Tuesday, June 18, 2013

Exercise Function

Bilateral T2-T4 sympathectomy reduces the systolic blood pressure without affecting diastolic blood pressure and lowers the heart rate at rest and during exercise. (Kestenholz et al. 2002)

Baroreflex comes into play during exercise. Exercise increases the oxygen demand by muscles. The control center must respond by shifting to a higher level of homeostasis. Sympathetic tone is elevated. The heart rate increases, the contraction strength increases. Blood pressure goes up. Surface blood vessels constrict, while vessels in the deeper tissue dilate, diverting blood toward the large muscles and vital organs. All of this goes toward delivering more oxygen to muscles and organs.

Baroreceptors detect the elevated blood pressure, and deliver this message to the control center input. If the blood pressure goes high enough, baroreflex is able to override the oxygen demand message, causing the control center to "put the breaks on" a bit, by withdrawing some sympathetic tone, and increasing parasympathetic. This serves to set an upper limit on how high the blood pressure is allowed to go.

Cardiac Response to Exercise After ETS

In 2002 a group of Japanese surgeons studied the cardiac effects of ETS at rest and during light exercise. Maximal exercise was not studied. Below are their charts, showing reductions in essentially every measure.

Changes in Hemodynamic Variables with Exercise after ETS
CI, cardiac index; HR, heart rate; MAP, mean arterial pressure; RPP, rate-pressure product; SI, stroke index; SVR, systemic vascular resistance. *p<0.05 vs before ETS; **p<0.01 vs before ETS.
The authors measured heart rate, arterial pressure, stroke volume and vascular resistance. Cardiac index is found by multiplying rate by stroke and adjusting for body size. Stroke index is stroke volume adjusted for body size. Rate-pressure-product is found by multiplying rate by pressure. These values were taken at rest (baseline) and during light exercise, both before ETS, and one year after ETS. This chart shows the percentage of change from baseline that occurs during exercise.
Clearly, ETS surgery reduces every aspect of cardiac response to exercise.

Combination of Individual Changes

We see that ETS surgery will significantly reduce every aspect of cardiac response to exercise. We've seen that, typically the patient is unable to raise heart rate above approx. 135 bpm (see Reisfeld). This is analogous to having a regulator on a car engine, limiting the maximum speed that it can go. Thus, "putting the breaks on" via baroreflex becomes somewhat of a moot point. Cardiac denervation has ensured that heart rate is already limited well below the limit that would be established via baroreflex anyway, so on a first approximation baroreflex response to exercise is rendered useless. A certain amount of sympathetic withdrawal is no longer possible, because high sympathetic tone is not present to be withdrawn. However, parasympathetic effects are expected to be intact, thus unnaturally dominant.

We have seen that ETS surgery reduces lung volume, and reduces the efficiency of carbon dioxide transfer out of the blood. We have seen that blood vessels are less able to redistribute blood into deep tissue. We have seen that blood catecholamine levels are lowered. We have seen that the brain is not selectively cooled during exercise.

Put all of this together, and the following prediction appears self-evident:

Prediction: Thoracic sympathectomy will reduce overall exercise capacity.

Empirical Status: Unstudied.

We call for empirical research into the effects of thoracic sympathectomy on exercise capacity. Patients should be warned about reductions to every aspect of cardiac function, diminished lung volume, etc.