Non- invasive vascular testing Pros and Cons of ABIs and Alternative Physiologic Assessments
Non- Invasive Physiologic Arterial Studies Segmental Systolic Pressure Measurements ABIs, TBIs, and full segmentals Doppler Wave Form Evaluation Transcutaneous Oxygen Tension (TcpO2) Pulse Volume Recording Laser Doppler Flowmetry
Cuff deflation slowly while listening for the return of the doppler signal defines the systolic perfusion pressure at the level of the cuff.
Segmental Systolic Pressure Measurements Can be measured at multiple levels of an extremity High thigh, low thigh, calf, and ankle Can define level of occlusive disease Ankle- brachial index (ABI) Most common test done at a single level Ankle systolic pressure/arm systolic pressure >0.90 Normal 0.60 to 0.90 Claudication 0.40 to 0.60 Severe Claudication/Rest pain <0.40 - Rest pain/tissue loss
Segmental Systolic pressures ABI Inaccurate in diabetics, and renal failure patients calcified vessels prevent compression of the vessel The cuff occlusion pressure is more dependent on the artery wall stiffness than on the pressure of the blood in the vessel Toe pressure measurements or TBIs are not affected by vessel calcification
Toe Pressures and TBIs Uses a photo phlethysmograph (ppg) sensor to detect pulsatile perfusion of the toe A small cuff on the toe is inflated to the point of compression that stops flow The pressure at which toe pulsations return in the toe pressure
Toe Pressures and TBIs TBI is the toe pressure/brachial pressure TBI is typically 0.20 lower than the corresponding ABI in an extremity without calcified vessels Toe pressures and healing > 60 mm Hg - adequate for healing 40-60 mm Hg - marginal for healing < 40 mm Hg - inadequate for healing Main limitation is when toe is not available
Doppler Waveform Analysis
Doppler Waveform Analysis Less dependent on calcification Severe calcification can distort the waveform and cause loss of the dicrotic notch Is operator/technique dependent Angle of the Doppler probe can increase or decrease the waveform Waveform peak/size can be altered by changing the gain setting Pt movement can interfere with accuracy Interpretation of morphology very variable
Transcutaneous Oximetry Mechanism
Transcutaneous Oximetry Mechanism of Action Clark polarographic electrode heats the underlying tissue to create a local hyperemia Heat also softens cells in the epidermal layer making the skin permeable to gas diffusion Local Oxygen tension measured at the skin surface > 40 mm Hg - perfusion adequate for healing 20-40 mm Hg - indeterminate healing < 20 mm Hg - non healing anticipated
Transcutaneous Oxygen Measurement Reflects metabolic state of the skin Allows for assessment of lower extremity arterial perfusion as long as other factors that impact the metabolic state of the skin are in steady state Temperature, edema, infection Dermal vasomotor status Smoking Medications
Pulse Volume Recordings - PVRs Uses air- filled cuffs that are inflated to 65mm Hg Represents arterial inflow during systole Inflated to 35-40mm Hg for digits
PVRs More sensitive than segmental pressures Able to detect mild disease Less operator dependent Not affected by calcified vessels, obesity or edema Qualitative and Consistent Useful in sequential studies of a single patient Not Quantitative
Laser Doppler Skin Perfusion Pressures (SSP) Quantitative evaluation of microcirculatory perfusion in the skin Measured using a laser Doppler sensor and an occlusive pressure cuff
Laser Doppler Skin Perfusion Pressures (SSP) Laser of 785 nm wavelength penetrates the skin up to 1.5 mm Laser light is either Doppler shifted by moving red cells or is reflected back to the sensor After decompression of an occlusive cuff on the foot the laser detects a shift or decrease in the returned light to the sensor as blood returns to the skin
Laser Doppler SSP The pressure at which the shift or change in the light reflect to the sensor occurs is the systolic perfusion pressure of the skin at that location Additional information observable from the graph includes percentage perfusion increase above baseline, total response time, perfusion reappearance time, and perfusion contour
Summary ABIs are very useful screening tool in patients without calcified tibial vessels but should be viewed with caution in diabetics and renal failure patients Toe pressures are essential adjunct to ABIs in calcified vessel patients Waveforms and PVRs are qualitative not quantitative Transcutaneous oxygen measurements are affected by many different factors Laser Doppler is an effective measurement of skin perfusion equivalent to toe pressures in predicting healing of foot/toe wounds