Calf or foot blood flow can be recorded with a mercury-in-Silastic strain gauge. The electrical resistance of the mercury column depends on the length of the gauge, and when the gauge is wrapped around the limb, small changes in gauge length are proportional to changes in the volume of the extremity. However, there are typically no significant differences in resting calf or foot blood flow between normal subjects and patients with intermittent claudication, even when the symptoms are severe.¹ Hyperemic volume flow is often lower in limbs of patients with arterial occlusive disease, but reactive hyperemia testing can be quite painful for the patient.² Therefore, measurements of volume flow are not very useful in the clinical evaluation of lower extremity ischemia.

Air plethysmography can be used to generate pulse volume waveforms, a technique referred to as pulse volume recording (PVR).³ PVR waveforms are obtained with partially inflated air-filled cuffs that detect volume changes sequentially down a limb. The arterial pulse wave causes volume changes in the limb and produces small pressure changes within the cuffs; these changes are displayed as waveforms with the use of appropriate transducers. A normal pulse volume waveform has a sharp systolic upstroke, a narrow peak, and a downstroke that bows toward the baseline and contains a prominent dicrotic notch (Fig. 13.1A). As proximal arterial occlusive disease progresses, the systolic upstroke is delayed, the peak becomes rounded, and the dicrotic notch is lost as the downstroke bows away from the baseline (Fig. 13.1B). With more severe disease, the pulse volume waveform decreases in amplitude and the upstroke and down-stroke times become nearly equal (Fig. 13.1C and D). With very severe proximal disease, the pulse volume waveform is absent.⁴ Evaluation of pulse volume waveforms is qualitative and based on the shape of the curve, the presence or absence of the dicrotic notch, and the amplitude.⁵ Lack of more quantitative data limits the utility of PVR. An example of normal and abnormal lower extremity segmental PVR examinations is shown in Figure 13.2.

One of the best current applications for plethysmography is in the evaluation of digit blood flow. Air plethysmography, strain-gauge plethysmography, and PPG can all be adapted for this purpose; however, PPG is probably the most widely used technique (Fig. 13.3). It should be noted that the PPG does not record a true volume change and therefore it is not, strictly speaking, a plethysmograph. It consists of an infrared light-emitting diode and a photosensor that detects the light reflected from the tissue. Because red light is attenuated in proportion to the blood content of tissue, the pulse waveform produced by the photosensor has the same features as the true plethysmographic methods. This technique is easily used in combination with pneumatic cuffs to measure digit systolic pressures, as discussed in Chapter 14 for the upper extremity. Digit pressures, as well as comparisons between digit pressures and brachial pressures, are utilized in clinical practice. The toe-to-brachial artery systolic pressure ratio (toe-brachial index or TBI) in normal subjects is greater than 0.70. An absolute digit pressure of less than 50 mm Hg indicates critical ischemia.

PPG-derived digit pressures are particularly useful in patients with highly calcified, noncompressible tibial arteries at the ankle. Digital arteries are less subject to calcification, and in the setting of calcified ankle arteries, toe pressures are a better indicator of the degree of distal ischemia than measurement of ankle pressures. The presence of normal digit PPG waveforms in patients with calcified proximal arteries indicates minimal restriction to blood flow despite the calcific arterial disease. Conversely, an obstructive digit waveform in the presence of normal ankle arterial pulses is frequently an indicator of pedal artery occlusive disease, a situation encountered in many patients with diabetes and those with atheroembolism or connective tissue disorders.

A simple continuous-wave Doppler flow detector can be used along with a set of pneumatic cuffs to measure systolic blood pressures in the lower limb. The site of pressure measurement is determined by cuff placement, and any arterial Doppler signal distal to the cuff can be used. The ratio of ankle systolic pressure to brachial systolic pressure is a very useful measure of overall lower extremity perfusion. This test is performed with the patient supine and after resting for a few minutes to make sure that limb blood flow is at baseline levels. A pneumatic cuff is placed just above the ankle, and the continuous-wave Doppler probe is positioned over the posterior tibial or dorsalis pedis artery distal to the cuff to monitor the flow signal. The cuff is then inflated to suprasystolic levels until the Doppler signal disappears and slowly deflated; the pressure in the cuff when the Doppler signal returns is recorded as the systolic pressure. This sequence is repeated with the other artery at that ankle, and the values compared with the highest brachial artery systolic pressure, also
obtained with the Doppler. For clinical purposes, the higher ipsilateral dorsalis pedis or posterior tibial artery pressure is divided by the higher brachial artery systolic pressure, yielding the ankle-brachial index (ABI) for that lower extremity. The use of a ratio makes the test relatively independent of day-to-day variations in systemic arterial blood pressure and facilitates serial follow-up and comparisons between patients. As discussed in Chapter 5, a normal ABI is in the range of 1.0 to 1.2, with progressively lower values corresponding to worsening arterial occlusive disease (Table 13.1).⁶ A decreased ABI also correlates with an increased risk of cardiovascular death, with the risk increasing as the ABI falls, as discussed in Chapter 26.