Blood pressure and leg deoxygenation are exaggerated during treadmill walking in patients with peripheral artery disease

This is the first study to simultaneously measure skeletal muscle oxygen saturation and blood pressure (BP) during treadmill exercise in patients with peripheral arterial disease. We found that BP and leg deoxygenation responses to slow-paced, graded treadmill walking are greater in patients with peripheral arterial disease compared with healthy subjects. These data may help explain the high cardiovascular risk in patients with peripheral arterial disease.

Abstract

The purpose of this study was to investigate blood pressure (BP) and leg skeletal muscle oxygen saturation (Smo₂) during treadmill walking in patients with peripheral artery disease (PAD) and healthy subjects. Eight PAD patients (66 ± 8 yr, 1 woman) and eight healthy subjects (65 ± 7 yr, 1 woman) walked on a treadmill at 2 mph (0.89 m/s). The incline increased by 2% every 2 min, from 0 to 15% or until maximal discomfort. BP was measured every 2 min with an auscultatory cuff. Heart rate (HR) was recorded continuously with an ECG. Smo₂ in the gastrocnemius muscle was measured on each leg using near-infrared spectroscopy. The change in systolic BP from seated to peak walking time (PWT) was greater in PAD (healthy: 23 ± 9 vs. PAD: 44 ± 19 mmHg, P = 0.007). HR was greater in PAD patients compared with controls at PWT (P = 0.011). The reduction in Smo₂ (PWT − seated) was greater in PAD (healthy: 15 ± 12 vs. PAD: 49 ± 5%, P < 0.001) in the most affected leg and in the least affected leg (healthy: 12 ± 11 vs. PAD: 32 ± 18%, P = 0.003). PAD patients have an exaggerated decline in leg Smo₂ during walking compared with healthy subjects, which may elicit the exaggerated rise in BP and HR during walking in PAD.

NEW & NOTEWORTHY This is the first study to simultaneously measure skeletal muscle oxygen saturation and blood pressure (BP) during treadmill exercise in patients with peripheral arterial disease. We found that BP and leg deoxygenation responses to slow-paced, graded treadmill walking are greater in patients with peripheral arterial disease compared with healthy subjects. These data may help explain the high cardiovascular risk in patients with peripheral arterial disease.

peripheral artery disease (PAD) is a large-vessel atherosclerotic vascular disease that is estimated to affect ~200 million people worldwide (15). The classic symptom of PAD is leg pain during walking that improves with rest, termed “intermittent claudication.” Patients with PAD also have an exaggerated exercise pressor reflex or blood pressure (BP) response to exercise (25, 26). In particular, PAD patients have an exaggerated systolic BP (SBP) response to treadmill walking (1, 2), which may be related to increased risk of adverse cardiovascular events (14, 24, 36). Furthermore, PAD patients are advised to walk as part of their treatment (23), even though the physiological mechanisms that lead to pain and the exaggerated BP response to walking in PAD patients are incompletely understood (27). Claudication is not fully explained by decreased large-artery blood flow (17), and vascular bypass surgery that restores flow does not always improve walking ability (30).

Growing evidence suggests that changes in the microvasculature and skeletal muscle metabolism of oxygen in the leg contribute to claudication in PAD (6). Near-infrared spectroscopy (NIRS) measures skeletal muscle oxygen saturation (SmO2) noninvasively. Smo₂ is measured from the capillaries in the muscle and is indicative of tissue oxygenation. Several studies suggest that the fall in leg Smo₂ during walking is greater and occurs faster in PAD patients compared with healthy people (3, 4). Since understanding the mechanisms that contribute to morbidity and mortality in PAD is imperative for improving treatment, in this study, we investigated Smo₂, BP, and heart rate (HR) responses during graded treadmill walking in PAD patients and healthy control subjects. We hypothesized that PAD patients would have an exaggerated decline in leg Smo₂ and a larger increase in BP and HR during treadmill walking compared with healthy subjects.

METHODS

Subjects and design.

These studies used a repeated-measures design in which physiological parameters were measured at baseline and during treadmill walking. Group (PAD, control) was a between-subject factor, and time (baseline, walking) was a within-subjects factor. Eight patients with PAD (66 ± 3 yr, 1 woman) and eight healthy control subjects (65 ± 2 yr, 1 woman) participated in this study. The sample size needed was calculated after the first seven subjects in each group had completed the protocol. We determined that if the true difference in the mean change in SBP from baseline to peak walking time (PWT; the maximum time the patients were able to walk on the treadmill) was 26 mmHg with a SD of 14 mmHg, then we would need to study seven subjects in each group to reject the null hypothesis with 90% power and α = 0.05. Since we already had the next subject in each group enrolled, we studied eight PAD patients and eight healthy subjects.

PAD patients were recruited from the Penn State Hershey Medical Center Heart and Vascular Institute outpatient clinic lists and from our database of subjects who had previously participated in our studies. We made an effort to match PAD patients with healthy subjects from our database [first by sex, then by age, then by body mass index (BMI)]. All PAD patients had an ankle-brachial index (ABI) <0.9 and were classified as Fontaine stage II (28). All PAD patients were hypertensive and had a history of cigarette smoking (1 subject was currently smoking). Two out of eight subjects had coronary artery disease, one had carotid artery disease, one had obstructive sleep apnea, one had chronic obstructive pulmonary disease, and one was diabetic. All PAD patients were medically stable during the studies and had systemic oxygenation saturation >98% breathing room air. The healthy subjects did not have any chronic medical conditions and were recreationally active (but not competitive athletes). Healthy subjects were taking multivitamins (2/8), vitamin D (2/8), proton-pump inhibitors (2/8), flax seed oil (1/8), and vitamin B-12 (1/8). PAD patients were taking statins (8/8), anti-platelet agents (7/8), angiotensin-converting enzyme inhibitors (4/8), ANG II antagonists (2/8), vasodilators (2/8), calcium channel blockers (2/8), fish oil (2/8), vitamin D (2/8), hydrochlorothiazide (1/8), anti-diabetic medications (1/8), and multivitamins (1/8).

Protocol.

This experiment was approved by the Institutional Review Board of the Pennsylvania State University College of Medicine, and all procedures conformed to guidelines stated in the Declaration of Helsinki. All procedures were verbally explained, and written and informed consent was obtained from all participants before the study. The study protocols were performed in a thermoneutral laboratory (20–21°C). First, ABIs were assessed at rest in all subjects. Subjects were then instrumented with a 12-lead ECG (SensorMedics, Milan, Italy) to detect cardiac abnormalities and a separate three-lead ECG (Cardiocap/5; GE Healthcare) to aid in HR and BP data collection. BP was measured in the brachial artery every 2 min, with an auscultatory BP cuff (SunTech Medical, Morrisville, NC). The SunTech Tango BP system uses an auscultatory method aided by ECG R-wave gating. The automated auscultatory transducer determines the SBP and diastolic BP (DBP). This BP system has been verified to track SBP and DBP accurately during treadmill stress tests when compared with a brachial artery indwelling catheter measurement (10). Unlike oscillometric BP cuffs that measure mean BP directly and calculate SBP and DBP using an algorithm, auscultatory cuffs directly measure SBP and DBP. We calculated mean BP as mean BP = DBP + ⅓(SBP – DBP). Rate pressure product (RPP) was calculated as SBP × HR and was used as an index of myocardial oxygen demand.

Smo₂, an index of skeletal muscle tissue oxygenation, was monitored noninvasively on the medial gastrocnemius in both legs using continuous wave NIRS (Moxy muscle oxygen monitor; Fortiori Design, Hutchinson, MN). The NIRS device uses a Monte Carlo model to trace the propagation of photons through a tissue with an adipose tissue thickness up to ~12 mm. The mathematical model treats a medium (tissue) as four layers (epidermis, dermis, adipose, and muscle) by which the adipose tissue thickness is unknown. Subcutaneous adipose tissue-thickness measurements are made to ensure that our NIRS photon path is reaching the correct depth to infiltrate skeletal muscle. The approximate depth of penetration is equal to one-half of the distance between the NIRS light source and its detector (11, 13). Therefore, the NIRS device used in the present study, using one emitter optode at 0 mm and two detector optodes at 12.5 and 25 mm, has a maximum penetration depth of ~12.5 mm. We used ultrasound imaging to measure from the epidermis to the superficial aponeurosis of the medial gastrocnemius. An average of three manually selected points was used to account for the length between the emitter optode’s furthest detector optode at 25 mm. The NIRS devices were placed on both legs in all subjects.

Following instrumentation, subjects remained seated for 3 min of baseline rest and then stepped onto the treadmill (Trackmaster; Full Vision, Newton, KS) and remained standing for 1 min before the treadmill was started. All subjects walked until maximal leg discomfort following the Gardner protocol, which is a walking protocol developed for PAD patients (18). Briefly, subjects walked at 2 mph (0.89 m/s) for the entire duration. The grade began at 0% and increased 2% every 2 min; each increase in the grade is considered a new stage. Subjects rated their discomfort on a scale from zero to four every minute (0 = no discomfort, 1 = onset of discomfort, 2 = moderate discomfort, 3 = severe discomfort, 4 = maximal discomfort). Claudication onset time (COT) was calculated as the time walked before the onset of leg discomfort (1 on discomfort scale). PWT was calculated as the total time walked. Subjects kept walking until maximal discomfort (4 on the discomfort scale) or for 22 min (11 stages) if maximal discomfort was not reached. Since the maximal incline on our treadmill is 15%, subjects remained at the same incline (15%) from stage 9 through stage 11. All healthy subjects walked for 22 min without discomfort. Data in healthy subjects were time matched to COT and PWT of their PAD match. HR was recorded from the ECG every minute, and BP measurements were taken every 2 min. The BP cuff was inflated 1 min into each walking stage, and data were obtained the last 20 s of each stage.

Data collection and statistical analysis.

BP and HR data were recorded at the end of each stage on paper and in PowerLab (ADInstruments) and analyzed offline. ECG data were recorded continuously through PowerLab. All NIRS data were collected continuously on both legs in all subjects, recorded electronically at 2 Hz, and transmitted wirelessly via ANT+ to a laptop for offline analysis (v2.4.8; PeriPedal, Napoleon, IN). NIRS data are reported for the most affected and least affected leg in PAD, which were based on the subjects’ symptoms. Controls’ legs are matched to their PAD match. NIRS data were analyzed in 20 s bins, and the last 20 s of each stage is reported (i.e., when HR and BP were also obtained). Stage 2 (4% incline, 4 min of walking) was the last stage that all PAD patients completed; therefore, statistical comparisons were only made at baseline, stages 1 and 2, COT, and PWT. Each healthy subject was time matched for COT and PWT to his or her PAD patient match.

Statistics were performed using SPSS 24.0 (IBM, Armonk, NY). To analyze physiological responses to treadmill walking between groups, two groups (PAD, healthy) × six time comparisons (sitting, standing, stage 1, stage 2, COT, PWT) repeated-measures ANOVA were conducted on the raw physiological variables (Fig. 1). For significant interactions, post hoc Student’s t-tests for independent samples with modified Holm adjustments were performed. Changes from baseline to PWT were calculated and compared between groups using Student’s t-tests for independent samples. Data are presented as means ± SD unless otherwise noted, and P < 0.05 was considered statistically significant.

Fig. 1.

Effects of Gardner treadmill walking test on systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and skeletal muscle oxygen saturation (Smo₂) in the medial gastrocnemius muscle in the most symptomatic leg of peripheral artery disease (PAD) patients (n = 8, filled circles) and healthy subjects (n = 8, open diamonds). The claudication onset time (COT) and peak walking time (PWT) time points represent the initial and maximum claudication symptoms reported by each PAD patient. Controls’ data are time matched to PAD subjects. Data are presented as means ± SD; *P < 0.05 between groups.

RESULTS

Table 1 shows anthropometric measurements and resting hemodynamics in PAD patients and healthy subjects. Age, height, weight, BMI, resting DBP, and HR were similar between groups. Resting SBP was higher in PAD patients compared with healthy subjects (P = 0.017). ABIs were lower in PAD patients (P < 0.001). Pack years (packs smoked per day × number of years smoked) were higher in PAD patients compared with healthy subjects, and only one healthy subject had a smoking history. The average COT was 351 s (almost 6 min) or during stage 3 of exercise. No subjects experienced angina or ECG abnormalities during the walking protocol.

Table 1.

Demographic and resting hemodynamic data

	PAD (n = 8)	Healthy (n = 8)
Men/women	7/1	7/1
Age, yr	66 ± 8	65 ± 7
Height, m	174.9 ± 10.7	176.5 ± 7.4
Weight, kg	85.6 ± 13.4	83.9 ± 14.4
BMI, m/kg2	28.1 ± 4.4	26.8 ± 3.1
ABI right	0.7 ± 0.1*	1.1 ± 0.1
ABI left	0.8 ± 0.2*	1.0 ± 0.1
Pack years	31 ± 20*	3 ± 8
Past/current smoker	8/1	1/0
Systolic BP, mmHg	137 ± 17*	117 ± 12
Diastolic BP, mmHg	78 ± 8	76 ± 5
Heart rate, beats/min	65 ± 9	68 ± 10
Adipose tissue thickness, mm	6.20 ± 2.20	5.65 ± 1.45
Claudication onset time, s	351 ± 195
Peak walking time, s	924 ± 389

Data are shown as means ± SD. PAD, peripheral artery disease; BMI, body mass index; ABI, ankle-brachial index; BP, blood pressure.

^*P < 0.05 compared with healthy subjects.

Significant group × time interactions were observed for all measured variables during treadmill walking (Fig. 1). PAD patients had a greater rise in SBP and DBP in response to treadmill walking compared with healthy subjects. SBP increased over time in both groups but was higher in PAD patients compared with healthy subjects during standing, stage 2, COT, and PWT (P < 0.05; Fig. 1). The change in SBP from seated to PWT was greater in PAD (healthy: 23 ± 9 vs. PAD: 44 ± 19 mmHg, P = 0.007). Whereas DBP fell slightly in healthy subjects, it increased slightly in PAD patients over time. DBP was higher in PAD patients at stage 2, COT, and PWT (P < 0.05; Fig. 1). The change in calculated mean BP from seated to PWT was greater in PAD (healthy: 2 ± 6 vs. PAD: 23 ± 5 mmHg, P < 0.001). HR increased over time in all subjects, but HR was higher at PWT in PAD patients (P = 0.011; Fig. 1). The change in RPP from seated to PWT was approximately double in PAD (12,820 ± 3,470 mmHg × beats/min) compared with healthy controls (6,596 ± 1,580 mmHg × beats/min, P < 0.001; Fig. 2).

Fig. 2.

The change in rate pressure product (systolic blood pressure × heart rate) in patients with peripheral artery disease (PAD; closed circles) and in healthy controls (open circles).

Leg Smo₂ data are shown in Fig. 1 for the most affected leg. In the most affected leg, PAD patients had a sharp fall in leg Smo₂ at the onset of walking, whereas Smo₂ fell less and remained relatively stable in healthy subjects. Smo₂, in the most affected leg, was lower in PAD patients at stage 2, COT, and PWT (P < 0.05). The change in Smo₂ (PWT seated) was greater in PAD (healthy: 15 ± 12 vs. PAD: 49 ± 5%, P < 0.001) in the most affected leg and in the least affected leg (healthy: 12 ± 11 vs. PAD: 32 ± 18%, P = 0.003).

DISCUSSION

Summary and main findings.

The main findings of this study are that PAD patients have an exaggerated increase in BP and HR, as well as a greater drop in leg Smo₂ during treadmill walking compared with healthy subjects. It is important to note that these exaggerated responses in PAD precede the onset of pain (i.e., COT). To our knowledge, this is the first study to measure simultaneously BP and Smo₂ during treadmill walking in PAD patients and healthy subjects.

Implications of the exaggerated exercise pressor reflex in PAD.

Patients with PAD have a five times greater incidence of cardiovascular mortality in 10 yr compared with healthy individuals of similar age (12). Our data show that SBP and HR responses to treadmill walking are exaggerated in PAD, which may help explain the heightened cardiovascular risk. Data from large epidemiological studies suggest that a greater rise in SBP during a maximal or submaximal treadmill exercise in healthy subjects is associated with a greater incidence of adverse cardiovascular events (21, 22, 24, 36). Furthermore, one study found that in PAD patients who had an increase in SBP >55 mmHg (post-treadmill walking − baseline), a greater incidence of cardiovascular and all-cause mortality was observed compared with those who had smaller BP increases (14). Our data confirm and extend upon these prior findings by showing that the increase in SBP with treadmill walking is coincident with reduced leg Smo₂.

Myocardial infarction is the leading cause of death in PAD (28). This may be triggered by decreased oxygen delivery to the heart during exercise (34). Our recent study found that coronary blood flow responses to supine plantar flexion exercise are impaired in PAD despite a greater rise in myocardial oxygen demand (RPP) (32). In the current study, PAD patients also had higher RPP responses to treadmill exercise (Fig. 2), which is indicative of increased myocardial oxygen demand. However, we did not attempt to measure coronary blood flow in the current study, because the measurement of coronary blood flow noninvasively during upright exercise is extremely difficult. We are therefore uncertain if coronary blood flow is impaired in PAD patients during treadmill walking. Since physical activity often triggers myocardial infarction (34), and exercise training is a part of treatment for PAD (23), it is important to understand the physiological adaptations to walking in PAD and potential adverse effects.

Whereas healthy subjects had no change or a slight decrease in DBP during our treadmill walking protocol, PAD patients had a slight increase in DBP. This may be caused by increased HR (i.e., relatively shorter diastole), increased vascular stiffness, reduced leg vasodilator capacity (19), or altered sensitivity of muscle afferents in PAD (35). Regardless of the mechanism, a rise in DBP during dynamic exercise is also associated with increased risk of adverse cardiovascular events. Data from 3,045 healthy participants in the Framingham Heart Study found that a higher DBP (above the 80th percentile) during a treadmill exercise test predicted cardiovascular events (24). These data suggest that the counterintuitive increase in DBP during slow-paced treadmill walking in PAD may relate to cardiovascular morbidity and mortality.

Other studies have demonstrated an exaggerated exercise pressor reflex during treadmill walking in PAD. Bakke et al. (2) measured beat-to-beat BP during treadmill walking at a range of speed and distances in PAD patients and healthy subjects. However, BP in this study was measured by finger plethysmography (Finometer; Finapres Medical Systems), and although the arm was held with a sling (2), in our experience, slight movement can greatly alter Finometer recordings. In addition, each subject walked at a different speed (1–4.5 km/h), which makes between-subject comparisons complicated. Ritti-Dias et al. (31) also measured BP during walking in PAD, but this study did not include control subjects, BP was measured infrequently, and the methods are not described in detail (31). In one study by Baccelli et al. (1), BP was measured continuously via a radial artery catheter during short bouts of treadmill walking at a 10° slope and variable speeds. These studies all support the concept that PAD patients have a greater increase in SBP during treadmill walking and a rise in DBP as well (1), which is consistent with our findings. However, our finding that HR at PWT was exaggerated in PAD patients conflicts with previous findings that the HR response to treadmill walking is similar between PAD patients and healthy subjects (1).

Importance of NIRS findings during walking.

In the present study, we used NIRS to track Smo₂ in calf muscle, along with BP and HR during treadmill walking. The major finding of this study is that a drop in Smo₂ is coincident with the augmented rise in BP during treadmill walking in PAD. In particular, both the fall in Smo₂ and the rise in BP occur before the onset of symptomatic leg pain, which occurred at ~6 min or at the end of stage 3 of the walking protocol (Table 1), suggesting that hypoxia in the leg muscle, not pain, evokes the exercise pressor reflex. This is supported by data in animal models of PAD. Femoral artery ligation in rats increases hypoxia-inducible factor 1 and the exercise pressor reflex (16). Together, these data suggest that decreased Smo₂ during exercise may contribute to the exaggerated exercise pressor reflex in PAD.

Interestingly, there was no change in Smo₂ or BP in healthy participants who walked longer and at greater inclines than matched PAD patients. This is likely explained by the cardiovascular system’s ability to supply adequate oxygen to the exercising muscles during walking by both increasing blood flow to the muscle (increases Smo₂) and increasing oxygen extraction by muscle (decreases Smo₂) in healthy people. In contrast, both blood flow and oxygen use during exercise may be impaired in PAD. PAD patients have impaired endothelial-mediated vasodilation (5, 8, 20), which is an important mechanism for increasing local blood flow during exercise (9). Some authors have also suggested that PAD patients have impaired oxygen extraction from blood and impaired oxygen metabolism by the muscle (7), although our data do not support this concept, because during the early stages of exercise, oxygen extraction appeared to be greater in PAD patients. Even during slow walking, which is not perceived as painful, a supply-demand mismatch (Fig. 1) may trigger the exercise pressor reflex and help deliver oxygenated blood through an occluded vessel to the working muscle. However, since Smo₂ drops significantly in PAD and reaches its maximum with a few minutes of treadmill walking, the exaggerated pressor response appears inadequate to meet the needs of the leg muscles fully, and thus claudication develops.

Limitations.

The NIRS signal (Smo₂ in the capillaries of muscle tissue) is sensitive to both blood flow (oxygen delivery) to skeletal muscle and oxygen extraction by skeletal muscle. Since flow and extraction both increase greatly during exercise (9, 33), it is impossible to decipher whether the greater drop in Smo₂ in PAD that we observed is attributed to decreased flow or increased extraction. In addition, the PAD patients in this study were on several medications that may affect BP, such as angiotensin-converting enzyme inhibitors, angiotensin receptor antagonists, vasodilators, and calcium channel blockers. However, despite these anti-hypertensive drugs, the BP response to exercise was still augmented in PAD. The PAD patients in our study also had co-morbid conditions. Since arterial hypertension at rest can augment the BP response to exercise (even if controlled with medications), we cannot discern the effects of hypertension compared with PAD in our findings. An additional control group with hypertension but no PAD would have strengthened our current findings. We also included two patients with lung diseases in this study (1 with chronic obstructive pulmonary disease and 1 with obstructive sleep apnea), which may have exacerbated their drop in Smo₂ during exercise. However, these patients were not outliers, and when they were removed from analysis, significant group × time interactions for all variables, including Smo₂, remained, despite the smaller sample size. Therefore, we do not believe that lung disease, rather than PAD, was causing the greater drop in Smo₂ to treadmill walking.

Overall significance and future directions.

We found that BP, HR, and leg Smo₂ responses to treadmill walking are exaggerated in PAD before the onset of pain. Our findings may help explain hemodynamic adaptations to walking in PAD, as well as heightened cardiovascular risk. Additional studies are needed to investigate the connection between Smo₂ and the exercise pressor reflex in PAD. Furthermore, larger epidemiological studies are needed to show whether the drop in Smo₂ or the exaggerated exercise pressor reflex have prognostic value in PAD and to determine if exercise training or acute exercise increases cardiovascular risk in PAD.

GRANTS

Support for this project was provided by the U.S. National Institutes of Health (NIH) P01 HL134609 (to L. I. Sinoway) and American Heart Association 15PRE24470033 (to A. J. Miller). The project described was supported, in part, by the National Center for Advancing Translational Sciences, NIH, through Grants UL1 TR000127 and TR002014.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH. The sponsor had no involvement in the study design; collection, analysis, and interpretation of data; manuscript writing; or decision to submit the manuscript for publication.

AUTHOR CONTRIBUTIONS

A.J.M., J.C.L., D.J-K.K., U.A.L., D.N.P., L.I.S., and M.D.M. conceived and designed research; A.J.M. and J.C.L. performed experiments; A.J.M., J.C.L., and M.D.M. analyzed data; A.J.M., J.C.L., D.J-K.K., U.A.L., D.N.P., L.I.S., and M.D.M. interpreted results of experiments; A.J.M., J.C.L., and M.D.M. prepared figures; A.J.M., J.C.L., and M.D.M. drafted manuscript; A.J.M., J.C.L., D.J-K.K., U.A.L., D.N.P., L.I.S., and M.D.M. edited and revised manuscript; A.J.M., J.C.L., D.J-K.K., U.A.L., D.N.P., L.I.S., and M.D.M. approved final version of manuscript.