Respiratory failure resulting from unilateral pathology of lungs when their compliance and resistance markedly differ may require special methods of artificial ventilation. In such cases, the use of conventional techniques of ventilation is likely to cause overdistension of the non-affected lung (resultant emphysema) or hypoventilation of the affected lung [1, 2, 3]. The optimal strategy is independent ventilation of the left and right lung, which requires endotracheal intubation and two synchronized ventilators. The parameters of ventilators ought to be adjusted to the conditions in both lungs. In addition to two ventilators, which occupies much space and limits the access to patients, the method requires increased numbers of the medical staff [4, 5, 6]. Another method involves the use of a single ventilator [7, 8, 9] and a volume splitter [10, 11, 12]. The splitter is attached to the ventilator’s breathing system and delivers gases independently to each lung in the chosen ratio ranging from 1:1 to 1:5. The independent tidal volumes reach both lungs through two separate breathing systems and a double lumen endotracheal tube. Expiratory gases are evacuated from lungs by independent expiratory systems. The assumed tidal volume and number of breaths are set via the ventilator and the splitter mediates the distribution of gases in the set proportion. The delivery of gases to both lungs is synchronized: consecutive phases of the respiratory cycle occur simultaneously in both lungs. The splitter can be additionally equipped with PEEP valves, thanks to which different end-expiratory pressures are obtained in the right and left lung.

In the majority of anaesthesias for thoracic surgical procedures, endobronchial intubation is used, which separates and isolates the airways [13], providing favourable operative conditions and protecting the non-affected lung against possible blood or secretion spillage from the affected lung. The separation of airways enables ventilation of one lung or independent ventilation of both lungs with different tidal volumes. However, under such circumstances, ventilation of one lung may lead to gas exchange abnormalities increasing the intrapulmonary shunt of unoxygenated blood [14, 15, 16]. Therefore, independent, synchronized ventilation in which tidal volumes are adjusted to each lung separately seems to be the optimal method of management during anaesthesia for thoracic surgery.

The aim of the present study was to assess the distribution of tidal volumes and airway pressures using independent lung ventilation  (ILV) during general anaesthesia for thoracic surgery.

METHODS

The study was approved by the Bioethical Committee of the Medical University of Lublin and included patients meeting ASA I-II criteria scheduled for elective thoracic surgical procedures under combined general anaesthesia requiring endobronchial intubation. The exclusion criteria were an airway pathology, COPD  or infeasible endobronchial intubation.

All patients were premedicated one hour before surgery with diazepam 10 mg. Before induction of anaesthesia, after preoxygenation, fentanyl 5 µg kg-1 and atropine 0.5 mg were administered. Anaesthesia was induced using thiopentone 5 mg kg-1 and suxamethonium 1 mg kg-1 and patients were intubated with double lumen Robertshaw tubes whose proper placement was verified with the fiberoscope. The volume-controlled IPPV was carried out using the Primus (Dräger, Germany) anaesthetic machine. VT was set at 6-10 mL kg-1, f 12-15 min-1 and FIO2 1.0. Anaesthesia was maintained with fractionated doses of fentanyl 0.001mg kg-1 and sevoflurane 1.5-3%. Muscle relaxation was provided using vecuronium 0.1 mg kg-1. The following parameters were routinely monitored during anaesthesia: SAP, DAP, HR, SpO2, ETCO2 and ET sevoflurane.

Independent lung ventilation was carried out using the splitter of tidal volumes constructed at the Institute of Biocybernetics and Biomedical Engineering, Polish Academy of Sciences (IBiIB PAN) [17].

The examinations were performed sequentially before the onset of surgery with the thoracic cavity closed. The measurements were commenced in the supine position with free distribution of respiratory gases between both lungs (stage I) and with forced distribution (1:1) for each lung separately (stage II). After the change into the lateral decubitus position, free distribution ventilation was restored (stage III) followed by ventilation with forced distribution (1:1) (stage IV). In each case, data were recorded 10 min after the institution of a particular stage. The following parameters were assessed: tidal volumes of both lungs (VTtot), of the right lung (VTP), of the left lung (VTL), peak airway pressures of the right (PmaxP) and of the left lung (PmaxL). The data were recorded using the Florian (Acutronic, Germany) analyser.

Data were statistically analysed using the structure index expressed as percentage, mean and standard deviation. Differences in means were tested using the Student’s t test for normal distribution; in the remaining cases, the Wilcoxon signed-rank test was applied for dependent samples and the Mann-Whitney test for independent samples. In all tests, p<0.05 was considered statistically significant.

RESULTS

The study involved 34 patients, 11 females and 23 males aged 19-78 years and weighing 55-106 kg. The types of surgery are listed in Table 1. The mean VTtot during anaesthesia was 625±82 mL. In the supine position, gases were distributed in equal proportions to each lung both at stage I and at II. In the lateral position (18 patients on the left and 16 patients on the right side), at stage III the distribution was always unequal: the „lower” lung (non-affected, dependent) received 47.4±6.8% whereas the “upper” lung (affected, nondependent) – 52.6±6.8% of the VTtot, and the differences were significant (p=0.035). With forced distribution (stage IV), no differences in gas distribution were observed between both lungs (Table 2).

Pmax in the supine position ranged from 16.0±4.0 cm H2O to 16.4±3.7 cm H2O (1.6±0,4 kPa) and in the lateral position - 16.8±3.8 cm H2O - 17.7±5.0 cm H2O (1.6± 0.4-1.7± 0.5 kPa) (Table 3). The use of forced distribution did not cause significant changes in Pmax in both lungs, irrespective of the patient`s position (Table 4).

In all patients, the course of anaesthesia and surgery was uneventful. After the completion of surgery and routine awakening, patients were transferred to the postoperative ward for further treatment. 

DISCUSSION

The presented assessment of usefulness of independent, synchronized lung ventilation is the first such study in our country. The study was carried out in patients in good general condition, without extensive lung pathologies and need for artificial lung ventilation during the postoperative period. A unique splitter of flow volumes and one ventilator were applited.

The development of respiratory failure caused by unilateral lung pathology justifies the use of alternative methods of artificial lung ventilation [3, 18]. Non-symmetrical distribution of respiratory gases during routine ventilation leads to overdistension of alveoli of the non-affected lung; the blood flow is delivered to poorly ventilated alveoli of the affected lung and the shunt of unoxygenated blood in the lungs increases [6, 19, 20].

In order to maintain proper oxygenation when the difference in compliance between the left and right lung is big, independent lung ventilation should be applied. Non-synchronized ILV with two ventilators is likely to worsen the circulatory disturbances (venous cardiac return and cardiac output are decreased) [21]. Ventilators may work in the volume- or pressure-controlled modes. A variety of modifications of systems were described that enable ILV with a single ventilator and various PEEP values for the left and right lung [3, 8].

The choice of artificial ventilation parameters depends on the functional status of lungs. The best therapeutic effects were observed when values of VT for both lungs were similar; however, under such circumstances airway pressures in the affected lung were markedly higher, posing the risk of barotrauma. For these reasons, the tidal volume was adjusted to the plateau pressure in the affected lung [22]. Another method to choose proper VT was based on the equalization of differences in ETCO2 values and shapes of capnographic curves of both lungs [23]. 

In the present study, the method of independent lung ventilation used was different. Thanks to the splitter of respiratory gases, tidal volumes were delivered to each lung separately and free and forced ventilations compared.

The analysis of free distribution demonstrated that in the supine position there was no significant difference in distribution of respiratory gases to each lung. Similar results were described in two other studies [24, 25]. Their findings are markedly different than those from other publications which showed that 45-46% of tidal volume was delivered to the left lung and 54-55% to the right lung [4, 20]. Even bigger discrepancies concern lung ventilation in patients in the lateral decubitus position. Our findings demonstrated a significant difference in tidal volumes distributed to the dependent lung compared to the nondependent one during free flow of respiratory gases.  Our data are markedly different from those reported by other authors. According to Bindslev and co-workers [23], who analysed free distribution of respiratory gases in the left lateral position, 39% of total tidal volume was delivered to the dependent lung and 61% to the nondependent one. Baehrendtz and colleagues [24] reported 34% of distribution to the dependent and 66% to the nondependent lung. Some other studies in ITU patients demonstrated that 30% of total tidal volume was delivered to the dependent and 70% to the nondependent lung [4, 20]. It should be emphasized, however, that the studies mentioned were carried out in small populations of patients (from 7 to 11 individuals). Moreover, individual volumes were estimated using the method of gas dilution and a mass spectrometer. The method is extremely laborious in the clinical setting, requires particular meticulousness and may result in numerous errors.

In our study, once the splitter with the 1:1 distribution of tidal volumes was incorporated, almost equal distribution to each lung was achieved both in the supine and lateral position. Similar results were reported by other authors when both lungs were ventilated independently using two synchronized ventilators [4, 20].

Lung ventilation in the lateral position is varied. In the “lower” lung, ventilation is decreased and perfusion increased. Simultaneously, in the “upper” lung, the peak pressure in the airways may exceed the alveolar perfusion pressure, thus increasing the 1st West zone in gas exchange. Independent ventilation eliminates those unfavourable phenomena. Independent ventilation was demonstrated to improve cardiac output, to reduce the venous blood admixture, thus to increase the oxygenation of arterial blood. On the other hand, this kind of ventilation is accompanied by increased pressures in airways of the dependent lung and the opposite effect in the nondependent lung [4, 20, 24].

Our results do not conform to those reports: maximum pressures in airways did not exceed safe limits both in the supine and in lateral position. Moreover, forced 1:1 ventilation did not induce significant Pmax changes in both lungs, irrespective of the patient`s position. This appears to be associated with the fact that the study was conducted in patients, whose high proportion had no significant abnormalities of lung compliance whereas the studies mentioned earlier concerned patients with bilateral lung pathologies treated in ITUs [4, 20]. The comparison of results is hindered by the lack of detailed descriptions of lateral positions of patients in some reports [20, 24].  In our study, some patients were placed on the left whereas some on the right side, which was not always taken into account while designing similar studies [4, 23]. Since the reports concerning ILV during general anaesthesia are not numerous, further thorough studies are needed.

The use of tidal volume splitters may be particularly useful for thoracic surgeries when proper ventilation and perfusion of lungs are difficult to maintain during lateral thoracotomies. Ventilation set optimally for each lung is likely to ensure safe management of patients during the procedure and anaesthesia. 

CONCLUSIONS

During artificial lung ventilation with free flow of respiratory gases in the supine position, gas volumes are equally distributed to the left and right lung.

In the lateral decubitus position, with free flow of gases, the volume delivered to the nondependent  lung is higher compared to the dependent lung.

Independent synchronized lung ventilation in the ratio of 1:1 equalizes tidal volumes delivered to both lungs in the supine and lateral position.

Independent synchronized lung ventilation in the ratio of 1:1 does not cause significant changes in peak airway pressures of both lungs, irrespective of the patient`s position.

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Address:

*Sławomir Sawulski
Klinika Anestezjologii
i Intensywnej Terapii
UM w Lublinie
ul. Jaczewskiego 8, 20-950 Lublin
tel.: 0-81 724 43 32
e-mail: anest@am.lublin.pl

Received: 18.10.2009
Accepted: 20.01.2010