One commonly used tool to evaluate mastectomy flap viability intra-operatively is the intravenous sodium fluorescein test (Wood's lamp method). This involves intravenous injection of fluorescein followed by intra-operative evaluation with a Wood's lamp. Although it has been available since 1931, its application is prone to subjective errors, and is limited to over/under reading by as much as 30% . It is also a test of vascularity - not viability, and subject to changes in vascularity such as vasospasm, intravascular clotting, or alterations in the distribution of the microcirculation. Alternatively the use of infrared spectroscopy takes into account metabolic changes of the dissected tissue, and potentially allows trends to be followed for flap evaluation post-operatively.
The arterial supply of the breast is generally defined as an anastomotic plexus of vessels originating from the axillary artery, the internal mammary artery, the intercostal arteries, and lateral thoracic artery. The contribution of each individual artery and the consequences of vascular interruption are poorly understood, but the course of the nerves and vessels may be related to the ligamentous apparatus . One such horizontal ligamentous suspension originates from the pectoral fascia along the 5th rib . Our finding that the decrease in perfusion from the inferior portion of the breast most accurately predicted post-operative epidermolysis may be supportive of this finding.
In addition, there currently does not exist any standardized method for measuring mastectomy skin flap thickness during an operation, further refinements in this technique-i.e. the use of calipers, may be helpful for future trials.
Traditionally, surgeons are careful to avoid transection of medial perforators. Consistent with this, our data demonstrate an increased likelihood of necrosis in the patient who had a significant decrease in medial StO2 measurements. This may be particularly important in those patients who undergo disruption of the medial perforators secondary to internal mammary node dissection.
There are significant limitations to this study. Most notable is the small sample size. Contributions from underlying co-morbidities (coronary artery disease, diabetes) may be more readily apparent with a larger sample size. In addition, this study population was predominately a minority population; there is an under-representation of Caucasian patients. Although the ViOptix T.Ox Tissue Oximeter system has been validated in several racial groups, there may be variability in StO2 measurements between races which can only be further elucidated with a large sample size. For further studies, assuming a 10% necrosis rate, a sample of 40 patients will provide more than 90% power to detect a two standard deviation difference of the mean StO2 measures (significance level is held at 0.05, two sided). Clearly a group of patients undergoing skin-sparing mastectomy with immediate reconstruction would provide the most useful clinical information as these patients are more likely to have difficulties with wound healing and face the greatest consequences (implant extrusion, flap failure) from poor wound healing.
It is known that the perfusion to the subdermal plexus of the skin is controlled by the autonomic nervous system in response to variations in metabolic demands and environment. All patients in this study were stable intra-operatively. However, the actual oxygen saturation and blood pressure measurements at the time of StO2 measurement were not evaluated, the influence of these factors will be examined in future studies. The patient with necrosis had drops in StO2 measurement, which also may be an indicator of failure to compensate for injury, whereas the patients who did not have necrosis, for the most part, had increased StO2 levels after dissection, potentially indicating an ability to increase perfusion appropriately to the area of injury.
Similarly, wound healing is a complicated process. Factors contributing to or complicating the wound healing process include body habitus, age, co-morbidities, prolonged operative time, collagen disorders, infection, history of radiation exposure, immune status, and steroid use [13–15].
Lastly, a review of the patient response to the ViOptix T.Ox Tissue Oximeter system indicates that the patient having necrosis also had a longer flap length. This would appear to be consistent with the concept that the blood supply of longer flaps is more tenuous, likely due to the greater area of vascular disruption required when a mastectomy is performed.