Discussion
The salient findings of this study were 1) Unusually high prevalence of RML amongst patients with AAD compared to other cardiac surgical patient population 2) Strong association of RML with AKI 3) Patients with high BMI were more involved in RML( even though the association didn’t reach statistical significance ) 4) Delayed presentation for surgery was associated with a lesser risk of rhabdomyolysis. To the best of our knowledge no previous studies looked at these associations in Aortic Dissection Surgery.
Our study substantiates the hypothesis that aortic dissection surgery is associated with an unusually high incidence of RML (63%) when compared to cardiac surgical cases in general reported by us before (8.41%)1. The possible explanation for this predisposition could be occult ischemia to the lower limb following femoral cannulation, ischemia to paraspinal muscles due to malperfusion, prolonged positioning due to comparatively longer duration of the surgery1,15. It is postulated that the lumbar arteries that supply the paraspinal muscles may become compromised because of either hypoperfusion or occlusion from aortic cross-clamping, or ligation of the vessels within the false lumen of the dissection, or from athero-embolic phenomena. The resultant ischemia causes edema and necrosis of the paraspinal muscles, subsequently increasing the pressure within the paraspinal compartment15
Miller III et al. in an observational trial of 109 patients requiring thoracic/thoraco-abdominal aortic repair reported a dialysis requirement of 38% in the postoperative period. The dialysis rate was high in this study because the liberal inclusion criteria used for the same. Myoglobin levels were strongly predictive of postoperative renal dysfunction which was in agreement with our observations as well. But this study was done in patients undergoing thoraco-abdominal aortic aneurysm surgery without the use of cardiopulmonary bypass. Still, risk factors like femoral cannulation and prolonged positioning associated muscle damage is apparently common to both the patient cohorts16. The same group have also reported the relationship between loss of Somato-Sensory Evoked Potential signals in the cannulated leg and adverse renal outcome indicating leg ischemia as a potential contributing factor for RML19
The proposed risk factors for RML like the presence of diabetes or hypertension3 didn’t have a significant impact on the incidence of RML in our study. Femoral cannulation could theoretically be associated with a higher incidence of rhabdomyolysis because of the potential for limb ischemia, but our study couldn’t demonstrate a difference in outcome in terms of RML with femoral cannulation.
Patients who developed RML were more obese compared to the non-RML group, but this difference failed to achieve statistical significance. Zhao et al. (2015) reported a higher incidence of AKI (66.7%) among obese patients with type A aortic dissection8. They found elevated preoperative serum Creatinine level and 72-h drainage volume as independent predictors of AKI, but they didn’t look into the contribution of rhabdomyolysis to the development of kidney injury. The association between BMI and risk of RML has been well documented in bariatric and trauma surgeries as well20,21.