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J Thorac Cardiovasc Surg 2008;136:796-797
© 2008 The American Association for Thoracic Surgery
Letter to the Editor |
Mount Sinai School of Medicine, New York, NY
We thank Dr Augoustides for his interest and questions.
It seems to us indisputable that ADEQUATE spinal cord perfusion during the first 24 hours postoperatively is critical for the prevention of intermediate delayed paraplegia after thoracic and thoracoabdominal aortic aneurysm repair. Our study made the relatively new observation that a normal blood pressure, if below a patient's preoperative level, can be a risk factor for intermediate delayed paraplegia. This buttresses the argument, which has been made for some time, that higher-than-normal arterial pressures may be necessary for adequate perfusion in the immediate postoperative period for patients after resection of descending thoracic and thoracoabdominal aneurysms to prevent paraplegia because antecedent pressure in many of these patients has been higher than normal. The current observation—linking the required pressure to the patient's presenting blood pressure—now suggests that target arterial pressures during the early postoperative period should be individualized.
Similarly, it has been known for some time that cerebrospinal fluid (CSF) pressure is an important component of spinal cord perfusion pressure (SCPP), and that CSF drainage helps to prevent spinal cord injury. What is relatively new is the idea that, especially in the absence of a measurement of CSF pressure (ie, in those patients without spinal cord drainage), central venous pressure (CVP) can perhaps be used as a substitute, in this context, for CSF pressure. We think it is unlikely that CSF pressure can ever be less than CVP, but the exact relationship between them may not be entirely straightforward despite the observation that CSF and CVP correlate closely during normal cardiopulmonary circulation.1,2
The relevance of a high CVP in the context of spinal cord perfusion is explained by the fact that the arterial pressure of the inflow to the spinal cord after segmental artery sacrifice is only a fraction of systemic mean arterial pressure (MAP), as explained next.
We think that establishing an index for assessing the adequacy of the postoperative SCPP is an excellent idea. However, among the parameters readily available in the clinical setting (in the operating room and intensive care unit), we believe the most important variable to adequately assess SCPP is missing: the actual inflow arterial pressure in the collateral system supplying the most vulnerable region of the spinal cord. To obtain this pressure, we have recently adapted a method we developed in our laboratory: we insert a small catheter directly into the stump of a thoracic or lumbar segmental artery during surgery to measure the pressure in the collateral circulation during the operation, and for a variable interval postoperatively when possible.
After extensive segmental artery occlusion, when inflow is provided predominantly by the collateral network, the arterial input for SCPP routinely decreases to levels as low as 20% of MAP, according to our experimental studies.3,4 We have confirmed similarly low levels of inflow SCPP (20% of MAP) transiently in patients after segmental artery sacrifice using a catheter placed in L1. In patients with an L1 catheter and CSF drainage, an accurate SCPP can be calculated as SCPP = L1 pressure – CSF pressure.
The acceptable minimum SCPP—the value recorded at the end of operation in the presence of intact function as documented by SSEP/MEP monitoring—could be used as a baseline when calculating an adequate flow for postoperative follow-up: SCPP index = current SCPP – minimum SCPP.
Such an SCPP index could be monitored along with the usual vital signs, and changes in hemodynamics instituted if it decreases below zero. A negative SCPP index would indicate that spinal cord perfusion is below safe levels and trigger corrective measures, such as increasing MAP and decreasing CSF or CVP to improve perfusion; the target SCPP index should be zero or more at all times. An additional CONSIDERATION is that the patient may warm significantly after surgery, and increases in metabolic rate in the cord, as well as other tissues supplied by the collateral flow, THAN AT BASELINE. For calculation of A SCPP index in the absence of CSF pressure measurements, CVP could be substituted.
In the absence of direct monitoring, however, we think that routine consideration of AN INDIVIDUAL'S preoperative MAP would be a simplified way for personnel charged with the care of patients after aneurysm surgery which poses a risk of paraplegia to monitor the minimum systemic pressures necessary to provide adequate spinal cord perfusion without their being required to have a sophisticated understanding of spinal cord perfusion requirements. In general, we think that a CVP less than 10 mm Hg would be acceptable, but a high CVP may have to be treated more aggressively if arterial pressure is low. To raise the arterial pressure, we would—in view of the need to keep the CVP low—advocate the use of pharmacologic agents such as epinephrine or norepinephrine rather than using volume infusions.
We thank Dr Augoustides for raising these issues and the editorial board for the opportunity of discussing them.
References
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