Predictive factors reducing arterial oxygenation (Pa[O.sub.2]) during one lung anaesthesia: a cross-sectional study.
One Lung Anaesthesia (OLA) is used in thoracic surgery for a variety of indications.  It ranges from prevention of spillage of blood and pus into the healthy lung, as in lung abscess and haemorrhage to facilitation of surgical exposure as in oesophageal surgery.
During one lung anaesthesia, the non-dependent lung is collapsed and the dependent lung is selectively ventilated using a double lumen tube. Since the collapsed lung continues to be perfused, there is a large right to left shunt at this level which is the major cause of hypoxaemia  in OLA. In addition, the ventilation of the dependent lung is hampered by the weight of the mediastinum, collection of secretions and the wedge under the lower thorax. The other probable factors affecting OLA Pa[O.sub.2] are age, obesity and smoking. All these factors also contribute to the hypoxaemia.
During OLA when there is arterial desaturation, various measures can be initiated to increase the oxygenation. These measures are increasing the FiO2, non-dependent lung CPAP and dependent lung PEEP. [3,4] To carry out this, we need an additional oxygen source and circuit in the case of CPAP and a retard valve for PEEP. CPAP is not desirable in all the patients prophylactically, as it may interfere with the surgery. Therefore, if we can predict which patients will have the need for these measures it will enable us to be prepared to use them before hypoxaemia occurs without having to use it for all patients as routine. This study was an attempt to find out the predictive factors for decreasing Pa[O.sub.2] and to find out if it was possible to predict which patient will have a clinically important decrease of Pa[O.sub.2] during OLA for thoracic surgery based on these factors. [5,6,7]
Objectives of the study are to find out the potential factors that reduce arterial oxygenation (Pa[O.sub.2]) during One Lung Anaesthesia (OLA) and to find out the possibility of predicting the Pa[O.sub.2] during One Lung Anaesthesia (OLA) based on these potential factors.
MATERIALS AND METHODS
A cross-sectional study was conducted among 34 patients who came for lung or non-lung surgery in the CMC Vellore during a period of two years. All the ASA grade I and II patients who needed One Lung Anaesthesia (OLA) during the study period either for lung or non-lung surgery were included in this study. The exclusion criteria were patients less than 15 years, presence of cardiovascular disease, anticipated difficult intubation and inability to isolate the lungs properly after double lumen tube insertion. On the patient arrival in the operating room peripheral and central intravenous cannulations and percutaneous arterial line insertion were done under local anaesthesia. Blood samples for preoperative Pa[O.sub.2], Pa[O.sub.2] during two lung ventilation (TLV), Pa[O.sub.2] at 10 mins. and 25 mins. after starting OLA were taken and assessed. Alteration in Pa[O.sub.2] related to side, smoking and gender was measured by mean[+ or -]SD. The data were analysed by multiple linear regression to assess the independent contribution of each of these 9 predictors with OLA Pa[O.sub.2] at 25 mins.
Total number of patients in this study were 34. Out of this 22 (64.7%) were males and 12 (35.29%) were females. Patients undergone right thoracotomy was 13 (38.23%) and left thoracotomy was 21 (61.76%). Among the 34 patients 24 (70.59%) underwent lung surgery, while 10 (29.41%) underwent non-lung surgery. Mean Pa[O.sub.2] at different stages of surgery is described in Table No. 1. Table No. 2 shows that there is a significant increase in the Pa[O.sub.2] at 25 minutes in patients who had their left lung collapsed during surgery. Under the same conditions, Pa[O.sub.2] with the right lung collapsed shows a drop. In Table No. 3 non-smokers showed a lower Pa[O.sub.2] at 25 minutes of one lung ventilation when compared to smokers. The females in the study showed a significantly higher Pa[O.sub.2] at 25 minutes of one lung ventilation (Table No. 4). Multiple linear regression analysis was done to assess the independent contribution of each of the 9 predictors in explaining the variability in the drop in Pa[O.sub.2] at 25 minutes of OLA. These predictors or potential factors affecting Pa[O.sub.2] are the side of operation, [FEV.sub.1]% vital capacity; Pa[O.sub.2] on age, sex, weight and age of the patient, effect of smoking and whether the patient had undergone lung or non-lung surgery. From the Table No. 5, it was noted that only 3 factors were of significance in predicting the Pa[O.sub.2]. These were the side of the surgery, sex of the patients and smoking. When the operative procedure was done on the left sided lung, Pa[O.sub.2] at 25 mins. had significantly (p = 0.01) increased. Women had significantly (p = 0.04) higher Pa[O.sub.2] at 25 mins. as compared to men. The non-smokers had lower Pa[O.sub.2] at 25 minutes (p = 0.03) compared to smokers. Based on this data, the following regression equation could be used for predicting Pa[O.sub.2] at 25 mins. This method was used to construct a predictive equation for Pa[O.sub.2] on OLV at 25 mins. by using the most highly significant predictors.
The Predictive Equation
The predictive equation may be derived as follows: Pa[O.sub.2] (25 mins.)
=65.0 (constant) + 25.6 (side) + 27.7 (sex)--26.1 (smoking). Points being given in relation to significance of predictors.
For side-1 for Right Thoracotomy, 2 for Left Thoracotomy.
For sex-1 for males, 2 for females.
For smoking-1 for smokers, 2 for non-smokers.
Example of Prediction of Pa[O.sub.2] during One Lung Anaesthesia at 25 mins.
Patient Age--20 yrs., Sex--Male, Side of Thoracotomy--Left, Non-Smoker.
By substituting out equation.
Predicted Pa[O.sub.2] at 25 mins in this patient = 65+25.6 (side) + 27.7 (sex) - 26.1 (smoking).
= 65+ 25.6 (left) + 27.7 (male) - 26.1 (non-smoker).
= 65 +_25.6 (2) + 27.7(1) - 26.1 (2).
= 65 + 51.2 + 27.7 - 52.2.
Pa[O.sub.2] at 25 mins. = 91.70.
Predicted Pa[O.sub.2] (25 mins.) during one lung anaesthesia is 91.70.
Observed Pa[O.sub.2] (25 mins.) during one lung anaesthesia is 91.00.
It has been generally appreciated that the side of surgery affects the Pa[O.sub.2] during OLA. The left lung receives 10% less perfusion than the right lung. In addition, the dependent lung receives 10% more than its usual blood flow. So it seems feasible that patients having left thoracotomy with the left lung collapsed will have better oxygenation than when the right lung is collapsed.  Based on the data of this study, a patient having left thoracotomy had a significantly higher Pa[O.sub.2] at 25 minutes than the patient with right-sided thoracotomy. Therefore, the side of the operation is a statistically significant factor in predicting the Pa[O.sub.2] during OLA. Slingers in this study  also got similar results with regard to the side of the operation. The females (n=12) in our study had a much higher Pa[O.sub.2] than the males at 25 minutes (Mean increases 16.6 mmHg). This result is statistically significant in this study. Therefore, this would be one of the variables in the predictive equation. We do not have any explanation for this phenomenon. This may be due to having fewer number of females during the study period and larger number will be required to present this evidence as conclusive. The mean pre-operative Pa[O.sub.2] in smoker was 89.98 mmHg and non-smoker was 94.10 mmHg and Pa[O.sub.2] at 25 minutes also showed a higher Pa[O.sub.2] in smokers (Table No. 5). This study has shown a significant negative correlation in the smokers. Even though we expect smokers to have a lower Pa[O.sub.2], they had higher Pa[O.sub.2] than the non-smokers. All smokers in this study had stopped smoking for at least two weeks prior to surgery and had bronchodilator therapy and physiotherapy when indicated. Cessation of smoking is considered to reverse the airway abnormalities in smokers. It has been shown that bronchodilators improves not only air flow obstruction and lung mechanism, but also gas exchange abnormalities.  The non-smokers did not have any bronchodilators or physiotherapy preoperatively, as their clinical signs did not warrant this line of treatment. This may explain why the smokers in this study had relatively higher Pa[O.sub.2]. Another study may be planned where both smokers and non-smoker received similar bronchodilator and physiotherapy in the pre-operative period and the outcome studied. From this study, pre-operative Pa[O.sub.2] does not emerge as a significant factor which would help to predict the intraoperative Pa[O.sub.2] during OLA. [10,11] We should not disregard this factor completely since a larger number of cases in the study could have definitely shown if pre-operative Pa[O.sub.2] could be significant or not. Similarly, the weight of the patients also has a P value of 0.10. Again, larger numbers in the study may have been more conclusive. The other factors like lung or non-lung surgery, age, [FEV.sub.1] and vital capacity did not show any correlation in the ability to predict the Pa[O.sub.2] at 25 minutes. Clinically, it is often seen that in patients with obstructive lung disease there is surprisingly good oxygenation during OLA.  But this study did not establish any predictable change in the Pa[O.sub.2] of the patients who had surgery for lung disease, i.e. lobectomy or pneumonectomy when compared to the patients who had non-lung surgeries. For example, oesophagectomy where one lung was collapsed to facilitate surgery.  The lung function tests that were done, [FEV.sub.1] and vital capacity pre-operatively did not give any indication of the Pa[O.sub.2] levels to be expected at 25 minutes of OLV,  even though [FEV.sub.1] seemed to be a significant contributory factor in Slinger's study.  There was no change in the Pa[O.sub.2] at 25 minutes whether the lung functions were normal or abnormal. Some studies show that the patients with better pre-operative pulmonary function tends to have a lower Pa[O.sub.2] value during OLA. 
We found only three predictive factors in altering the Pa[O.sub.2] were statistically significant. They were the side of the operation, the sex of the patient and history of smoking. The other 6 factors (which were not significant in our study) were pre-op Pa[O.sub.2], lung or non-lung operation, age and weight of the patient, [FEV.sub.1] and vital capacity. In this study, a predictive equation was formulated from the parameters which are available routinely which would tell us the patients at risk of hypoxaemia with OLA on FiO2 of 0.5. Here, the side of the operation, sex and the smoking were the significant predictors for predicting hypoxaemia at 25 minutes. Therefore, the predictive equation was based on these factors. The equation derived in this study can be applied reliably only to patients undergoing similar anaesthetic. The usefulness of this equation for other anaesthetic techniques has to be studied. The accuracy of the predictive equation in this study is 40% as compared to Slinger's study which was 53%. This may be so because intraoperative Pa[O.sub.2] with two lung ventilation was a significant variable in the equation in their study. We hoped that using pre-operative Pa[O.sub.2] levels as a predictive factor, we would be able to predict the degree of hypoxaemia before the patient was anaesthetised. This we felt would help in getting the additional equipment required to provide CPAP or PEP as required. However, we did not find pre-operative Pa[O.sub.2] value as a good predictor.
I express my heartfelt gratitude for her painstaking efforts in completion of the study in a better way and also to my all other teachers, colleagues in the department.
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Ezhil Rajan V (1), Suresh Rajkumar (2)
(1) Professor, Department of Anaesthesiology, Aarupadai Veedu Medical College, Puducherry.
(2) Associate Professor, Department of Anaesthesiology, Aarupadai Veedu Medical College, Puducherry.
Financial or Other, Competing Interest: None.
Submission 12-08-2016, Peer Review 06-09-2016, Acceptance 12-09-2016, Published 26-10-2016.
Dr. Ezhil Rajan V, Professor, Department of Anaesthesiology, Aarupadai Veedu Medical College, Puducherry.
Table 1: Pa[O.sub.2] Values at Different Stages of Surgical Procedure Pa[O.sub.2] Levels Mean S.D. Minimum Maximum Pre-op Pa[O.sub.2] 91.2 11.21 70.80 112.00 TLV Pa[O.sub.2] 150.69 47.14 63.70 266.80 OLV Pa[O.sub.2] 10 mins. 111.32 42.89 45.60 216.70 OLV Pa[O.sub.2] 25 mins. 100.21 43.71 58.90 199.40 TLV = Two lung Ventilation. OLV = One Lung Ventilation. Table 2: Pa[O.sub.2] Levels and the Side of Thoracotomy Variable No. Pre-op Pa[O.sub.2] Pa[O.sub.2] of Pts. at 25 mins. Right Side 13 91.3 [+ or -] 11.21 86.74 [+ or -] 18.00 Left Side 21 91.33 [+ or -] 21.22 108.54 [+ or -] 39.00 P < 0.05, t test 3.43 Table 3: Pa[O.sub.2] Levels and Smoking Variable No. Pre-op Pa[O.sub.2] Pa[O.sub.2] at 25 mins. of Pts. Smoker 11 89.98 [+ or -] 12.32 98.44 [+ or -] 36.43 Non-Smoker 23 94.10 [+ or -] 08.25 86.74 [+ or -] 18.00 P < 0.05, t test 2.41 Table 4: Pa[O.sub.2] Levels and Sex Variable No. Pre-op Pa[O.sub.2] Pa[O.sub.2] at 25 mins. of Pts. Males 22 90.71 [+ or -] 10.36 94.34 [+ or -] 27.23 Females 12 92.42 [+ or -] 33.04 110.95 [+ or -] 44.12 P < 0.05, t test 3.29 Table 5: Predictive Factors in Altering Pa[O.sub.2] Levels-Multiple Linear Regression Sl. No. Factors Unstandardised P Value regr. coeffic 1 Pre-op Pa[O.sub.2] -.90 0.13 2 Side of Surgery 34.83 0.01 3 Sex 31.42 0.04 4 Lung Surgery -14.25 0.41 5 Weight -1.27 0.10 6 Age .27 0.64 7 Smoking/Non -36.00 0.03 8 FEVI 31.91 0.25 9 Vital Capacity -10.26 0.66
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|Title Annotation:||Original Research Article|
|Author:||Ezhil, Rajan V.; Rajkumar, Suresh|
|Publication:||Journal of Evolution of Medical and Dental Sciences|
|Date:||Oct 27, 2016|
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