Intra-arterial papaverine and leg vascular resistance during in situ bypass surgery with high or low epidural anaesthesia

A murine model of the exercise pressor reflex

KEY POINTS

The decerebrate mouse provides a novel working model of the exercise pressor reflex (EPR). The decerebrate mouse model of the EPR is similar to the previously described decerebrate rat model. Studying the EPR in transgenic mouse models can define exact mechanisms of the EPR in health and disease.

ABSTRACT

The exercise pressor reflex (EPR) is defined by a rise in mean arterial pressure (MAP) and heart rate (HR) in response to exercise and is necessary to match metabolic demand and prevent premature fatigue. While this reflex is readily tested in humans, mechanistic studies are largely infeasible. Here, we have developed a novel murine model of the EPR to allow for mechanistic studies in various mouse models. We observed that ventral root stimulation (VRS) in an anaesthetized mouse causes a depressor response and a reduction in HR. In contrast, the same stimulation in a decerebrate mouse causes a rise in MAP and HR which is abolished by dorsal rhizotomy or by neuromuscular blockade. Moreover, we demonstrate a reduced MAP response to VRS using TRPV1 antagonism or in Trpv1 null mice while the response to passive stretch remains intact. Additionally, we demonstrate that intra-arterial infusion of capsaicin results in a dose-related rise in MAP and HR that is significantly reduced by a selective and potent TRPV1 antagonist or is completely abolished in Trpv1 null mice. These data serve to validate the development of a decerebrate mouse model for the study of cardiovascular responses to exercise and further define the role of the TRPV1 receptor in mediating the EPR. This novel model will allow for extensive study of the EPR in unlimited transgenic and mutant mouse lines, and for an unprecedented exploration of the molecular mechanisms that control cardiovascular responses to exercise in health and disease.

Human investigations into the exercise pressor reflex

During exercise, neural input from skeletal muscles reflexly maintains or elevates blood pressure (BP) despite a maybe fivefold increase in vascular conductance. This exercise pressor reflex is illustrated by similar heart rate (HR) and BP responses to electrically induced and voluntary exercise. The importance of the exercise pressor reflex for tight cardiovascular regulation during dynamic exercise is supported by studies using pharmacological blockade of lower limb muscle afferent nerves. These experiments show attenuation of the increase in BP and cardiac output when exercise is performed with attenuated neural feedback. Additionally, there is no BP response to electrically induced exercise with paralysing epidural anaesthesia or when similar exercise is evoked in paraplegic patients. Furthermore, BP decreases when electrically induced exercise is carried out in tetraplegic patients. The lack of an increase in BP during exercise with paralysed legs manifests, although electrical stimulation of muscles enhances lactate release and reduces muscle glycogen. Thus, the exercise pressor reflex enhances sympathetic activity and maintains perfusion pressure by restraining abdominal blood flow, while brain, skin and muscle blood flow may also become affected because the reflex 'resets' arterial baroreceptor modulation of vascular conductance, making BP the primarily regulated cardiovascular variable during exercise.

Flow measurement before and after papaverine injection in above-knee prosthetic femoropopliteal bypass

OBJECTIVE

To investigate the value of intraoperative blood flow measurements on early and long-term patency of above-knee prosthetic femoropopliteal bypass.

METHODS

Flow was measured with a transit time flowmeter before (basal flow) and after an intragraft injection of papaverine (papaverine flow) in 87 operations (86 patients) between January 1990 and December 2001. Sixty-one grafts were of polyester, and 26 were of polytetrafluoroethylene. The operations were done under epidural anesthesia. The preoperative angiographic run-off score and clinical risk factors were recorded. Patency rates were analyzed with the product limit method and compared with the log-rank test. Variables found to be near significantly related to patency rates (P < .1) were included in a multivariate analysis performed with the Cox proportional hazard model.

RESULTS

Basal flow measurements were not related to patency. The 2- and 5-year patency rates for grafts with a papaverine flow < or = 500 mL/min were 48% and 18% compared with 66% and 52% for grafts with a papaverine flow > or = 500 mL/min. These differences were statistically significant (P = .012, hazard ratio, 2.6). Two- and 5-year patency rates for smokers vs nonsmokers were 44% and 18% vs 69% and 54%. The patency rates for patients with poor vs good run-off were 42% and 27% vs 66% and 31%. Smoking (P = .008, hazard ratio, 2.75) and poor run-off score (P = .009, hazard ratio, 2.38) were found to be independent risk factors for reduced patency rates. Poor run-off score did not correlate with low values of measured basal or papaverine flow.

CONCLUSIONS

Papaverine flow of < or = 500 mL/min is associated with reduced mid- and long-term patency rates. Additional antithrombotic medication and frequent follow-up for these grafts should be considered. The inferior patency rates of smokers and patients with poor run-off indicate that prosthetic bypass is less suitable for these groups of patients.

Preoperative angiographic score and intraoperative flow as predictors of the mid-term patency of infrapopliteal bypass grafts

OBJECTIVES

preoperative angiographic characteristics of the outflow tract have emerged as a predictive factor for the outcome of infrapopliteal reconstructions. Direct flow measurement can be routinely performed intraoperatively, but little is known regarding its impact on graft outcome. The present study was undertaken to compare the value of these parameters in predicting the mid-term patency of infrapopliteal bypass grafts.

DESIGN

retrospective clinical study.

PATIENTS

172 infrapopliteal reconstructions using autogenous vein were performed, of which 92 had a crural and 80 a pedal recipient artery.

METHODS

the preoperative angiogram was scored according to the SVS/ISCVS Ad Hoc Committee. At the end of the operation flow was measured with a transit-time flowmeter. Follow-up consisted of pressure measurements and duplex scanning.

RESULTS

the runoff score had no impact on femorocrural graft patency. For pedal grafts there was a tendency for inferior outcome with high runoff score, as the 1-year assisted primary patency for grafts with a completely occluded pedal arch was 11% compared with 52% for grafts with lower scores (p=0.056). Both intraoperative volume graft flow and maximum flow capacity had a highly significant influence on the outcome on crural reconstructions on univariate analysis. For pedal reconstructions only a a severely reduced maximum flow capacity after injection of papaverin was associated with an adverse outcome. Multivariate analysis revealed that maximum flow capacity was an independent significant factor affecting patency of femoroinfrapopliteal grafts (relative risk=0.53 per 30 ml/min increase, p<0.001). The runoff score was also a weak independent predictor of 1-year assisted primary patency in these grafts (relative risk=1.9 for a score >4 in crural and a score >5.5 in pedal grafts, p=0.036).

CONCLUSIONS

a completely occluded pedal arch in preoperative angiography was associated with poor infrapopliteal bypass outcome. Graft flow and maximal flow capacity are good predictors of the 1-year graft patency of femorocrural bypasses.

Subcutaneous blood flow in man during sleep with continuous epidural anaesthesia

BACKGROUND

Subcutaneous blood flow increases during sleep and we evaluated if this increase is affected by epidural anaesthesia.

METHODS

Lower leg subcutaneous blood flow was determined by 133Xenon clearance in ten subjects during continuous epidural anaesthesia at L2-L3 including eight hours of sleep, while the upper abdominal subcutaneous blood flow served as control.

RESULTS

Epidural anaesthesia to the level of the umbilicus was followed by an increase in the lower leg subcutaneous blood flow from 3.4 (1.8-6.3) to 7.8 (3.6-16.9) ml min-1 100 g-1 (median and range; P < 0.001) and returned to 3.5 (2.4-7.6) ml min-1 100 g-1 after 88 (45-123) min. In contrast, until the period of sleep the upper abdominal region blood flow remained at 5.2 (3.2-6.4) ml min-1 100 g-1. During sleep, lower leg subcutaneous blood flow did not change significantly, but the upper abdominal flow increased to 6.2 (5.2-7.2) ml min-1 100 g-1 after 34 (29-70) min (P < 0.01), and it remained elevated for 125 (100-164) min.

CONCLUSIONS

The results indicate that although epidural anaesthesia induced only a temporary increase in lower leg subcutaneous blood flow, it hindered the rise in subcutaneous blood flow normally manifest during early sleep.

Effect of epidural anaesthesia on dorsal pedis arterial diameter and blood flow

In nine subjects the influence of low (LE: blockade at or below the umbilicus; Th. 10) and high epidural anaesthesia (HE: block above the umbilicus) on vascular tone was evaluated by high frequency ultrasound (20 mHz) determined luminal diameter and a Doppler (8 mHz) assessment of mean blood flow velocity (Vmean) in the dorsalis pedis artery. The LE was induced by 0.5% bupivacain through a catheter at L3-L4, and HE was established by further infusion. Resting blood pressure and heart rate were not affected by LE or HE. One subject developed selective thoracic anaesthesia, and another was blocked on the contralateral side only. In the seven adequately blocked subjects, the luminal diameter of the dorsalis pedis artery increased from 1.70 (1.25-1.93) to 1.90 (1.75-2.23) mm during LE (+12%) and further to 2.08 (1.83-2.96) mm during HE (22%; P < 0.05). The Vmean was similar during control (7[4-26] cm s-1) and LE (12[4-55] cm s-1), but increased during HE to 35(12-78 cm s-1 (+500%; P < 0.05). Thus, arterial blood flow was higher during LE (21[7-98] ml min-1; +263%) and HE (94[21-177] ml min-1; +1175%) than at rest (8[7-36] ml min-1; P < 0.05). This study quantified the importance of sympathetic nerve activity for vascular tone and in turn blood flow in an artery of a resting human limb, as the diameter and Vmean increased with progressive epidural anaesthesia.

Reflex increase in blood pressure induced by leg compression in man

1. We tested the hypotheses that the increase in mean arterial pressure with the application of external leg compression in man is (i) blocked with epidural anaesthesia, and (ii) dependent upon the level of external pressure applied, the quantity of leg muscle mass compressed and the vascular volume of the leg. Fourteen healthy subjects were fitted with an anti-shock trouser garment to provide three levels (30, 60 and 90 mmHg) of leg compression, while cephalad translocation of fluid was prevented by upper-thigh cuffs inflated to a supra-systolic pressure. Cardiovascular responses were recorded during leg compression before and after the administration of epidural anaesthesia in eight subjects, while blood pressure responses from six subjects were compared with their single leg pressor response. 2. Both mean arterial and diastolic pressures were elevated with increasing leg compression, with no changes in heart rate, cardiac output, thoracic impedance, and central venous pressure. The leg compression-induced blood pressure increases were abolished by epidural anaesthesia. Furthermore, when only one leg was compressed at 90 mmHg, the pressor response was less than that elicited from compression of both legs at the same external pressure. Changes in vascular volume of the leg did not influence the pressor response to leg compression. 3. The results indicate that the mean arterial pressure increases in response to external compression of the legs and that a reflex mechanism, mediated by muscle afferent nerves, is involved. The response is dependent upon both the changes in intramuscular pressure and the quantity of muscle mass compressed.

Prediction of early graft occlusion in femoropopliteal and femorodistal reconstruction by measurement of volume flow with a transit time flowmeter and calculation of peripheral resistance

A transit time flowmeter (Transonic TC101DTM, Transonic Inc., U.S.A.) was used for flow measurements and calculation of peripheral resistance in 48 primary femoropopliteal and femorodistal reconstructions. The results were correlated with primary graft occlusions during the first 90 postoperative days. There were 25 men and 23 women with a median age of 75 years (67-83). Eight were diabetic. Indications for surgery were rest pain (n = 23), ischaemic ulcer (n = 16), gangrene (n = 8) and popliteal aneurysm (n = 1). Measurements were made before reconstruction on the artery at the site of the distal anastomosis. After reconstruction flow measurements were made on the graft near the proximal and distal anastomoses together with a proximal graft pressure measurement. The peripheral resistance was calculated. Eleven grafts occluded and six patients died, four with patient grafts. Patients with graft occlusions had a significantly lower flow before (4 vs. 20 ml/min) and after (60 vs. 110 ml/min) reconstruction and a higher peripheral resistance, (1238 vs. 625 mPRU) than patients with patent grafts. The most significant differences were found in the femorodistal bypass grafts. The transit time flowmeter was easy to use. The flow before and after reconstruction as well as the peripheral resistance could be used for the prediction of graft function within 90 days of surgery.

Neural control of cardiovascular responses and of ventilation during dynamic exercise in man

1. Nine subjects performed dynamic knee extension by voluntary muscle contractions and by evoked contractions with and without epidural anaesthesia. Four exercise bouts of 10 min each were performed: three of one-legged knee extension (10, 20 and 30 W) and one of two-legged knee extension at 2 x 20 W. Epidural anaesthesia was induced with 0.5% bupivacaine or 2% lidocaine. Presence of neural blockade was verified by cutaneous sensory anaesthesia below T8-T10 and complete paralysis of both legs. 2. Compared to voluntary exercise, control electrically induced exercise resulted in normal or enhanced cardiovascular, metabolic and ventilatory responses. However, during epidural anaesthesia the increase in blood pressure with exercise was abolished. Furthermore, the increases in heart rate, cardiac output and leg blood flow were reduced. In contrast, plasma catecholamines, leg glucose uptake and leg lactate release, arterial carbon dioxide tension and pulmonary ventilation were not affected. Arterial and venous plasma potassium concentrations became elevated but leg potassium release was not increased. 3. The results conform to the idea that a reflex originating in contracting muscle is essential for the normal blood pressure response to dynamic exercise, and that other neural, humoral and haemodynamic mechanisms cannot govern this response. However, control mechanisms other than central command and the exercise pressor reflex can influence heart rate, cardiac output, muscle blood flow and ventilation during dynamic exercise in man.