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Nashville, TN
Dr. Smith is Professor of Anesthesiology Emeritus, Adjunct Professor of Anesthesiology, Vanderbilt University School of Medicine.
Correspondence to: Bradley E. Smith, MD, Professor of Anesthesiology Emeritus, Adjunct Professor of Anesthesiology, Vanderbilt University School of Medicine, PO Box 128543, Nashville, TN 37212-4853; e-mail: bradley.smith{at}vanderbilt.edu
In of this issue of CHEST (see page 1564), Lundstrøm and colleagues1 present a prospective, randomized, and controlled study of the incidence of hypoxemic episodes following surgery for coronary artery bypass graft (CABG) surgery utilizing extracorporeal circulation (ECC). (Note: CABG without ECC is not discussed here.) Control subjects (CONs) received general anesthesia followed by postsurgical pain control therapy with opioid-based medications. In addition, the "test" group also received anesthesia and pain control medication via an indwelling catheter in the thoracic epidural space (TEAA). These authors failed to demonstrate any protective effect of the use of the indwelling catheter in the TEAA on postsurgical hypoxemic episodes. In fact, on the third postsurgical night, hypoxemic episodes occurred in 100% of the patients in the TEAA group and in only 76% of the subjects in the CON group (p < 0.05).
Despite enthusiastic claims for use of the catheter in the TEAA,23 significant benefit as measured by outcome analysis thus far has been difficult to demonstrate. Numerous and sometimes passionate discussion of the use of the catheter in the TEAA continue to appear.45 However, a recent metaanalysis6 of 15 prospective randomized controlled trials of the use of catheters in the TEAA compared with standard management failed to show that mortality, major morbidity, new myocardial infarction, new myocardial ischemia, or hospital discharge time is improved by use of a catheter in the TEAA. Time to tracheal extubation, incidence of cardiac arrhythmias, respiratory complications, and pain control were more favorable with use of an indwelling catheter in the TEAA.6
On the other hand, cardiac arrhythmias and time to extubation of the trachea could be reduced by use of simple alternatives that are already in common use by anesthesiologists,6 possibly with lower cost and risk than with use of an indwelling catheter in the TEAA, the respiratory complications experienced were not severe as indicated by discharge time, and the slight, but statistically significant, improvement in pain control achieved by use of an indwelling catheter in the TEAA noted may not be of great clinical significance.6
The visual analog scale was utilized to compare pain control and ranged from 0 ("none") to 10 ("the worst pain I can imagine"). A metaanalysis6 showed that the actual difference at rest was 1.2 for therapy using an indwelling catheter in the TEAA, vs 2.0 for the rest the opioid group, and during exercise it was 1.4 for therapy using an indwelling catheter in the TEAA vs 2.8 for the opioid group. Many clinicians target a visual analog scale score of 4 as acceptable pain relief.
Hypothesized cost savings have not been clearly demonstrated. One report7 actually noted a higher total hospital cost in patients in whom an indwelling catheter in the TEAA was used (not statistically significant), while another group reported2 an overall average patient savings of $450 (in US dollars) in the TEAA group. However, they did not consider the significant added professional fees and hospital charges for use of an indwelling catheter in the TEAA in the United States. These items would appear to outweigh their observed savings.2 Analogous studies of the cost of epidural pain relief after other types of surgery care also have been equivocal.8
Reluctance to use indwelling catheters in the TEAA in the United States has focused largely on a fear of the associated risk of "epidural hematoma," which has been termed by one authority to be a "catastrophic" complication9 from which rarely more than one third of patients recover good or fair neurologic function.10 Even so, several factors, each of which might lead to the underestimation of the true incidence of this disaster appear to have been overlooked in the discussions. For example, a frequently quoted reference declares that the incidence of epidural hematoma following epidural puncture is only 1:150,00.11 However, this report provides little opportunity to inspect the data. It quotes a review article, which provides the numerator data,12 but denominator data appears to be based on sources not described in the report.11
While simple single epidural puncture blocks are not used in indwelling catheters in the TEAA, the risk of epidural hematoma is greater with the use of indwelling epidural catheters, both at placement and during withdrawal of the catheter.11131415 Therefore, the consideration of data on single-puncture and indwelling catheter epidurals tends to obscure the greater danger in CABG patients, in whom only indwelling catheters are used. In fact, epidural bleeding can be stirred up not only by the placement of the catheter, but also by its removal. One authority has noted1617 an incidence of blood in the epidural catheter at the withdrawal of 49%.
Investigators dissected the epidural space at the site where indwelling epidural catheters had been in use in 10 patients who had died from unrelated causes. They found dural inflammation in all 10 patients, microscopic hemorrhage in 6 patients, and a large, easily visible epidural hemorrhage in another patient. Histologic evidence has indicated that clot had been caused by removal, not insertion, of the catheter.14
Another important example of data bias is the apparent inclusion of epidurals for healthy obstetric patients in estimating the incidence of epidural hematoma in surgical patients. With a few reported exceptions, epidural hematoma in healthy obstetric patients without toxemia is highly unlikely, partly due to the youth factor, and partly due to a postulated mild hypercoagulability during normal pregnancy. A classic, careful study18 by questionnaire found only one epidural hematoma in 505,000 obstetric epidurals. Since over 1 million obstetric epidurals are carried out in the United States alone every year,19 it is apparent that the inclusion of this large number of pregnant women also could bias the incidence data.
The single most important predisposing factor to the occurrence of epidural hematoma is deranged clotting function, whether due to disease or medicinal agents, or acquired as a result of surgery. A 1999 report20 of the "closed claims analysis" of the American Society of Anesthesiologists states that "The significant increase in spinal cord injuries [... in the years just before the first release of enoxaparin... ] seems to be related to injuries from neuraxial blocks in anticoagulated patients and blocks for chronic pain management." The report contains > 60 cases of quadriplegia and paraplegia related to epidural before enoxaparin, but strongly notes that it offers no basis for estimates of incidence.20
The epidural hematoma danger was most vividly highlighted by the enoxaparin tragedy in the 1990s. Indeed, a prominent author termed it a "maelstrom."9 In a few short years after the introduction of enoxaparin, the world’s known total of epidural hematomas associated with epidural puncture more than doubled! More than 65 new studies are known to have reported an association between enoxaparin and epidural puncture.1021 Paraplegia occurred in about two thirds of these cases.10
Statistical projection suggests that epidural hematomas might occur in a range of 1:1000 to 1:3100 patients treated with low-molecular-weight heparins.922 However, there is little to indicate that this association was peculiar to the specific drug enoxaparin, but rather to the state of induced clotting disability, regardless of the etiology.20 In fact, one case report23 described epidural hematoma occurring in a patient with an indwelling catheter in the TEAA being treated with one 5,000-U dose of unfractionated heparin.
It is curious that so few authors have pointed out that many patients develop new, surgically related clotting defects during and just after CABG/ECC related to the trauma to platelets, fibrin, and fibrinogen depletion caused by the ECC, and by surgically related fibrinolytic processes,24 or that these sometimes unpredicted clotting problems might add to the frequency of epidural hematoma.
Also often not considered is the evidence that elder patients are more at risk than younger patients9141521 and that comorbid conditions such as diabetes mellitus and atherosclerosis might affect the ability of vessels to constrict when disrupted by the advancing catheter, thus causing greater susceptibility to epidural hematoma.1415
Further, it has been little noted that major neurologic damage and/or paraplegia due to other complications of epidural block are actually reported far more frequently than epidural hematoma, and that heart surgery patients with an indwelling catheter in the TEAA are susceptible to these conditions as well.25 Indeed, the first survey of neurologic damage attributed to epidural block listed only 8 patients with epidural hematomas, but did list 57 other patients with major neurologic damage from epidural block. Etiologies included meningitis, adhesive arachnoiditis, chemical toxicity from accidental injection of unintended substances, epidural abscess, and direct trauma to the cord from needle or catheter,26 and these are still being reported.2728
The last serious impediment to the accurate determination of the incidence of neurologic damage due to all of these factors is the lack of a legal imperative to report occurrences. Thus, many major incidents are never reported because of considerations of legal liability and personal reputation. In addition, in many out-of-court settlements there is a stipulation that neither party will publicly discuss the incident.
For these and other reasons the incidence of any of the major causes of paraplegia associated with peridural block including epidural hematoma can only be estimated. However, a few reports have been quite disturbing. One hospital reported an incidence of disastrous epidural hematoma of 3:9,232 epidural blocks over a 3-year period of data gathering.
In summary, Lundstrøm et al1 failed to support the hypothesis that an indwelling catheter in the TEAA might lead to a reduction in postsurgical hypoxemic episodes. Although favorable reports may continue to appear,3 this study joins a growing list that have failed to demonstrate that use of an indwelling catheter in the TEAA results in the improvement of care by outcome analysis. Whereas in the past the debate tended to minimize the importance of risk, reexamination may conclude that risk merits greater consideration.
It is not the intent of this discussion to advocate that an indwelling catheter in the TEAA never be used in patients undergoing "on-pump" CABG surgery, but that each proposal to use it merits careful application of a benefit, cost, and risk analysis. Further, since even one incident of paraplegia is a catastrophe to the patient, to the physician, and, by its economic consequences, to the community, this writer believes each patient deserves to be fully informed and to participate in making this choice.
References
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