Mini-ventilation for improved oxygenation during lung resection surgery.
The purpose of this study was to examine an alternative ventilation modality to treat hypoxaemia and keep arterial oxygen saturation of 90% or greater with minimum impairment of the surgical field during OLV. In this study, the non-dependent lung was ventilated alternately, in crossover fashion, by continuous positive airway pressure (CPAP) and by a portable ventilator with low rate and pressure. We anticipated that a low ventilation rate and pressure would produce the least interference with the surgeon's exposure but give better matching of ventilation and perfusion and improve oxygenation.
PATIENTS AND METHODS
With the approval of our local Ethics Committee, we conducted a prospective randomised singleblinded crossover study of adult patients who were scheduled for open lobe resection with combined epidural/general anaesthesia. All patients signed informed consent to participate in the study. Exclusion criteria included American Society of Anesthesiologists physical status classification [greater than or equal to] IV, pregnancy, weight below 50 kg or above 120 kg and abnormal preoperative pulmonary function tests. Oxygen saturation below 85% at any time was set as an end-point at which the study would be stopped. The primary outcome variables were the [P.sub.a][O.sub.2] measurements, while the secondary outcome variables were the spirometric measurements and the surgeons' evaluation of the surgical field.
All patients had oral premedication including diazepam 10 mg and metoclopramide 10 mg one hour before anaesthesia. On arrival at the operating room, an intravenous line was placed. Each patient was continuously monitored with electrocardiogram, pulse oximeter, invasive blood pressure, end-tidal capnography, urinary catheter and oesophageal thermometer. A thoracic epidural catheter (T5 to T8) was placed and then general anaesthesia was induced. A left-sided double-lumen endotracheal tube (Mallinckrodt Medical, Athlone, Ireland) was placed and its location was verified by auscultation and fibreoptic bronchoscopy. The dependent lung was ventilated by the same anaesthesia machine (Drager Narkomed 2A, North American Drager, Telford, PA, USA) in all patients and isoflurane was used for maintenance of anaesthesia.
Ventilation parameters by the main anaesthesia machine were as follows: tidal volume was set to 8 to 10 ml/kg during two-lung ventilation (TLV) and reduced to 6 to 8 ml/kg during OLV; respiratory rate was 10 per minute during TLV and was increased to 12 to 15 per minute during OLV, adjusted to keep [P.sub.a]C[O.sub.2] between 40 to 50 mmHg; inspired oxygen was 50% during TLV and was increased to 60 to 100% during OLV, adjusted to maintain saturation above 94%; positive end-expiratory pressure was 2 cm[H.sub.2]O, first in both lungs and then in the dependent lung during OLV. All patients were placed in the horizontal lateral decubitus position with padded rolls under the dependent axilla.
When OLV was initiated, the non-dependent lung was open to the atmosphere until 15 minutes after starting OLV, when the operated lung was either connected to a CPAP system or to a small portable time-cycled paraPAC-2D ventilator (SIMS pneuPAC Ltd, Luton, UK), each using 100% oxygen. The order of intervention was randomised, using computerised software that simulates tossing a coin. The CPAP pressure was set to 5 cm[H.sub.2]O and the mini-ventilation of the lung was set at a rate of 8 breaths/minute. Peak pressure and tidal volume were set to the lowest available values, resulting in peak pressure of 10 cm[H.sub.2]O and tidal volume of 100
to 150 ml. The treatment of the operated lung was alternated every 15 minutes with a five-minute interval between, without additional ventilatory support or oxygen insufflations and with the lung open to air, to avoid the influence of one treatment modality upon the other. The study intervention, mini-ventilation or CPAP, was terminated when lung separation was ended.
Data were collected by one of the researchers who was not blinded to the trial. Arterial oxygen partial pressures ([P.sub.a][O.sub.2]), peak inspiratory pressure and dynamic compliance were measured after 15 minutes each of TLV, OLV, OLV+mini-ventilation and OLV+CPAP. Peak inspiratory pressure and compliance were recorded from the anaesthesia machine spirometer. At the same time, the surgeon was asked to comment on his satisfaction with the surgical conditions. Surgeon satisfaction was graded from 0 (too much interference) to 3 (very good surgical field conditions). The surgeons were not entirely blinded to the ventilation mode since the portable ventilator has a distinctive sound.
Sample size was calculated assuming a difference in [P.sub.a][O.sub.2] between CPAP and mini-ventilation of 50 mmHg; a safety level of 95%; standard deviation of 60; intensity of 80% and measurement ratio of 1:1. Based on these assumptions, the minimal study size was 24 participants. Parametric data are presented as mean [+ or -] standard deviation. Statistical analysis was performed with Statistica (StatSoft, Tulsa OK, USA). Paired t-test and Wilcoxon signed rank test were used to compare the [P.sub.a][O.sub.2] and compliance data, and repeated measures were applied as needed. Differences were considered statistically significant at P <0.05.
Thirty-eight patients were included in the study; their data are summarised in Table 1. No patient was excluded from the study. The duration of surgery was 89 [+ or -] 30 minutes.
[P.sub.a][O.sub.2] decreased below 90 mmHg in one patient during TLV, but improved after repositioning of the double-lumen tube. [P.sub.a][O.sub.2] decreased below 90 mmHg in two patients during OLV without CPAP or miniventilation, in three during OLV+CPAP and in one during OLV+mini-ventilation, although the double-lumen tube position was verified in all these patients. The ratio between [P.sub.a][O.sub.2] and the fraction of inspiratory oxygen ([P.sub.a][O.sub.2]/FiO2 ratio) was significantly higher during TLV compared to OLV (P <0.001) and during mini-ventilation compared to CPAP (P <0.001). Oxygenation during OLV+CPAP was not significantly higher than during OLV.
There was no difference in peak inspiratory pressure and in dynamic compliance in the dependent lung between OLV+CPAP and OLV+miniventilation. No difference was found between right and left thoracotomy. Respiratory parameters and [P.sub.a] [O.sub.2] values are listed in Table 2.
In 18 patients (53%), surgeon satisfaction was the same between the two methods of ventilation. In 15 patients (44%), the surgeon preferred CPAP while in one (3%) mini-ventilation provided better surgical exposure.
Hypoxaemia is a well-known problem during OLV and may occur in 6 to 11% of cases (7, 8). Several solutions are offered to overcome the problem3, such as using CPAP to the non-dependent lung and positive end-expiratory pressure to the dependent lung. Another option is intermittent TLV (2, 3, 5). However, most of these techniques interfere with the surgical conditions. In this study, we demonstrated that the anaesthetist could significantly improve oxygenation during OLV by using mini-ventilation. The explanation as to why mini-ventilation was found to produce superior oxygenation could be that reducing the partial pressure of alveolar C[O.sub.2] resulted in an increased partial pressure of alveolar [O.sub.2]. This hypothesis could have been verified by C[O.sub.2] measurements during the different ventilation modes, but these were not carried out. However, from a practical point of view, we found that mini-ventilation can be helpful since it is easily performed with a small portable ventilator placed near the anaesthesia machine, delivering low tidal volumes and low-pressure ventilation. We propose mini-ventilation as an alternative solution for hypoxaemia during OLV, when CPAP is not effective. Mini-ventilation may also be used in cases where the use of 100% oxygen might be hazardous (10). Using mini-ventilation on the non-dependent lung may facilitate reducing the inspiratory fraction of oxygen in the dependent lung.
We did not find a significant difference in patient oxygenation between OLV and OLV+CPAP. A possible explanation may be that the five-minute interval between the different ventilation modes was not long enough to let the lung collapse completely. During OLV and OLV+CPAP the airway pressure was similar. In addition, CPAP was set to only 5 cm[H.sub.2] O and this may not have been enough to make a difference.
The perceived disadvantage of mini-ventilation, as we found in this study, is its interference with the surgical exposure. In 44% of the cases in our study, the surgeon preferred CPAP to mini-ventilation, while in only one case (3%) was mini-ventilation the preferred method. The surgeon indicated that the surgical field was not stationary during miniventilation as opposed to CPAP. It may be possible to use mini-ventilation with an even smaller tidal volume, for example, reducing from 100 to 150 ml to about 50 ml, thereby creating less interference with surgical conditions. Russell (9) inflated the lung with 66 ml of oxygen and was successful in treating hypoxaemia without interfering with the surgery. Furthermore, hypoxaemia during OLV may also be treated without impairment of the surgical field using fibreoptic bronchoscopic segmental oxygen insufflation11. However, this technique requires a skilled bronchoscopist and high-quality equipment, while minimum ventilation with a portable ventilator is easy to operate and more usable.
There were several limitations to our study. There was a five-minute interval between the study conditions, which may not have been enough time to recover from the previous intervention. However, this argument stands for both ventilation modes and their effects, if they exist, cause similar changes on all results. Another problem is the dynamic nature of the shunt fraction during different stages of the surgery, which may be reflected by changes in arterial oxygenation. To overcome this problem we randomly assigned the first intervention in each patient. The fact that the surgeon was not blinded to the ventilation mode, because of the distinctive noise of the portable ventilator, is an additional limitation of this study which we could not overcome technically.
In summary, mini-ventilation of the nondependent lung is superior to CPAP in improving oxygenation during lung resection, although it may interfere with surgical conditions.
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M. Y. SHECHTMAN *, A. ZISER ([dagger]), M. BARAK ([dagger]), A. BEN-NUN ([double dagger]) Department of Anesthesiology, Rambam Health Care Campus Teaching Hospital, Haifa, Israel
* M.D., Staff Anesthesiologist, Department of Anesthesiology, Chaim Sheba Medical Center, Tel Hashomer.
([dagger]) M.D., Anesthesiologist, Department of Anesthesiology, Rambam Health Care Campus and the Bruce Rappaport Faculty of Medicine, Technion--Institute of Technology.
([double dagger]) M.D., Head of Department, Department of Thoracic Surgery, Chaim Sheba Medical Center, Tel Hashomer.
Address for correspondence: Dr M. Barak, Department of Anesthesiology, Rambam Health Care Campus, 8 Ha'Aliyah Street, Haifa 35254 Israel.
Accepted for publication on January 15, 2011.
TABLE 1 Patients' data Age, y 59.0 [+ or -] 18.5 Weight, kg 70.8 [+ or -] 16.0 BMI, kg/[m.sup.2] 24.9 [+ or -] 4.7 Thoracotomy side Right 23 (60.5) Left 15 (39.5) Data are mean [+ or -] standard deviation or number (%). BMI=body mass index. TABLE 2 Pa[O.sub.2]/F[iO.sub.2] values and respiratory parameters Pa[O.sub.2]/F[iO.sub.2] TLV 448.5 [+ or -] 160.4 P <0.001 OLV 232.2 [+ or -] 107.0 CPAP 228.0 [+ or -] 131.7 P <0.001 Mini-ventilation 378.5 [+ or -] 179.6 PIP (cm[H.sub.2]O) TLV 20.2 [+ or -] 4.2 P <0.001 OLV 25.2 [+ or -] 3.8 OLV+CPAP 24.8 [+ or -] 5.0 OLV+mini ventilation 25.0 [+ or -] 4.6 NS Dynamic compliance CPAP 16.9 [+ or -] 5.8 NS Mini-ventilation 16.46.2 Data are mean [+ or -] standard deviation. TLV=two-lung ventilation, OLV=one-lung ventilation, CPAP=continuous positive airway pressure, PIP=peak inspiratory pressure, NS=not significant.
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|Author:||Shechtman, M.Y.; Ziser, A.; Barak, M.; Ben-Nun, A.|
|Publication:||Anaesthesia and Intensive Care|
|Date:||May 1, 2011|
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