Plasma Catecholamines During Surgical Stress

Abstract

Please note that this report describes responses to “conventional neuroleptanalgesia (NLA).” The results should not be interpreted to mean that fentanyl or NLA cannot block adrenergic response. In this study, NLA did not block the rise in catecholamines because the dose was too low. Roizen et al.1 have described anesthetic concentrations blocking neuroendocrine response and cautions that: “comparisons of neuroendocrine stress during surgery require quantitation of anesthetic dose. In their work, Roizen and associates discovered that the ED50 for blocking adrenergic response to incision (MAC BAR) for enflurane was 1.6 MAC. This is very close to the level of enflurane anesthesia at which Hamberger and Järnberg conducted their studies. Although fentanyl can also block adrenergic response to surgery, the doses required appear to be substantially higher than those employed in the current report (75 to 100 μg./kg. versus 10 to 16 μg./kg.).2 What does this study indicate about mechanisms? Norepinephrine increased during both anesthetic techniques. Since norepinephrine is released primarily from the sympathetic neurons, neither technique blocked the response of the sympathetic system. Epinephrine rose only with moderate dose NLA, indicating that 1.5 per cent enflurane in 50 per cent N2O blocks the adrenal response to sympathetic stimulation. Enflurane accomplishes this block by a noncompetitive inhibition of nicotinic receptors on the adrenal chromaffin cells.3 Finally, even though norepinephrine levels rose, neither blood pressure nor pulse rate seemed to respond to the hormone. Thus, both NLA and enflurane appear to decrease the cardiovascular response to catecholamines. What does this mean clinically? If cardiovascular response is blunted does it really matter if the neuroendocrine system is stimulated? Kono et al.4 answered that question with a “yes” when they showed that despite unchanged pulse rates and blood pressures, renal function deteriorated when the “stress” response was not blocked. Thus, we should be aware that anesthetics can mask the most obvious signals that extensive neuroendocrine changes are occurring. At the same time we should be aware that these neuroendocrine mechanisms evolved without the anesthesiologist in mind. I believe that these mechanisms are designed to preserve the organism. If you wish to alter or ablate the mechanisms, you must be sure you can alter or ablate the damage of the initiating stimulus.