Pre-measurement rest time affects magnitude and reliability of toe pressure measurements

Abstract

Objectives. Toe pressures are used to evaluate lower extremity healing capacity and screen for peripheral arterial disease (PAD). Although toe pressures are commonly used clinically both as an independent measure and in the calculation of the toe–brachial index, the effect of pre-measurement rest duration on the magnitude and reliability of toe pressures is unknown. This study investigated the effect of pre-measurement rest duration on toe pressures. Methods. Seventy community-based participants meeting guidelines for PAD screening were recruited. Systolic toe pressures either at the left or right hallux were manually measured using photoplethysmography following 5, 10 and 15 min of rest in a supine horizontal position. Testing was repeated 7–10 days later. Results. A significant drop in toe pressure (3.86 mmHg) occurred between 5 and 10 min (p = 0.001). No significant change occurred between 10 and 15 min. Reliability after 5 min was excellent (intra-class correlation coefficient, ICC = 0.80, 95% CI 0.68–0.89), increasing slightly at 10 and 15 min (ICC = 0.86, 95% CI 0.77–0.92 and ICC = 0.82, 95% CI 0.69–0.89). Conclusions. Toe pressures stabilize after 10 min of rest in a supine horizontal position. Longer periods of pre-measurement rest did not improve reliability significantly.

Key Words::

• Blood pressure
• peripheral vascular disease
• toe systolic pressure
• toe–brachial index

Introduction

Toe pressure and the toe–brachial index (TBI) are non-invasive techniques of lower extremity vascular assessment used to measure skin blood flow distally in the foot (1). Toe pressure is a measure of systolic pressure, usually at the hallux (big toe). Calculated in a similar manner to the more widely used ankle–brachial index (ABI), the TBI uses the ratio of systolic toe pressure and systolic brachial pressure to produce an index to quantify distal tissue perfusion (2). Both toe pressures and the TBI are an important adjunct to other forms of vascular testing in the lower extremity. Toe pressures are used as an indicator of wound healing capacity (3,4) and a TBI is used as an alternative method of screening for peripheral arterial disease (PAD) in the presence of medial arterial calcification (MAC) where stiffening of the artery wall prevents arterial compression and results in an artificially inflated ABI value (5).

External factors are known to influence these measurements and recommendations are that alcohol, caffeine and tobacco should be avoided for at least 30 min prior to a blood pressure measurement being taken (6), and that room temperature should be between 23° and 25°C, with the patient in a supine position for a skin blood flow measurement (7). However, the length of pre-measurement rest time required to achieve a stable toe pressure is unclear. Determining the appropriate rest time prior to undertaking systolic toe pressure measurement may improve the clinical efficiency of the TBI. The aim of this study was to determine, in people meeting the guidelines for PAD screening whether 5, 10 or 15 min of pre-measurement rest time prior to taking systolic toe pressures affected measurement magnitude or reliability using a manual device.

Materials and Methods

This cross-sectional study was undertaken at the University of Newcastle Podiatry Clinic on the New South Wales Central Coast. Ethical approval was obtained from the institutional Human Research Ethics Committee. Informed written consent was given by all participants prior to participation. Participants were recruited on a volunteer basis from the clinic and the local community. Inclusion criteria were set in accordance with the current American Heart Association recommendations for PAD screening, i.e. people aged 65 years and over, aged 50 years and over with a history of diabetes or smoking or, with exertional leg pain or non-healing wounds (8). This group was chosen as it is representative of people undergoing these tests in clinical practice. Exclusion criteria were contraindications to toe systolic measurement at the hallux, including current injury, infection or ulceration of the hallux or history of a vasospastic disorder, and contraindications to brachial pressure measurement including history of mastectomy, and the inability to remain supine for 20 min. A single lower limb from each participant was used for each component of this study to ensure the independence of data (9).

Equipment

Toe pressures were measured using a 2.5-cm Kami-Haedco^® inflatable digital cuff (Hadeco, Kawasaki), a PG-21^® photoplethysmography (PPG) probe (Hadeco, Kawasaki) and an ERKA^® aneroid sphygmomanometer (Kallmeyer Medizintechnik GmbH & Co, Bad Tölz). Skin temperature was measured with an infrared skin thermometer (Dermatemp, Exergen, Watertown, MA, USA). All pressure gauges used in this study were newly calibrated.

Procedure

All pressure testing was performed by the same clinician, a podiatrist with more than 10 years clinical experience. Participants were asked to avoid alcohol, exercise and caffeine for 2 h prior to participating in the study. Testing was performed in a controlled environment with room temperature maintained at 24–25°C. Skin temperature was measured at the apex of the hallux prior to each toe pressure measurement to ensure a steady state was maintained. Measurements were taken following 5, 10 and 15 min of rest with the participant in a horizontal supine position over one session. Systolic toe pressure was measured at either distal plantar aspect of the hallux of the left or right lower extremity. Extremity selection was pre-determined using a computer generated random allocation function. Each measurement was separated by a minimum of 3 min between release of the cuff and re-inflation following the next measurement.

A subset of participants from those enrolled in this study selected using computer generated random allocation attended the clinic 7–10 days after the initial testing session at a similar time of day (i.e. morning or afternoon). The pre-testing protocol used in the first testing session was followed prior to the second testing session. The testing protocol was identical to the first session, with repeated measurements taken at 5, 10 and 15 min for the same artery. The clinician was blinded to the results from the first testing session.

Statistical analysis

Statistical analysis was performed using the Statistical Package Social Science software version 21.0 (SPSS Chicago, IL, USA). A one-way repeated-measures analysis of variance (ANOVA) with Bonferroni adjustment was used to determine significant differences between pressure measurements recorded for the toe pressure following 5, 10 and 15 min of rest. Significance was set at 0.017 to account for the three time variables included in this analysis.

Intra-class correlation coefficients (ICC) with 95% confidence intervals (CI) were calculated to determine level of agreement between test and retest for the toe pressure following 5, 10 and 15 min of rest after a 7-day interval. All ICC values for intra-tester reliability were interpreted according to cut-offs suggested by Fleiss (10). Paired t-tests were performed for the test and retest pressures to determine whether a statistically significant difference existed between scores. The 95% limits of agreement (LOA) were calculated to determine the interval in which 95% of the differences between the measurements performed at the first and second testing session would be expected to lie (11).

Results

Seventy participants were recruited to this study. Seventy people participated in the initial assessment of pre-measurement rest time and 50 participants attended the clinic 7–10 days later for repeat testing. Participant characteristics are reported in Table I.

Table I. Participant characteristics.

Characteristic	Pre-measurement rest time	Reliability
Total participants	70	50
Age	68.12 ± 3.77 years	67.06 ± 4.16 years
Gender	39 males (56%)	26 males (52%)
	31 females	24 females
Body mass index	27.19 ± 2.63	25.26 ± 1.87
Smoking history (current and past history)	24 (34%)	18 (36%)
Diabetes	34 (49%)	20 (40%)

Effect of pre-measurement rest time

Mean toe pressures were significantly affected by duration of pre-measurement rest time (p = 0.001). Post hoc testing demonstrated mean toe pressures fell significantly between 5 and 10 min from 93.51 to 89.65 mmHg (mean change 3.86 mmHg). Between 10 and 15 min of rest, pressures stabilized with a non-significant mean reduction of 0.71 mmHg (Table II).

Table II. Effect of pre-measurement rest time on toe pressure (n = 70).

Toe pressure	n	Mean (SD)	p-value
5–10 min	70	93.51 (19.06): 89.65 (21.68)	0.001
5 and 15 min	70	93.51 (19.06): 88.74 (20.58)	0.001
10 and 15 min	70	89.45 (21.68): 88.74 (20.58)	0.35

SD, standard deviation.

Test–retest reliability

All pressure testing demonstrated excellent test– retest reliability, indicated by ICCs of 0.80 or greater (Table III). Test–retest reliability was slightly higher after 10 min of rest than after 5 min of rest (ICC = 0.86, 95% CI 0.77–0.92 and ICC = 0.80, 95% CI 0.68–0.89). Further pre-measurement rest time did not change the test–retest reliability. Paired samples t-tests demonstrated no significant differences between test and retest pressures at each time interval. The 95% LOA were narrowest for test– retest reliability after 10 min of rest (− 22.24 to 22.56) (Table III).

Table III. Test–retest reliability of systolic toe pressure measurements after 5, 10 and 15 min of pre-measurement rest (n = 50).

	Test mean and SD (mmHg)	Retest mean and SD (mmHg)	ICC	95% CI	Paired t-test, p-value	95% LOA(mmHg)
5 min	96.70 (22.51)	97.18 (23.52)	0.80	0.68–0.89	0.36	− 23.68 to 27.04
10 min	94.28 (22.04)	94.12 (21.47)	0.86	0.77–0.92	0.92	− 22.24 to 22.56
15 min	93.82 (22.91)	93.68 (22.12)	0.82	0.69–0.89	0.94	− 23.00 to 23.28

ICC, intra-class correlation coefficient; 95% CI, 95% confidence interval; 95% LOA, 95% limits of agreement.

Discussion

The results of this study support previous research demonstrating a minimum of 10 min rest is required to achieve a stable state prior to peripheral vascular assessment (12). In a community-based population, we have demonstrated systolic toe pressure falls by approximately 4 mmHg between 5 and 10 min. A pre-measurement rest period of 15 min was not associated with any further significant change in systolic toe pressures with a mean change of less than 1 mmHg. Previous research has demonstrated a small increase in systolic toe pressure between 5 and 10 min with stabilization of pressure beyond 10 min (13). The inconsistency of this finding with the results of this present study may be due to our use of a manual sphygmomanometer allowing for a lower maximum inflation (20–30 mmHg beyond a visible waveform compared with up to 300 mmHg for the automated device) and rapid release of pressure following toe pressure measurement. Use of a set maximum inflation pressure designed to be able to measure both high and low toe pressures and a more gradual release of pressure in an automated device is likely to create a longer period of arterial occlusion than produced by the manual devices used for this study. In the case of an automated device, the prolonged arterial occlusion may have resulted in measurement of post-occlusive reactive hyperaemia raising the toe systolic pressure between the 5- and 10-min time points.

Our findings are consistent with research assessing the effect of pre-measurement rest time on systolic ankle and brachial pressure, with stabilization of pressures occurring after 10 min of rest (12,14) The magnitude of the change systolic toe pressure we measured is slightly smaller than the drop in ankle pressure reported between 5 and 10 min (5.02 mmHg) (12). However, as normal systolic toe pressure measurements are approximately 75% of brachial pressures, a change of 3.86 mmHg between 5 and 10 min may represent a clinically relevant error if the pressure were to be used in a TBI calculation. This would be particularly relevant if a change of this magnitude occurred in the presence of arterial pathology where the toe pressure is low. Furthermore, it is unknown from this present study if there was an additional fall in pressure between 0 and 5 min and it is likely the overall fall is greater than 4 mmHg.

Test–retest reliability for toe systolic pressure was found to be excellent at 5, 10 and 15 min, although was slightly higher at the 10 and 15 minute time points (ICC range: 0.80–0.84). The 95% LOAs varied between 22 and 27 mmHg and are consistent with those reported in the literature previously (15). The magnitude of the LOA's represents a relatively large margin of error and suggests that the usefulness of systolic toe pressures as a screening tool for PAD may be limited. However, it is noteworthy that LOAs were found to be narrowest when the measurement was taken following 10 min of rest, suggesting ensuring pressure has stabilized contributes to a more accurate measurement.

There was no significant change in mean systolic toe pressure between test and retest at any time point, demonstrating that differences in toe systolic pressure particularly between 5 and 10 min were replicated at the second testing session. The reliability shown in this study is consistent with results of test–retest reliability of toe pressure performed with manual devices in both a diabetes cohort (15) and people with PAD (16). The results of this present study further support the use of systolic toe pressure measurements as a reliable method of vascular assessment in the clinical environment. However, slightly greater reliability may be achieved if the measurement is performed following a minimum of 10 min of rest.

Limitations

The assessment of the effect of pre-measurement rest time performed in this study assessed changes in systolic toe pressure following 5 mins of rest onwards. As a baseline measure (at 0 min) was not taken, we are unable to determine the total drop in systolic toe pressure over 15 min. It is therefore likely the overall drop in systolic toe pressure between 0 and 15 min was more or less than 3.86 mmHg, depending on the direction of change in the first 5 min. Given the wide LOAs associated with measures of systolic toe pressure, it is possible the change in systolic toe pressure between 5 and 10 min of rest may have limited clinical significance. However, our results do suggest waiting 10 min may reduce measurement error.

The results of this study may not reflect changes in systolic toe pressure occurring over similar rest periods in younger people free from risk factors for, or signs and symptoms of, PAD. The older community-based population used in this study was chosen for its clinical relevance as all participants met the criteria to undergo routine assessment for PAD. It is possible that changes in toe systolic pressures over time differ in participants without arterial pathology or of younger age.

Conclusion

Our study shows that in a community-based older population systolic toe pressures fall by approximately 4 mmHg between 5 and 10 min and remain stable beyond this time. Clinically, 10 min of pre-measurement rest time is most appropriate for taking systolic toe pressures and gives highest test–retest reliability.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.