Predictors of Postoperative Ventilation in Scoliosis Surgery: A Retrospective Analysis

Abstract

Background:

Patients undergoing corrective surgery for scoliosis may require postoperative ventilation for various reasons.

Aim:

The aim was to study the correlation of preoperative (pulmonary function test [PFT], etiology, and Cobb's angle) and intraoperative factors (type of surgery, number of spinal segments involved, blood transfusion, and temperature at the end of surgery) on postoperative ventilation following scoliosis surgery.

Settings and Design:

patients' medical records of scoliosis surgery at a tertiary care center during 2010–2016 were retrospectively analyzed.

Materials and Methods:

We studied retrospectively 108 scoliosis surgeries done in our institute during this period by the same group of anesthetists using standardized anesthesia technique. We analyzed preoperative (etiology, preoperative PFT, and Cobb's angle) and intraoperative factors (type of surgery, number of spinal segments involved, blood transfusion, and temperature) influencing postoperative ventilation.

Statistical Analysis:

For all the continuous variables, the results are either given in mean ± standard deviation, and for categorical variables as a percentage. To obtain the association of categorical variables, Chi-square test was applied.

Results:

Patients with Cobb's angle above 76° and spinal segment involvement of 11 ± 3 required postoperative ventilation. Forced expiratory volume in 1 s (FEV1%) <38 and forced vital capacity (FVC%) <38.23 of the predicted could not be extubated. Increased blood transfusion and hypothermia were found to affect postoperative ventilation.

Conclusion:

Preoperative factors such as etiology of scoliosis, Cobb's angle, spirometric values FEV1% and FVC% of predicted and intraoperative factors like number of spinal segments involved, affect postoperative ventilation following scoliosis surgery. Increased blood transfusion and hypothermia are the preventable factors leading to ventilation.

INTRODUCTION

Scoliosis is a progressive abnormal lateral curvature of spine >10° in the coronal plane. The etiology of scoliosis varies, the most common being idiopathic (70%). Nonidiopathic scoliosis is due to congenital causes, neuromuscular diseases, mesenchymal disorder and secondary to infection or trauma. As scoliosis progresses, there may be serious alterations in cardiac and respiratory function. Direct reduction of chest wall compliance and indirect progressive reduction in lung compliance may lead to the development of chronic respiratory failure, pulmonary artery hypertension, and right ventricular failure.[1] Scoliosis correction surgery is complex, many patients will need postoperative ventilation and extended intensive care stay due to various reasons resulting in increased medical expenses. There are very few studies focusing on the factors leading to postoperative ventilation following scoliosis surgery. From wide literature search, we identified few suggested preoperative and intraoperative predictors for postoperative ventilation from different patient cohorts.[2] We aimed to identify the common factors leading to postoperative ventilation after scoliosis surgery from a single center over 7 years.

MATERIALS AND METHODS

This retrospective study was conducted at a tertiary care center. After obtaining approval from the Institutional Ethics Committee in 2017, we retrospectively collected the perioperative data of all patients who underwent corrective scoliosis surgery during the period from January 2010 to December 2016. A total of 120 patients underwent scoliosis correction during this period anesthetized by the same group of specialists. Twelve patients were excluded from the study due to poor documentation. Data were collected from the hospital information system and written records such as anesthesia charts. We focused only on those parameters that are theoretically associated with postoperative ventilation. In our institute, all scoliosis patients undergo pulmonary function tests (PFTs) if feasible, apart from other routine investigations. We have studied the effect of preoperative factors such as spirometric values (forced expiratory volume in 1 s [FEV1%] and forced vital capacity [FVC%] of the predicted), Cobb's angle, type of scoliosis and intraoperative factors leading to postoperative ventilation.

Apart from standard monitoring, intra-arterial blood pressure, central venous pressure, urine output, core temperature, and procedure specific neurological monitoring with somatosensory evoked potential (SSEP) and motor evoked potential (MEP) were monitored routinely. Induction and intubation were done according to standardized technique. A very few patients with anterior scoliosis correction needed double lumen tube. The depth of anesthesia was maintained using inhalational agent isoflurane (minimum alveolar concentration <1), opioid and either dexmedetomidine or propofol infusion, with BIS value targeted at 40–60. Muscle relaxants were used for intubation and later avoided during maintenance for facilitating MEP monitoring. Blood loss was assessed and if found > 20% of the estimated blood volume, transfusion was done. Antifibrinolytic agents were routinely employed to reduce blood loss (tranexamic acid 15 mg/kg followed by infusion at the rate of 1 mg/kg/h). Body and fluid warmers were used to prevent hypothermia. The correlation with factors such as types of surgery, number of spinal segments involved, blood transfusion, and temperature at the end of surgery was analyzed. For statistical analysis, patients were divided into two groups as follows: extubated (E) and ventilated (V) groups.

Only hemodynamically stable and normothermic patients having adequate oxygenation and ventilation with full reversal of muscle relaxant were extubated.

Statistical analysis

Statistical analysis was performed using IBM SPSS software version 20 (SPSS Inc., Chicago, IL, USA). For all the continuous variables, the results are presented as mean ± standard deviation and for categorical variables as a percentage. To obtain the association of categorical variables, Chi-square test was applied. To compare the mean difference of numerical variables between groups, independent two sample t-test was applied for parametric data and Mann–Whitney U-test for nonparametric data. A value of P < 0.05 was considered as statistically significant. To find the strongest predictor for ventilation multiple logistic regression analysis and odds ratio (OR) were used.

RESULTS

We retrospectively studied 108 patients who underwent scoliosis surgery over a period of 7 years from January 2010 to December 2016. The majority of patients were female - 88% (95 out of 108)[3] [Table 1]. Age of patients varied from 5 to 24 years with a median age of 14.20 ± 3.26. There was no statistically significant difference in the age group of patients between Groups E and V (P = 0.236) [Table 2].

Table 1.

Age and Sex

Table 2.

Factors affecting post-operative ventilation

The most common type of scoliosis in our study was adolescent idiopathic scoliosis (74%) followed by congenital (9%) and juvenile idiopathic scoliosis (7%). In these subgroups of patients, all of the infantile idiopathic scoliosis patients were in Group V (required postoperative ventilation), while 66.6% of the neuromuscular scoliosis (NMS), 62.5% of the juvenile idiopathic scoliosis, 50% of the congenital, 40% of the neurofibromatosis and only 12.5% of the adolescent idiopathic patients were in Group V [Table 3].

Table 3.

Type of scoliosis and postoperative ventilation

Cobb's angle was found to be significantly higher in Group V (76.00° ±20.940° with a value of P < 0.001), whereas in Group E, it was only 52.26° ±15.227° [Table 2 and Figure 1].

Figure 1.

Cobb's angle and postoperative ventilation

There was statistically significant difference (P < 0.001) in preoperative spirometric values of FEV1% and FVC% between the two groups, with values in group V much lower than in Group E [Table 2 and Figures 2, 3].

Figure 2.

FEV1% predicted and postoperative ventilation

Figure 3.

FVC% predicted and postoperative ventilation

Ventilated patients were found to have higher number of spinal segment involvement of 11.12 ± 2.197, compared to extubated patients (9.733 ± 2.744) with a value of P < 0.021 [Table 2 and Figure 4].

Figure 4.

Number of spinal segments involved and postoperative ventilation

In this study, there were only six patients with anterior correction; all of them could be extubated. Among the 102 posterior correction patients, 26 (25.5%) patients required ventilation [Table 4].

Table 4.

Type of surgery and postoperative ventilation

In both patient groups, there was a tendency toward hypothermia, with the temperature at conclusion of surgery being 34.03°C ± 0.75°C in Group V and 34.99°C ± 0.67°C in Group E. This reached statistical significance with P < 0.001 [Table 2 and Figure 5].

Figure 5.

Temperature and postoperative ventilation

We found a varied need for packed red cell transfusion in the two groups. Analysis showed that Group V required nominally higher blood transfusion intraoperatively (1481.35 ml ± 1043.90) than Group E (1151.46 ml ± 487.078), which was statistically significant (P < 0.029) [Table 2 and Figure 6].

Figure 6.

Blood transfusion and postoperative ventilation

Statistical analysis with Pearson's correlation did not show preoperative factors influencing either blood transfusion or temperature.

Using multivariate analysis which included all significant factors in the univariate analysis, temperature was the strongest predictor of postoperative ventilation (P < 0.001, OR - 10.36). The OR of FVC% was 1.050 and Cobb's angle was 1.000, respectively [Table 5]. An OR is a measure of association between an exposure and an outcome.

Table 5.

Relative importance of the significant factors

DISCUSSION

This retrospective analysis showed that scoliosis predominantly affects females. Out of 108 patients who underwent scoliosis surgery 95 were female, which correlates with similar other reports from various studies with the predominance of scoliosis in females.[3,4] In this study, age was not a statistically significant factor between extubated and ventilated patients.

The most common etiology, we found was idiopathic scoliosis (infantile - 2, juvenile 8, and adolescent - 80 [total 90/108]). The two cases of infantile scoliosis and a significant percentage of juvenile scoliosis in the study needed postoperative ventilation, may be related to their lung hypoplasia which could be due to thoracic deformity during the period of very rapid lung growth and development.[5] During this period, we had only three NMS correction of which two patients required ventilation. In their study, Udink ten Cate et al. found higher incidence of prolonged mechanical ventilation in NMS and the major risk factor was a decreased preoperative pulmonary function.[6] Kang et al., in their study concluded that corrective surgery in NMS with abnormal PFT was associated with higher postoperative complications and intensive care unit (ICU) stay.[7]

The effect of preoperative PFT in predicting postoperative ventilation in scoliosis surgery has been evaluated in multiple studies. In our study, we have observed the statistically significant difference in the spirometric parameters such as FEV1% and FVC% between the two groups. Patients with FEV1% <37.99 and FVC% <38.23 needed mechanical ventilation. A study by Almenrader and Patel found that patients with preoperative FVC% <30% have increased tendency for postoperative ventilation.[8] Yuan et al. concluded that FEV1% <40% of predicted value and older age were associated with prolonged mechanical ventilation in scoliosis surgery.[9] Gibson in his review article has pointed out that FEV1% <40% is associated with postoperative pulmonary complication in scoliosis surgery.[10]

The influence of the type of surgery on extubation was difficult to comment as we had limited anterior surgical correction of scoliosis compared to posterior correction. The study by Gurajala et al. did not show any difference in mechanical ventilation associated with the type of surgery.[2] In their study, Hod-Feins et al. showed less postoperative complications and ICU stay in posterior correction.[11] A study by Gurajala et al. found direct relationship of increased spinal segment involvement of more than eight segments and postoperative ventilation.[2] In our study, patients with spinal segment involvement of 11 ± 3 required postoperative ventilation.

Hypothermia is one of the major predictors found to be associated with postoperative ventilation.[2] Hypothermia causes vasoconstriction, impaired coagulation, increased blood loss, metabolic abnormalities, and prolonged ventilation.[2,12] In this study, low temperature at the conclusion of the surgery was also a factor which led to postoperative ventilation. Lenhardt et al. found that a decrease in temperature by <2°C delayed postoperative recovery by 40 min.[13]

In the present study, the patients in group V required a higher amount of blood transfusion than E group. This may be due to the higher number of vertebral involvement, increased severity of scoliosis, or due to hypothermia.[2] Rajagopalan et al. concluded in their study that even mild hypothermia <1°C significantly increases blood loss by approximately 16% and increases the risk of transfusion by 22%.[14] Verma et al. in their study on adolescent idiopathic scoliosis found that antifibrinolytic drugs reduce operative blood loss but not transfusion rate.[15]

We found that among the leading factors influencing postoperative ventilation, the preoperative factors were the reflection of the severity of scoliosis. Blood transfusion and temperature were the two most influential intraoperative factors leading to postoperative ventilation. This study did not show any association between preoperative factors leading to increased blood transfusion or development of hypothermia. There is a need for focused and efficient temperature management plan for scoliosis as we found that even mild hypothermia may lead to extubation difficulties. These observations also emphasized that we need to review our blood conservation strategies and management of massive transfusion.

CONCLUSION

Preoperative factors such as etiology, Cobb's angle, and spirometric values FEV1% and FVC% of predicted, number of spinal segments involved and intraoperative factors such as blood transfusion and hypothermia are found to influence postoperative ventilation following scoliosis surgery. Blood transfusion and temperature are the two modifiable and controllable factors which affect postoperative ventilation. Even mild hypothermia may negatively influence the outcome of a well-balanced anesthetic technique in scoliosis correction surgeries.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.