Keywords

Lung recruitment, Multilevel lung ventilation, Pressure control ventilation

Introduction

Anaesthesia and extracorporeal circulation (ECC) during prolonged cardiac surgery often cause lung injury, which is usually represented by grade 1 ARDS (PaO₂/FiO₂ ratio = 200-300). This phenomenon is already well documented [1] and our own experience shows that the lung mechanics and transport of O₂ are significantly impaired during and after anaesthesia [2]. Placing the patient in a supine position during operation increases the dead space over tidal volume ventilation (VD/VT) by approximately 15-20%. The decrease in pulmonary compliance is 10-20% on average and up to 35% in obese persons. The geometry of the alveoli is usually impaired by their cyclical collapsing and reopening, which leads to surfactant damage [2,3]. The typical complications on the level of alveoli are atelectasis, atelectotrauma, biotrauma, impaired ventilation-to-perfusion ratio (Qs/Qt), increased permeability of the alveolar-capillary membrane for liquids and the increase in ventilation work after disconnecting from artificial lung ventilation (ALV). The application of high concentrations of O₂, i.e., FiO₂ > 0.6-0.9, leads to resorption atelectasis [4,5]. Because of considerable effect on the alveolar geometry, this condition may lead to significant postoperative complications. In many cases, the complications mentioned above can be prevented by a reduced usage of high O₂ concentration, retaining the alveolar geometry by maintaining positive end expiratory pressure (PEEP). Applying the principles of "protective ventilation" by performing a lung recruitment manoeuvre during the postoperative period by using necessary PEEP may help. However, a new ventilation technique called programmed multilevel ventilation (PMLV), in this case, "programmed three-level artificial lung ventilation (3-LV)", could be an even better solution. 3-LV is our original method developed especially for the recruitment and ventilation of inhomogeneously affected lungs, which is typical in ARDS of various origins. The pressure control ventilation (PCV) mode in ARDS appears to be less effective because even with the best frequency and pressure adjustment, it cannot achieve an optimal distribution of gases to the lung compartments, which are differently affected. Our objective was to determine the lung recruitment ability by using the 3-LV ventilation technique in comparison with the commonly used conventional PCV mode in postoperative cardiac surgery patients after weaning from ECC. In this paper, we evaluate the parameters of lung mechanics, oxygenation and postoperative adaptation to spontaneous ventilation in 3-LV compared with PCV.

Methods

Study design and patients

The study was approved by the ethics committee of the East Slovakian Institute of Cardiovascular Diseases: EK no. VZ/7 / KardO/2011. Head of Committee Stanislav Juhas, MD, PhD.

This prospective study was performed starting from February 2014 to January 2017. It comprises 88 cardiac surgery patients, with cardiac arrest lasting 50 min (79 ± 26 min) in ECC. After being disconnected from the ECC, the patients were randomly divided into two groups ventilated by alternating PCV and 3-LV techniques in different orders. Protocol: Group 1 (n = 44) starts with 1 hour of PCV followed by 3-LV mode in the second hour. Group 2 (n = 44) was ventilated in the reverse order, thus starting with 3-LV for 1 hour followed by PCV, as described in the time flow chart in Figure 1. Patients undergoing scheduled cardiac surgery were assigned. Patients undergoing operation on Monday, Wednesday and Friday were assigned to Group 1, while the ones in Group 2 underwent operation on Tuesday and Thursday. The exclusion criteria were a chronic lung disease (bronchial asthma, chronic obstructive pulmonary disease (COPD), ejection fraction of the left ventricle (LVEF) < 30% and low lung compliance at the beginning of anaesthesia with dynamic compliance (Cdyn) < 30 ml^*cm H₂O^-1. The demographic parameters can be found in Table 1.

Anaesthesia: Identical anaesthesia was applied to both groups in the following combination: Propofol (Fresenius) at the introduction to anaesthesia at a dose of 150 ± 41 mg, a 2.5- to 5-mg bolus of Midazolam (Accord) at the introduction, 15 - 24 µg of Sufentanyl (Janssen) at the introduction to anaesthesia, a continuous dose of Sufentanyl forte (Jansen) of 0.66 ± 0.12 µg^* kg^-1*h^-1, a 50-mg bolus of Atracurium (GSK) repeated as necessary, and a minimal alveolar concentration of Sevofluran (Abbott) of (MAC) = 0.6-1.1 or 0.6 ± 0.2 during ECC. The patients were intubated with an 8.0 mm orotracheal tube. Artificial lung ventilation (ALV) during anaesthesia was performed using the principles of protective ALV listed in Table 2. FiO₂ was maintained if possible at not higher than 0.5, and ETCO₂ was > 35 but ≤ 41 mmHg (5.6 kPa). Measured parameters: During anaesthesia and during ALV in the postoperative period, the ventilation parameters, parameters of lung mechanics, static compliance (Cst), airway resistance (Raw), oxygenation parameters, concentrations of blood gases and haemodynamic parameters were monitored. After admission to the intensive care unit (ICU), the patients were ventilated according to the study protocol. Ventilator Chirolog Servoventilator AURA-V with a computer assistance module ProfiLungs (Chirana Medical, Slovakia) was used. During ALV the changes in the parameters of lung mechanics, oxygenation index, values of ETCO₂, and minute ventilation (MV) were monitored. The pressures applied and the extent of changes of ventilation volumes during 3-LV were also monitored. The measured values of specific tidal volume/ml^*kg^-1/ (VTspec) fluctuated depending on the individual transition pressure levels of the 3-LV within the ranges of 1.9-2.8 ml^*kg^-1, 2.9-4.9 ml^*kg^-1 and 5.0-7.2 ml^*kg^-1 in 7-8%, 60 - 75% and 15 - 20% of the breathing cycles, respectively. 3-LV caused variable volume, pressure and time ventilation. After weaning from ventilation and extubation, the spontaneous breathing frequency as an indicator of adaptation to spontaneous breathing was observed and afterwards compared between both groups.

Explanation of basic principles of the three levels of programmed ventilation (3-LV)

It is well known that in classical ALV regimes, even the best frequency and pressure optimization procedure of ALV parameters cannot optimally distribute gases to individual but differently damaged compartments [6]. Hence, a single ventilation mode with the ventilation frequency (f), time of inspiration-to-expiration ratio (T_I: TE), respiratory volume (V_T), positive end-expiratory pressure (PEEP), and ventilation support pressure (Ppc) in pressure controlled mode as the fixed parameters cannot optimally ventilate inhomogeneous lungs [6,7]. Differently damaged compartments are dispersed in whole lungs. It is not possible to divide the flow of gases into each separate lung compartment using fixed ventilation parameters [2,5]. Fixed ventilation parameters allow optimal ventilation for only some compartments of the lungs with well-suited mechanical properties. Other compartments will be ventilated more or less sub-optimally. One possible solution is to use a ventilator with programmed 3-LV to generate variable volume, pressure and time ventilation cycles. It can improve the distribution of gases into the variably damaged lung compartments. The setting of the ventilation parameters of 3-LV is derived from continuously measured lung mechanics by a computer in the Aura-V ventilator with the ProfiLungs system. The principle of pressure changes during 3-LV compared with PCV is schematically depicted in Figure 2.

Statistics

The acquired parameters of lung mechanics and oxygenation index in both groups were statistically evaluated by Student's paired t-test (Microsoft Excel).

Results

Based on the data shown below, the 3-LV ventilation method achieved better parameters than PCV. We compared 3LV to PCV using the following parameters: Lung mechanics, oxygenation, carbon dioxide elimination and haemodynamic. For the assessment of recruitment efficiency, the adaptation to breathing after disconnecting from ALV, the oxygenation of arterial blood and Cst were evaluated. The effect of ALV on the circulatory system was also considered. After admission to the ICU, the acute lung injury score (LIS - Murray score) was in the range of 1.5 ± 0.25 points, confirming moderate lung impairment. The change in ALV methods does not cause a significant decrease in the mean arterial pressure (MAP). The average MAP before and after the application of 3LV was 79 ± 8 vs. 75 ± 6 mmHg and 83 ± 8 vs. 79 ± 7 mmHg in the first and second groups, respectively (p = NS. Cardiac output was measured using Monitor Vigileo (Arrow)), the influence of ventilation on haemodynamics was assessed. In all cases, as an indirect indicator of peripheral perfusion and CO, we measured the central venous blood oxygen saturation (ScvO₂). After the patient was introduced to ALV and disconnected from extracorporeal circulation (ECC), the arterio-alveolar difference in oxygen (A-aDO₂) significantly increased 2 to 4-fold, (average 201 ± 42 mmHg, p < 0.01). After the first hour of ALV, the average SaO₂ and SvO₂ were within the physiological range 94-100% and 67-77%, respectively. This indicates adequate peripheral perfusion and sufficient CO. The value of the PaO₂/FiO₂ ratio after weaning from ECC decreased significantly in both groups, to below 300 Torr, which corresponds to grade 1 ARDS (according to the Berlin classification). The PaO₂/FiO₂ ratio after admission to the ICU was 249 ± 24 mmHg in Group 1 and 266 ± 25 mmHg in Group 2 (p = NS). ALV lasted 3.3 ± 0.4 hr and then the patient was disconnected from ALV. The PaO₂ in Group 1 vs. Group 2 was 384 mmHg vs. 321 mmHg (p < 0.01). During 3-LV, we observed in both groups an increased EtCO₂ in the first 25 minutes of using 3-LV, which indicated an increasing area of gas exchange as a sign of lung recruitment (Figure 3). The Cst was monitored every 10 minutes and showed major improvement in Group 1 compared with Group 2. (p < 0.01). In all cases when 3-LV was applied, regardless of whether it was applied for the first or second time, the Cst was improved (+ 30%) compared with PCV (p < 0.01). This effect was observed regardless of the order in which the 3-LV technique was applied (Figure 4). At 60 and 120 minutes after extubation, the breathing frequency as an indicator of adaptation to spontaneous breathing was assessed. In both cases, it was higher in Group 2 than in Group 1 (p < 0.05). In Group 1, Cst was higher than that in Group 2. The improved adaptation in Group 1 was probably caused by the lower ventilation work (Table 3).

Discussion

It is well known that use of ECC during cardiac surgery is followed by substantial signs of lung impairment. The PaO₂/FiO₂ ratio decreases to values indicating the presence of grade 1 ARDS [1]. The acute lung injury score (LIS) after the surgical procedure fluctuated on average at 1.5 points, which indicates moderate lung impairment. The mechanism of impairment is multifactorial [1]. During anaesthesia, there is a decline in A-aDO₂ associated with moderate hypoxemia and caused by the increased disproportion of ventilation-perfusion due to the decreased ventilation of perfused parts of lungs [2]. A major disproportion of ventilation-perfusion was observed after weaning from ECC. Seventy percent of resorption atelectases arise predominantly in dependent lung zones, which are correlated with the number of hospitalization days [3]. The decrease in Cst in all monitored groups of patients after disconnection from ECC is a sign of development of atelectasis and impairment of alveolar geometry [3]. Mols, et al. [5] define atelectasis as a condition of the absence of gas in an alveolus causing its long-term collapse. Atelectotrauma as a cyclically repeated collapse and filling (opening) of the alveolus during a ventilation cycle. Alveoli open during inspiration and close during expiration. Alveolar recruitment is characterized by the re-opening of a previously collapsed alveolus, whereas derecruitment is the collapse of an alveolus previously opened by recruitment. Ongoing recruitment (lasting recruitment) is defined as a condition of permanent alveolar recruitment, which does not allow derecruitment. From a mechanical-pneumatic point of view, it is the opposite of atelectasis.

All groups of our patients demonstrated a decrease in Cst after being disconnected from ECC as a sign of atelectasis. Our opinion that early recruitment using adequate PEEP or another lung recruitment technique (in our case, 3-LV) has a beneficial effect was already described by Borges, et al. [8]. With increasing the FiO₂ value we are able to improve the PaO₂, but this may cause absorption atelectasis and hyperoxia; therefore, we did not use it [4,9]. The high FiO₂ may have an adverse effect on CO and peripheral vascular resistance (PVR) [10]. We applied this concentration of O₂ to maintain PaO₂ in a recommended range of 75 - 160 Torr, (10.0 - 21 kPa) [11]. It is well known that increased FiO₂ and PaO₂ in ICU patients increases mortality compared with patients with adequate PaO₂ [9,11]. Peak pressure applied during 3-LV had a minimal impact on circulation if adequate PEEP was used. During 3-LV with the application of adequate PEEP and the minimum possible peak airway pressure (Paw max), a moderate or insignificant decrease in MAP and the heart rate was observed. Similarly, no significant desaturations of venous blood as an indicator of a decrease in cardiac output (CO) was observed during 3-LV. Lung recruitment by 3-LV is probably achieved by irregular changes in respiratory volume, pressure and breathing frequency during each respiratory cycle. Non-constant respiratory volumes, inspiration times and ventilation pressures probably enhanced the opening of collapsed but recruitable compartments [12]. The benefit of 3LV ventilation could be confirmed by the improvement of PaO₂/FiO₂ and lung compliance as well as by a reduction of A-aDO₂ [13]. According to Johnson [2], the increase in Cst is a marker of lung recruitment effectiveness. This finding concurs with our results. The frequency of spontaneous breathing after disconnecting from a ventilator is a good indicator of spontaneous breathing work. Increasing breathing frequency is the most significant predictor of respiratory failure [14]. Therefore, we used this parameter to compare respiratory stability after disconnection from 3-LV or PCV.

Conclusion

1. We compared the effectiveness of PCV with the 3-LV ventilation method in postoperative cardiac surgery patients with ECC. Based on the monitored Cst, PaO₂/FiO₂, EtCO₂ we assessed the lung recruitment, impact on circulation and spontaneous breathing stability.

2. The application of 3-LV compared with PCV during postoperative period improved arterial blood oxygenation PaO₂/FiO₂, the elimination of CO₂, the stability of alveolar geometry (Cst) and the transition from artificial ventilation to spontaneous breathing.

3. The advantage of using 3-LV is effective lung recruitment without using a high PEEP. The mean airway pressure value was comparable with PCV group.

4. In the cases of inhomogeneous distribution of gases in the lungs, for example, ARDS, SARS - (Severe Acute Respiratory Syndrome), H1N1 pneumonia, and lung oedema, 3-LV could be successfully used for lung recruitment and the improvement of gas distribution in such damaged lungs.

5. The effectiveness of 3-LV compared with classic ventilation methods (PCV, PS, SIMV, etc.) will probably be higher.

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