applying the airway occlusion technique during PSV breaths, recognizing the presence of muscle relaxation by visual inspection. In spite of the many possible limitations of this method, we showed in a previous study 25that during PSV the airway pressure plateau during inspiratory occlusion is a reliable measurement of the relaxed elastic recoil pressure of the respiratory system in patients with acute lung injury or ARDS.

The average Plat^Sused in our study was 38 cm H²O. Using higher inflation pressure during a sigh might have resulted in more effective recruitment and further improvement in arterial oxygenation but also in increased risk of barotrauma. The Plat^Svalues that we used are similar to the inflation pressures that have been effective in improving gas exchange in other studies in which sighs were cyclically delivered. 11,12Moreover, in spontaneously breathing patients, inspiratory pressure levels may also be limited by subjective tolerability and stimulation of the cough reflex.

The modality used in our study to administer sigh differs in some ways from other studies in which cyclically delivered sighs successfully improved gas exchange. 11,12 

First, it was our goal to preserve spontaneous respiratory activity. During CPPV in paralyzed patients, gas distribution in the lung is determined mainly by regional compliance. In supine position, anterior (nondependent) regions of the lung are the most compliant, and ventilation in the posterior (dependent) regions may be compromised, leading to progressive atelectasis. It has been shown that preserving spontaneous ventilatory effort may improve gas exchange, possibly by enhancing ventilation of the dependent parts of the lung and preventing atelectasis. 14,26 

Second, in our study, sighs were pressure-controlled rather than volume-controlled. During pressure control ventilation, inspiratory flow stops when alveolar pressure reaches the set inspiratory pressure. If alveolar recruitment takes place, more gas is delivered to maintain plateau pressure, facilitating the filling and stabilization of newly opened alveoli. 27 

Third, regardless of the ventilatory mode, sigh pressure was maintained for a relatively long time, possibly favoring the filling of recruited alveoli also by gas redistribution.

Finally, compared to the study of Pelosi et al. , 11who used three consecutive sighs per minute, and the study of Foti et al. , 12who increased the PEEP for two consecutive breaths every 30 s, in our study a single sigh per minute proved beneficial. Identifying the lowest effective sigh rate is of primary importance because of the known drawbacks on lung parenchyma due to high distending volumes or pressures. However, Davies at al. 28showed that sighs delivered at a rate of two breaths every 10 min were of no benefit during PSV, despite the use of inflation pressures comparable with those of our study. The role of frequency on sigh effectiveness remains to be formally explored.

It is worth noting that Crs was slightly higher in our patients than in those studied by Pelosi et al.  11Hence, the tendency to develop alveolar collapse was possibly lower in our patients. However, the higher level of PEEP used in the study of Pelosi et al.  11should have prevented derecruitment more efficiently 29than in our study, where level of PEEP was limited also by the conscious patient's tolerability.

The introduction of sighs was associated with a decrease in respiratory drive, as shown by a limited but significant decrease in P^0.1, V^T,PSV, and T^I,PSV. It has been shown that with propofol general anesthesia a sigh reduces the V^Tof the ensuing breaths, possibly due to a post-sigh inhibition of peripheral chemoreceptors and lung mechanoreceptors. 30A similar mechanism may have been at play in our patients.

Several factors may have determined these changes. It has been shown that during PSV both V^Eand P^0.1decrease when oxygenation is improved. 13The additional V^Tprovided by the sigh supplied between 10 and 12% of baseline V^E, decreasing the amount of ventilation that patients had to provide.

The observed effect of sighs on the P^0.1may also be explained in part by the changes in EELV, 31which may directly affect the P^0.1measurement. However, we did not find any significant correlation between the effect of sigh on P^0.1and that on EELV.

Partial ventilatory techniques prevent paralysis and reduce the use of sedative drugs. It was our goal to learn whether by adding sigh it was possible to extend the indication for the use of partial ventilatory techniques to patients with early ARDS. In a recent study, acute lung injury was successfully managed with PSV in 38 of 48 patients, 11suggesting the possibility of safe use of PSV in the early phases of ARDS. PSV was more likely to fail for patients with high ventilatory needs and low Crs. By decreasing the respiratory drive, as in this study, sigh may increase the efficacy of PSV in this patient population.

Moreover, the achievement of a better Crs and alveolar recruitment may play a relevant role in increasing the success rate of PSV in ARDS patients. The response to PSV is also directly affected by pulmonary edema 17and hypoxia. 18The increase in arterial oxygenation associated with the introduction of the sigh may increase PSV efficacy in this patient population.

It is worth considering that we selected patients in the earlier phase of acute respiratory failure, when alveolar collapse and dependent atelectasis, mainly related to increased lung weight and superimposed pressure, have shown good recruitability. 5The same results may not be achieved in patients with late-stage ARDS. In addition, sighs may have a less pronounced effect in patients who are recovering from the acute phase of respiratory failure, 28where a fibroproliferative process has started and alveolar collapse is driven by an increased pulmonary elastance. Early addition of sighs may have the function of opening previously collapsed lung and preventing lung collapse associated with low V^Tventilation.

Our goal was to investigate the acute short-term physiologic effects of sigh administration. Indeed, no conclusion could be drawn about the outcomes for the patients. A different study design will be necessary to investigate the long-term effects of this approach.

In conclusion, the addiction of one sigh per minute, delivered as pressure-controlled breath to PSV in ARDS patients, improved oxygenation and lung mechanics. These findings suggest a possible role for clinical evaluation of periodic recruitment maneuvers during assisted breathing.

The authors thank Gianluca Greco, Mirco Nacoti, and Enrico Colombo, M.D. (Residents, Department of Anesthesia and Intensive Care, San Gerardo Hospital, Monza), for their help in collecting the data; and the nursing and medical staffs of the Intensive Care Unit of the San Gerardo Hospital for their patience and cooperation, without which the study could not have been performed.