Impact of Postoperative ABG Analysis and ICU Weaning Protocol in Surgical Outcome of Atlanto-Axial Dislocation: It's not the Towering Sail, but the Unseen Wind that Moves the Ship
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.355113
Source of Support: None, Conflict of Interest: None
Keywords: Arterial blood gas, atlanto-axial dislocation, intensive care unit weaning protocol, pulmonary function test
The Surgical nuances of various approaches for atlanto-axial dislocation (AAD) have been discussed in the literature a lot. Different school of thoughts propose their own modification and advantages. In our experience of operating more than 500 cases of AAD, we feel that surgical outcome depends on other unexplored factors. These factors, including preoperative respiratory reserve, body mass index (BMI), adequate preoperative respiratory optimization, postoperative intensive care protocol and physiotherapy, should be considered more important rather than surgical corridor and small technical alterations. In this study, we intend to analyze the preoperative spirometry and postoperative arterial blood gas (ABG) values in an operated patient of AAD; and find its correlation with intensive care unit (ICU) stay or surgical outcome. We herein propose our postoperative extubation protocol highlighting the importance of interpretation of blood gas values.
In this retrospective study, 66 patients of AAD, operated in our institute (January 2014 to November 2018), were analysed. Individual consent from the patient to use clinical and radiological details for publication was taken. Institutional ethical clearance was obtained, and there was no conflict of interest. Data analysis included, demographic data, clinical features, BMI, and radiological parameters were recorded from hospital case records, outpatient files, and the hospital information system.
We included all patients of AAD irrespective of the presence or absence of basilar invagination or Chiari-malformation. Patients were operated by posterior fusion ± distraction. The technical aspect was similar in all patients, and there is no difference in level of expertise among surgeons. Patients with Revanappa's modified Nurick grade 5 (chair-bound or bedridden) [n = 35] and traumatic AAD were excluded from study [n = 20]. Being a retrospective study, a majority of ABG [n = 145] could not be retrieved so these patients were excluded. None of the patients underwent isolated transoral decompression.
Preoperative pulmonary function assessment (PFT) and the ABG analysis were conducted on the first postoperative day were studied in our study. The PFT was conducted as part of the routine workup of AAD patients in our institute. The PFT was conducted in the Pulmonary Medicine Department by a qualified doctor. The laboratory had normative data for age and sex-matched subjects [Supplement 1]. The percentage of predicted values of the PFT parameters (based on the patient's height, age, and sex) was used for comparison.
The following parameters in the PFT were assessed:
In the ABG, we focussed at pH, pCO2, pHCO3, and base excess (BE).
The following outcomes were assessed in terms of PFT and ABG parameters:
The surgical outcome was assessed by Revanappa's modified Nurick grade (Grade 0 – signs or symptoms of root involvement but without evidence of spinal cord disease; Grade 1 – signs of spinal cord disease but no difficulty in walking; Grade 2 – slight difficulty in walking but can get up from squatting or sitting on ground without vertical support; Grade 3 – difficulty in walking and requires vertical support to get up from squatting or sitting on ground; Grade 4 – able to walk only with someone else's help or with the aid of a frame; and Grade 5 – chair-bound or bedridden).
All the patients underwent high cervical neural decompression and fusion surgery (anterior (transoral odontoidectomy or posterior C1-2 distraction followed by occipito – C1 − C2 fusion/C1 − C2 fusion) depend upon their radiological and clinical sign of cervical myelopathy.
The Statistical Package for the Social Sciences version 22.00, International Business Machines (IBM, New York, NY, USA) was used for statistical analysis. For comparison of mean distribution of PFT parameters, among groups, an independent t-test analysis and analysis of variance were conducted. ABG analysis was conducted on an individual basis depending on interquartile range. Chi-square test was used to find a correlation between outcome or study parameters. A P value less than 0.05 was considered significant.
Demography and clinical characteristics
The mean age was 25.7 ± 11.8 years [M:F = 49:17], ranging from 8 years to 58 years [Table 1]. In total, 89.4% of patients were below 40 years, and 42.4% even below 20 years. Patients with chronic progressive myelopathy (nearly 59% presented after 1-5 years) underwent AAD anomalies by Dynamic CT CVJ and MRI cervical spine. Nearly 41% (n = 27) of patients had BMI of less than 18.5, and 50% (n = 33) had their breath holding time of less than 20 minutes. The majority of our patients had spinal cord intensity changes in T2 weighted MRI images (80%) and basilar invagination (82%) on CT CVJ. There was improvement in mean Nurick grade from 3.17 ± 0.8 to 2.76 ± 0.7 in the last follow up after surgery. Therefore, in spite of the fact that we deal with a complex subset of patients, 80% of our patients had “better” or “good” outcome. There were 4 deaths in our study.
In total, 63.6% (n = 42) patients were extubated within 24 hours (Group A), and 36.45 patients (n = 24) were extubated after 24 hours. In total, 39.4% patients (n =26) had ICU duration of less than 48 hours (Group 1), and 60.6% patients (n = 40) had ICU duration of more than 48 hours (Group 2).
Effect of preoperative pulmonary reserve
The majority of the patients had moderate restriction [Supplement 2] preoperatively (FVC = 50-69 and FEV1 = 60-69). The mean IVC of 66 patients was 53.47 ± 20.0 (13-93), mean FVC was 59.98 ± 21.6 (11-111), mean FEV1 was 61.7 ± 21.9 (11-118), mean FEV1/FVC ratio was 104.1 ± 13.1 (43-121), and mean MEF25-75 54.9 ± 22.1 (9-122). On further subgroups analyses, the preoperative respiratory parameters were not statistical significant with either timing of extubation, ICU duration, nor surgical outcome. On analysing the patients, we found a trend showing patients with poor preoperative parameters have comparatively more ICU duration and worse outcome. The timing of extubation, however, does not correlate with the PFT parameters. Actually, all the patients (n = 66) were operated in elective settings, and were well optimized before taking them for surgery. Therefore, importance of optimizing the preoperative PFT is substantiated.
Effect of postoperative blood gas parameters
Mean pH of patients in our study was 7.36 ± 0.08 (7.0-8.0), mean pCO2 was 37.93 ± 7.9 (14-67), HCO3 20.98 ± 4.7 (10-34), and mean BE -3.87 ± 5.5 (-14 to 9). Most common arterial base gas disorder in our study is metabolic acidosis. [Table 2] shows a comparison of blood gas parameters among Group 1 and 2. The mean of parameters was not statistically significant (because these parameters have a normal range); both the extreme values are considered abnormal. There is a clinical trend. In patients with mild acid-base disorders (pH between 7.31 and 7.45), extubation was possible within 24 hours, whereas patients with moderate acid-base disorders or mild mixed disorders took more than 24 hours for extubation. Patients with BE more than -9 were extubated after 1 day. The similar trend was seen for surgical outcome also; patients with moderate acid-base disorders or mild mixed disorders have worse outcomes. On comparing the mean distribution of ABG parameters, we found a significant outcome for pCO2 (mean pCO2 of “good outcome group” was 37.8 ± 5.49 compared to 41.02 ± 16.36 of “worse outcome group”).
Out of 26 patients with ICU duration less than 2 days, 23 patients had “good” outcome, whereas ten out of 40 patients with ICU duration of more than or equal to 2 days had “bad” outcome (Pearson's Chi-square P = 0.00) [Table 3]. There was a positive correlation between timing of extubation and outcome (r = 0.2), extubation and ICU duration (r = 0.5), ICU duration and outcome (r = 0.3). There was no correlation between preoperative IVC and timing of extubation (r = 0.1), between BE and timing of extubation (r = 0.01), and between outcome and blood gas values (r = -0.1).
Analysis of all patients who had delayed extubation (more than 48 hours)
In our study, 16 patients were extubated after 48 hours. These patients needed prolonged postoperative ventilator support and optimization. [Table 4] shows ABG and PFT values of the 16 patients who had delayed extubation in our study group. Individual PFT analysis of these patients showed that 3 patients had normal PFT and 3 had a mild restrictive pattern, only 4 patients had a severed restrictive pattern, and 6 patients had a moderate restrictive pattern on preoperative PFT. The retrospective analysis of ABG also showed similar varied result; 4 patients had normal range ABG, 5 had a mild acid-base disorder, and only 2 patients had a severe metabolic acidosis. In addition, 3 patients had a mixed acid-base disorder, and 2 patients had moderate metabolic acidosis. There was an interesting pattern in our study, suggesting that 10 patients (in delayed extubation group) had BMI less than 10. Therefore, in a well-optimized patient, planned for elective surgery for AAD, independent preoperative PFT, ABG, or BMI may not affect the outcomes, may be because of the physiological compensatory reserve. However, when two or more of these parameters are affected, then the above compensation fails, and either outcome becomes worse or patients needed reintubation.
A novel grading system is advocated [Table 5] to predict the outcome in AAD patients including preoperative PFT, postoperative ABG, and BMI values. In our study, the number of patients with score 1-6 had good (n = 19) and better (n = 15) outcome; compared to patients with score 7-10 respectively. This grading system correlation showed a strong correlation with outcome (r = 1).
The above grading system was applied retrospectively on the 6 patients who needed re-intubation, because physiological compensation failed. [Table 6] shows that all these patients had poor predictive grade; with more than two parameters deranged.
The surgical outcome of patients operated for AAD is variably reported in the literature.,,,,, Apart from nonmodifiable factors such as preoperative disability or medical co-morbidity, other factors like preoperative respiratory capacity and postoperative intensive care management have convincing effects on patient outcome. It is evident from our study that these under focussed factors are the foretellers of a need for re-intubation, duration of ICU-stay, and the probable outcome. However, these factors need to be interpreted in the background of preoperative PFT, BMI, and renal status; not ignoring intraoperative variables such as duration of surgery, blood loss, type of surgery, and surgical complications.
In patients of AAD, an occult respiratory dysfunction may occur, due to direct compression of neural tissue (poor drive) or due to weak respiratory muscles (poor effort). As our departmental protocol, all patients underwent pulmonary function test in preoperative period. We found that these patients of AAD showed a restrictive pattern of PFT. A low preoperative IVC and FVC values were seen in these patients, which may explain longer ICU stay in these patients, although the results were statistically insignificant. In a similar study by Bhagavatula et al., there was a significant preoperative decrease in FVC and FEV1 which was responsible for intra- and postoperative respiratory insufficiency. They also observed that FVC was lower in patients with worse neurological grading and proposed some possible mechanisms for the same. The variously explained pathogenesis is being repetitive cord trauma, stagnant hypoxia secondary to venous stasis or vertebral, and spinal artery occlusion and preexisting microscopic intraaxial abnormalities. The discordant in our result may be due to our preoperative policy. We optimize the patients in ward and gradually build up IRV by incentive spirometry, and once sufficient capacity is achieved, then only post these patients for surgery. Uppar et al. concluded that preoperative FVC, FEV1, MVV, and PEFR are significantly reduced with AAD, most common being the AAD; and objective evaluation of respiratory dysfunction is necessary to improve postoperative pulmonary rehabilitation and thus ICU stay. There are no studies till date to establish the significance of preoperative PFT to ICU stay.
Comparing ABG is not possible by analysing any central statistical value. Rather we preferred to compare range, although mean partial pressure of carbon dioxide was significantly lower in patients with poor outcome group. The patients with ICU duration of more than 24 hours (Group 2) had carbon dioxide retention (respiratory acidosis). We also found that base deficit of more than 9-10 is an independent parameter for longer ICU duration and poor outcome. To the best of our knowledge, this is the first study to highlight the importance of postoperative blood gas analysis in patients with AAD. We would like to propose that immediate postoperative ABG reflect may guide the intensives to make a wise decision. We noted that there is a strong correlation between the three parameters (BMI + PFT + ABG) influencing the timing of extubation and the duration if ICU stays collectively.
One-third of our patients had either mixed acid-base disorder and/or moderate acidosis/alkalosis. These patients formed a group of delayed extubation. These patients remained uncompensated, either due to poor pulmonary reserve, blood loss during surgery, prolonged surgical duration, or prolonged persistent cervico-medullary compression. These all correlated with prolonged ICU stay and worse outcome.
A majority of our study patients (63.6%, n = 42) had mild metabolic acidosis or normal ABG study in the first day of ICU and were extubated uneventfully within the first 24 hours of ICU stay. This leads us to the finding that mild metabolic acidosis is well tolerated/well compensated and has minimal if at all effect on reintubation or worse outcome. The most common causes of metabolic acidosis in the immediate postoperative period could be attributed to hemodynamic instability, renal failure, hyperchlorinemia due to rapid saline infusion, and cation-exchange resins. Gunnerson et al. also showed that metabolic acidosis is the main ABG disorder of patients admitted in ICU setting and had a positive correlation for poor outcome with respect to progressive worsening of metabolic acidosis. Patients with severe metabolic acidosis had worse outcomes and vice versa. We also noticed that patients with mild metabolic alkalosis also had uneventful extubation within the first 24 hours of ICU stay. In a similar study by Cain et al., it was concluded that the patients with an ICU stay >5 days had significantly decreased PFT parameters (FVC, FEV1, FEF25-75, and PEFR) compared to those who stayed less than 5 days but the study was done in patients undergoing cardiac surgery.
Base deficit is the amount of base required to neutralize to titrate 1 litre of whole arterial blood to a pH 7.4 with a sample fully saturated with oxygen at 27°C and PaCO2 of 40 mmHg. It is an indirect marker of intravenous fluid deficit and is shown to be superior to pH for evaluating reversal of metabolic acidosis. Davis et al. defined 3 categories of BE: mild, moderate, and severe and said it to be an equally good marker as lactate in predicting the outcome/resuscitation of the patient. With moderate to severe base deficit, patient had bad outcomes. In our study, all patients with BE of less than 10, in the immediate postoperative setting had a poor outcome. More so, if the patient had a poor pulmonary/renal reserve, the patient can be doomed to have a poor outcome and calls for immediate intervention for its correction. Or else patients were seeming to have prolonged ICU stay, delayed extubation, worsened neurological outcome, tracheostomy, and ventilator dependence.
In a study by Smith et al., BE was more negative than 4 mmmol/L and had a mortality of 57.1% compared to 17.6% with BE more than -4 mmol/L. Santi et al. also showed that patients with BE less than -4 mmol/L had higher mortality than the contrary. Both these studies were defined in critically ill patients admitted to ICU due to different reasons. In our study, we have defined BE in AAD-operated patients in deciding the outcome in the form of the timing of extubation, and also -10 was the critical level deciding the outcome.
BE in AAD patients can occur due to excessive blood loss, persistent compression over the cord, hypovolemia, sepsis, neuromuscular weakness, renal pathology, and loss of carbon dioxide respiratory drive. In a study by Epstein et al., the failure rate in ICU ranged between 2% and 20%. These failed extubation pertain greater hospital morbidity and mortality and thereby increasing hospital duration. The patient is first subjected to spontaneous breathing trial, following sedation-off, and is reassessed for response frequently (Weaning protocol – [Annexure 1]). The patient should be evaluated for ABG before extubation. This ABG further guides the clinician, as to the decision for extubation. An abnormal ABG parameter is gradually compensated in-vivo by buffers viz. blood, lungs, and kidneys. An abnormal ABG is an indirect marker of inadequate buffer systems in the body; it could be either acidosis or alkalosis both of which when moderate to severe or of when present together would lead to delayed extubation. We would suggest to notice the alarming signs of moderate to severe ABG disorder, BE, and mixed disorders and to treat them appropriately. It also averts the cost of reintubation.
Nutrition is another factor which has been deceived of a spotlight. BMI is an indirect measure of the body's nutritional status and has become one of the factors of discussion in the present topic. In total, 40.9% of our patients were malnourished. We found that patients with poor nutrition (BMI < 20) had a poor outcome, respiratory compromise due to reduced muscle tone, and power of the respiratory muscles overlapped with the inability to bare surgical stress in view of a poor nutritional reserve.
Patients having moderate to severe primary or mixed acid-base disorder have a probability of reintubation or delayed extubation; hence, they should be optimized before the trial of extubation. In a well-optimized patient, posted for elective surgery, preoperative spirometry values do not effect ICU duration, timing of extubation, or even outcome. Physiological compensations of these patients crumble in situations of severe metabolic acidosis with a base deficit of more than -9 or even with mild mixed-acid base disorders. Independent breathing effort or ABG values may not be significant, but a strong correlation was seen with the novel grading system (grade > 6 had worse outcome). An algorithm for postoperative ICU protocol has been proposed depending upon parameters used in our grading system.
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Conflicts of interest
There are no conflicts of interest.
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]