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Ali et al. Plast Aesthet Res 2021;8:35 https://dx.doi.org/10.20517/2347-9264.2021.29 Page 11 of 15

flaps or flaps with a small cutaneous component.

With the head and neck FTT practice at our institution, given the number of free flaps that are mucosal,
that have small monitoring paddles, or that are entirely buried, we have not found it practical to invest in
this technology. Its application, in our estimation, is most useful for flaps with large cutaneous surfaces that
are easily accessible.

Laser doppler flowmetry
Laser doppler flowmetry (LDF) is a non-invasive technology that allows for continuous monitoring of
vascular perfusion. Briefly, the device consists of a specialized fiber optic probe that is secured to the
cutaneous surface of a FTT for the purpose of measuring microcirculatory changes via an induced doppler
[66]
shift . Centers that utilize LDF typically describe a 48 to 72 h monitoring window where nursing staff or
residents record flow values and notify the microvascular surgeons if there is a predetermined decrease in
[67]
flow over a particular period of time . Studies have determined that the flowmeter is able to output value
changes indicative of flap compromise 1 to 3 h ahead of a change in the clinical appearance of the flap,
[68]
thereby decreasing the time to revision . They additionally report on a low or non-existent false positive
rate [67,68] . Proponents of the LDF system cite its reliability, accuracy, and negligible overall false-positive and
false-negative rates . Disadvantages of the system include complexities in data interpretation, particularly
[67]
between arterial and venous thromboses, technical issues if affixed to a dynamic free flap, inability to
monitor buried flaps, and overall cost of adopting the system . At our institution, we have not utilized LDF
[69]
as an adjunct in free tissue monitoring.

Digital infrared surface thermometer monitoring
Gross temperature assessment via palpation has long been a principle of overall clinical flap monitoring.
Early proponents attempted to assess for temperature change via surface temperature monitoring.
Kannan were able to demonstrate a positive correlation between flap microcirculation and flap surface
[70]
temperature, although this was less reliable for venous thrombosis in comparison to arterial thrombosis. A
study by Khouri and Shaw demonstrated a sensitivity of 98% and a predictive value of 75% in identifying
[71]
a microvascular thrombosis. While surface temperature recordings are naturally affected by normal skin
temperature fluctuations, a previous case series by Baudet et al. noted that an acute drop in temperature
[72]
of 3 °C or more was found to be correlated with arterial compromise. With advances in technology, some
investigators who rely on temperature changes as a means for flap monitoring have begun utilizing non-
contact infrared thermometers in place of surface contact thermometers. A retrospective review of free flap
failures by Papillion et al. demonstrated a temperature difference between flaps that survived and those
[73]
that were ultimately unable to be salvaged. Specifically, the authors found a temperature difference of 2.6 to
5.4 °C between the FTT tissue and adjacent skin in the failing flap group . Additional studies have been
[73]
further able to correlate alterations in flap temperature obtained via a digital infrared thermometer to
[74]
changes in capillary blood flow and hemoglobin oxygenation . Clear advantages of this adjunct include
ease of use, lack of reliance on contact technology that could malfunction or become dislodged, relative
accuracy, and low overall adoption cost. Potential disadvantages include lack of standardized alarm
thresholds, a difficulty in accounting for natural surface temperature fluctuations or those fluctuations
associated with a systemic process, and that temperature change may potentially be a late finding in the
overall picture of flap compromise . At our institution, our residents and nursing staff are taught to
[73]
evaluate for gross temperature changes as part of their clinical assessment, however we do not formally
utilize surface contact thermometry or infrared thermometry as part of our adjunctive monitoring
measures.