“It’s faster” - “It’s easier” - “It’s cheaper”
Each of these statements, by vendors and the poorly informed alike, has been used to argue that skin perfusion pressure (SPP) testing eliminates any need for transcutaneous oximetry screening within the practice of hyperbaric medicine. They may indeed be considered correct if one limits any comparison to the actual data collection process. However, they in no way translate into any meaningful clinical value comparison during assessment of lower extremity wound healing deficiencies.
SPP assessment was introduced in the late 1960’s. It was initially an invasive technique involving radioisotope clearance and considered the “gold standard” at the time for predicting amputation site healing. Within two decades it had evolved to its current non-invasive form, using laser Doppler technology. SPP is defined as the minimum external pressure exerted by an inflated cuff above which skin blood flow ceases. Critically, SPP as a pressure test offers no information relating to a key hyperbaric work-up question; namely, the state of tissue/peri-lesion oxygenation, something transcutaneous oximetry uniquely does. And critical because when hyperbaric oxygen (HBO) therapy is under consideration to supplement oxygen delivery, its presumed basis is evidence of a local tissue/wound oxygen shortcoming. Blood pressure and oxygen delivery do not always correlate. This is important to emphasize because it is increasingly appreciated that limb viability is more a function of adequacy of oxygen than blood flow, per se. Local and systemic factors (calcified vessels, a VQ mismatch, pulmonary shunts, etc.) can result in a lowered blood oxygen tension during findings of seemingly normal blood pressure.
To adequately assess candidacy for HBO therapy one must go beyond identification of sub-optimal oxygenation. For a patient to be expected to benefit from a hyperbaric treatment course they must possess the physiologic capacity to respond locally (wound site) to centrally delivered/breathed oxygen. This necessitates determination of the health and perfusion status of the lungs, central and peripheral arterial vasculature. A normobaric 100% oxygen challenge identifies such capacity, or lack thereof, with further arterial workout indicated if the latter. SPP is incapable of assessing any impact of 100% oxygen inhalation.
There is hyperbaric data to indicate an anticipated healing likelihood when in-chamber peri-wound oxygen values exceed a certain threshold. This again is uniquely assessed by transcutaneous oximetry while patients are under pressure, to ensure the chamber is sufficiently pressurized. Not so SPP.
One authoritative clinical practice guideline requires a repeat transcutaneous oxygen study after 15 hyperbaric treatments to determine if patients are responding, via evidence of improved peri-wound oxygenation. This serves to avoid medically unnecessary and expensive treatments in non-responders. Any such change would be unlikely reflected on repeat SPP.
The final aspect of this screening algorithm is determination of the point of maximum hyperbaric benefit. An evidence-based, cost-effective, and responsible dosing approach is to treat only until a critical mass of angiogenesis has been induced. At this point, normal oxidative function is likely to have been restored and wound healing expected to continue spontaneously, supported by standard wound care. This contrasts with continued medically unnecessary and expensive HBO therapy until complete wound closure. Transcutaneous oximetry rather than SDPP betters guide stopping point decisions as it identifies when peri-wound oxygen levels have essentially normalized.
If hyperbaric wound referrals are to be screened and case-managed in a clinically appropriate and cost-effective manner, there needs to be:
Evidence of local hypoxia
Demonstration of its reversibility, per normobaric 100% oxygen inhalation
Confirmation of an early healing response
Identification of an anticipated therapeutic endpoint
The only technology able to address each of these steps is transcutaneous oximetry. It is by no means faster than SPP, nor is it easier to undertake or cheaper to acquire. But it does answer requisite questions that guide the appropriate use of HBO therapy in the healing deficient patient. This same value argument can be made regarding Doppler ultrasound, another non-invasive screening tool, this time limited to surrogate measurement of blood flow per circulating red blood cells.