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4.4 Discussion
We have investigated the incidence of long-term TS in patients undergoing
tracheostomy in a critical care setting. Additionally, we have estimated the incidence of peri-procedural complications (bleeding and wound infection) that we have postulated may have a role in the development of TS. When considering published RCTs and comparative observational studies reporting long-term outcomes we have found ST and all percutaneous techniques (CPDT, GWDF, STD, TLT, SSRD, BD) broadly similar in terms of TS and bleeding. In keeping with earlier meta-analyses we have also found a higher incidence of wound infection when comparing ST to
CPDT51. Despite the frequency with which percutaneous tracheostomies are
performed within the critical care setting there appear to be relatively few high quality studies assessing long term outcomes between techniques.
When considering our pooled proportions meta-analysis across all studies, the TS rate reported varies from 2.8 to 0.6% for ST and STD respectively with all percutaneous techniques being broadly comparable (table 4.2). The point estimate of rate for total bleeding episodes varies from 12.1% for STD to 2% for TLT (table 4.3) but the confidence intervals around some of these estimates are wide with a substantial degree of between study heterogeneity. Major bleeding ranges from 4.7% for ST to 0.6% for TLT, whilst wound infection for ST is 9.9% and 1.1% for CPDT, again with wide 95% CIs and substantial between study heterogeneity.
By incorporating cohort studies as well as RCTs into our analysis we have adopted a different methodology to earlier meta-analyses. The majority of studies incorporated into previous analyses were RCTs of relatively small samples sizes and as such, given the low incidence, were limited in their ability to detect a difference in risk of TS. Whilst the incorporation of non-randomised studies may introduce an element of bias,
work by Golder et al has suggested this effect may be minimal314. In a meta-analysis of meta-analyses they found a high degree of concordance between meta-analyses of adverse events comparing data solely from RCTs with those from both RCTs and observational studies (with less discrepancy for larger studies) concluding that meta- analyses of adverse events should not, necessarily, be confined to specific study types. In a sensitivity analysis, we excluded the non-randomised studies and compared meta- analysis results to those that include both RCT and prospective observational studies. Overall, results were similar but changed from being non- significant to statistically significant for bleeding complication for two comparisons, suggesting that there may be important differences between RCTs and observational studies (see below). Another potential limitation to accurate determination of the incidence of TS, following percutaneous tracheostomy, in the current study is the possibility that patients who are perceived to be predisposed to a difficult tracheostomy may be assigned to the ST group in non-randomised studies. In support of this concern, of the seven studies assessing ST, five were RCTs 297,298,303,305,310 reporting a TS incidence of 1.4% (4/328) and two were prospective non-randomised studies with a TS
incidence of 8.4% (7/90)300,311. In addition, the risk difference estimates for TS from the prospective non-randomised studies were each more extreme than corresponding estimates from RCTs in those comparisons between ST and percutaneous
tracheostomy. Of the non-randomised studies, the study by MacCallum (reporting 2 stenoses) provides no information on group allocation other than stating that
procedures were performed consecutively300. In contrast, Polderman (5 stenoses) reports that 7 patients were allocated to the ST group because of perceived difficult anatomy whilst the remainder of patients were allocated randomly.311 No information as to whether those patients with difficult anatomy were also the patients who
developed TS was provided. The choice between surgical and percutaneous
tracheostomy in most situations is not a random event with most operators opting for the surgical approach when potential difficulties or safety concerns are anticipated. It is probable, therefore, that many of the surgical tracheostomies described within the studies herein would be more prone to the occurrence of complications. It is possible, therefore, that the observed differences in complication rates we have demonstrated in our analyses could be due to patient factors rather than the operative technique per se. However, as discussed above, other than a potential role for tracheal ring fracture there is little evidence for peri-operative complications causing TS.
Accepting these potential limitations, in the critical care setting, there is a trend toward increased risk of TS for surgical tracheostomy patients.
Our data confirm our view that in the critical care setting, in common with previous authors, a percutaneous procedure should be the technique of choice as it is both safe and cost effective51.
It is somewhat surprising (and a common theme across all previous meta-analyses), given the frequency with which PDTs are now undertaken, that we have only
identified 29 studies reporting upon the long term outcomes of 5473 patients over the preceding 14 years. We find it concerning that some techniques in relatively
widespread use (SSRD, BD, TLT) have such sparse long-term outcome data. When specifically considering the issue of TS and the possibility of unrecognised sub- clinical TS in this patient population, very few studies have undertaken
comprehensive radiological imaging to determine the underlying prevalence of stenosis. Considering that the STD technique appears to be currently the most widely used approach,45,46,189 until recently there were only 14 patients described in the
literature who had undergone radiological imaging after STD tracheostomy to determine the underlying prevalence of both clinical and sub-clinical TS80.
Reporting of complications across studies remains far from standardised and posed significant problems during data extraction. We had initially set out to collect data for a significant number of secondary outcomes (appendix 1). However, lack of uniform reporting made this aim much more difficult and ultimately of little value as many complications are reported infrequently with marked differences in incidences across studies. Even reporting of our primary and secondary outcomes was inconsistent, with some studies failing to quote an incidence for TS, bleeding and wound infection. One study appeared to describe an apparent incidence of TS (presence of stridor with spirometry findings in keeping with upper airway obstruction) but failed to ascribe these findings to a diagnosis of TS.296 A lack of a consistent definition for significant versus minor bleeding lead us to analyse total bleeding episodes as well as those categorised as major. Additionally, one study reported stomal decay as an outcome, interpreted as infection for our analysis.297 Additionally, we were unable to explore the possible relationships between bleeding and stomal infection and TS. Whilst most of the studies included in the analysis reported TS, bleeding and stomal infection rates there was little, if any, supplemental data reported for us to ascertain whether these events were associated and occurring within the same patients or not. A number of other studies that detailed complications of multiple techniques reported composite outcomes where the complication rate for a given technique was impossible to ascertain. In this circumstance we wrote to the author for clarification. In some instances this resolved the problem either completely or partially. In others, where there was no reply from the corresponding author the study was excluded from the
analysis. These limitations highlight the vital need to develop and report standardised core outcome measures to improve the synthesis of trial data.315
Whilst RCTs to compare long term outcomes, particularly TS, following individual PDT techniques would be desirable it is highly unlikely that they will be performed. TS has a low incidence, critical illness mortality is high and long term follow up difficult. An adequately powered study would require at least 2000 patients to detect a reduction in TS from 2.8% to 1% (α = 0.05 β = 0.8) and this figure excludes drop out. Thus we believe our study, despite the limitations described, is currently best placed to inform clinicians in this area.