How temperature affects toe-brachial index measurements: the role of toe warming and environmental conditions

Accurate measurement of the toe-brachial index (TBI) is crucial for diagnosing peripheral arterial disease (PAD), especially in patients where ankle-brachial (ABI) measurements are inconclusive due to factors such as arterial calcification, non-compressible vessels, or anatomical variations. TBI shows better sensitivity in these “challenging” populations but is technically more challenging: emerging research highlights that toe temperature, affected by both environmental conditions and localized warming, significantly influences the reliability of these measurements.

Why temperature matters in TBI assessments

The TBI is calculated by dividing the systolic pressure at the toe by the systolic pressure at the arm. Since toes are more susceptible to temperature-induced vasoconstriction or dilation than arms, environmental or physiological changes in temperature can skew results.

A study published in Circulation [1] found that mean toe pressures at 10°C were significantly lower than those at 30°C or in routine conditions. Pressures increased progressively with temperatures from 27°C to 39°C, plateauing around 36°C to 39°C. This suggests that low toe temperatures can lead to falsely low toe systolic pressures readings, emphasizing the need for warming to achieve accurate assessments.

Evidence from clinical studies

1. Heating the toes: enhancing accuracy

A 2017 study [2]  about the influence of heating on toe pressures examined 46 patients with lower limb arterial disease (86 legs) with a laser Doppler device and found that heating the toes significantly increased toe systolic pressures when they heated up the toes to 40°C. During the baseline measurement the baseline mean toe systolic pressure was 58.5 mmHg and the toe skin temperature 24°C (standard deviation: 2.8). The mean room temperature during the measurement was 22.7°C.

They then heated up their toes for 5 minutes. The toe temperature after heating up the toes was 27.8°C (standard deviation: 2.8). The mean toe systolic pressure measurement rose to 62 mmHg. But when the toes were heated to 40 °C, the change to toe pressure rose to 71 mmHg.

Individual variation in differences was also observed. Variations between responses to the heating were present after the first heating and were greater from −34 mmHg (TP decreased from 74 to 40 mmHg) to +91 mmHg (toe pressure increased from 14 to 104 mmHg). After the first heating, the toe pressure decreased or remained the same in the second measurement in a total of 40 (46.5%) legs.

But, when the toes were heated to 40 °C, the change in toe pressure from the baseline varied between −28 and +103 mmHg. After the second heating, the toe pressure was unchanged or worse in 22 (25.6%) patients, and the proportion was the same in diabetics versus nondiabetics (25.6% vs 25.5%, respectively) and in symptomatic versus asymptomatic legs (20.0% in asymptomatic legs, 20.9% in claudicants, and 33.3% in CLI legs).

Data from this study indicates that there is a different response to heating in different clinical situations and in patients with different comorbidities. These results also indicate that warming the toes can enhance the accuracy of toe systolic pressure measurements by mitigating vasoconstriction effects. And that over-heating of the toes can falsely elevate toe pressure results.

Therefore, toe pressure measurements are a part of a holistic assessment: only the toe pressure or TBI result alone cannot give us a reliable overview of the patient’s condition.

2. Cooling of the toes: a risk of underestimation

Research [3] has shown that toe cooling significantly reduces systolic pressures in healthy patients. They have compared results with patients with Raynaud’s syndrome. The results showed that in patients with Raynaud's syndrome, body cooling led to a mean toe pressure to decrease of 58 mmHg, while the healthy patients in the control group had a drop of 24 mmHg. Applying local cooling of the toes from 30°C to 10°C  resulted in over a 40 mmHg decrease in both groups.

These findings underscore the importance of maintaining adequate toe temperature during assessments to avoid underestimating toe systolic pressures.

Additionally, a study [4] where they researched the effect of body heating and cooling on the ankle and toe systolic pressures in arterial disease, they found that very low toe temperatures,whether from local cold exposure or reduced blood flow due to systemic cooling,led to markedly reduced or even unmeasurable toe systolic pressures. The likely cause is increased vascular tone or altered arterial wall elasticity at low temperatures. Notably, limbs with the lowest toe temperatures during cooling had the lowest pressures. This temperature sensitivity introduces significant variability in toe systolic pressure measurements, exceeding normal reproducibility limits and emphasizing the need to control or normalize toe temperature for accurate vascular assessments.

3. Foot thermoregulation and second toe measurements

A study [5] on thermographic patterns of the upper and lower limbs revealed that the hallux (big toe) is consistently warmer than the other toes, with a progressive decrease in temperature from the second to the fourth toes. This pattern is attributed to the vascular anatomy of the foot.

But if a patient has an amputated big or first toe, then we can measure the systolic toe pressure also on the second toe. When researchers studied if the second toe systolic pressure measurement is a valid substitute for first toe systolic pressure measurements in diabetic patients, they found that second toe systolic pressure measurements are interchangeable with those of the first toe. Second toe pressures can be used in diabetic patients whose first toe pressures cannot be assessed. [6]

4. Reproducibility of noninvasive vascular measurements of the peripheral circulation

Studies investigating the reproducibility of noninvasive vascular measurements in the peripheral circulation[7] have shown that ankle-brachial index measurements are highly reliable. These studies also confirmed that a change in the ankle-brachial pressure index of more than 14% typically indicates a clinically significant pathological change.

At the same time, the research has also evaluated the reproducibility of toe pressure measurements [8] and found that the variability of the TBI is greater, reported anywhere from the same as the ABI to about twice that of the ABI. The higher variability of the TBI may be due to higher susceptibility of toe arteries to vasoreactivity.

To put this into perspective, several factors can influence the accuracy of toe pressure readings. These include not only toe and ambient temperatures, and the specific location of measurement, but also who is performing the measurement. Differences in technique between healthcare providers can significantly affect the results.

In conclusion, toe pressure measurement is a highly sensitive procedure, affected by multiple variables. For results to be consistently reliable, a holistic and standardized approach must be adopted, ideally under carefully controlled conditions.

Measurement protocol

To conclude this overview, the need for a standardized and well-established protocol for measuring toe systolic pressures is clear. This necessity is further emphasized in a recent study [9], where the authors highlighted the importance of using a consistent methodology to ensure both accuracy and reproducibility. The paper outlines key procedural steps and clinical considerations, reinforcing that a systematic approach is essential for obtaining reliable and meaningful results.

1. Test conditions

To ensure accurate and reproducible measurements, several environmental and physiological factors must be controlled:

Room temperature: Maintain a consistent room temperature between 22–24°C. Cold environments can induce vasoconstriction, which may artificially lower TSP readings.

Patient positioning: The patient should lie supine for a few minutes before the test begins and remain in this position throughout the procedure. This helps stabilize blood pressure and ensures that the toes are at the heart level.

Pre-test restrictions: Patients should avoid smoking, caffeine, and exercise for at least 2 hours before testing, as these factors can affect vascular tone and blood flow.

Minimize interference:

• Reduce limb tremors, sudden movements, and hyperaemia, as these can disrupt the photoplethysmography (PPG) signal.

• Be mindful of underlying vaso-neural conditions (e.g., Raynaud’s phenomenon) that may impact TSP due to altered vascular function.

2. Required equipment

PPG device: This is the most commonly used modality and operates using infrared light to detect blood volume changes, producing a readable waveform.

Cuffs: Appropriate cuff sizing is essential. Incorrectly sized cuffs can produce falsely high or low readings. Cuff selection should correspond to approximately 1.5 times the diameter of the digit being assessed. MESI mTABLET TBI cuffs feature the size indicator and the OK region to help determine the appropriate cuff size for the selected toe.

3. Measurement technique

Hygiene and preparation: Follow standard hygiene protocols. Wearing gloves is recommended, particularly when working with patients who have open wounds.

Cuff placement:

• Place the cuff around the base of the hallux, ensuring it is secure but not exerting pressure before inflation.

• Position it firmly enough to remain in place during the measurement.

PPG probe placement:

• Attach the probe to the distal pulp of the hallux so it is flush with the skin, preventing ambient light interference.

• Secure the probe using hypoallergenic tape, and ensure leads are fixed to prevent movement during the test.

Alternative sites:

• If the hallux is unavailable, the second toe may be used with similar reliability.

• Lesser digits are generally not recommended due to the difficulty in achieving consistent placement and limited validation.

• In such cases, consider alternative vascular assessments (e.g., ankle pressures or continuous wave Doppler) or refer for imaging or specialist vascular consultation.

Signal acquisition:

• Wait for a strong, cyclical waveform to appear on the monitor before proceeding. The waveform can be interpreted qualitatively; dampened, delayed, or diminished signals may indicate increasing severity of peripheral arterial disease (PAD).

• Inflate the cuff gradually. Avoid inflating above 250 mmHg to prevent cuff damage—waveforms typically flatten between 150–200 mmHg but may do so at lower pressures in severely diseased limbs.

• Once the waveform disappears, inflate an additional 20 mmHg.

• Begin deflation slowly at a rate of 2–5 mmHg per second.

Identifying TSP:

• The first small, regular cyclical waveform (blip) seen during deflation indicates that arterial pressure has exceeded cuff pressure—this is recorded as the TSP.

• Note: This initial waveform may be small and easily missed by inexperienced clinicians.

Repeat measurements:

• If the measurement must be repeated, wait at least 3 minutes before re-testing to avoid a hyperaemic response, which can temporarily elevate TSP.

While measuring toe pressures is an important component of vascular assessment, it should be integrated into a comprehensive evaluation of the limb. Ideally, foot wound and vascular care are delivered within a multidisciplinary limb salvage team, where assessments such as TSP and TBI are part of a coordinated, holistic approach to diagnosis and management. However, such integrated care is not always feasible in all clinical settings, highlighting the importance of applying best-practice protocols even in more limited environments.