Revisiting Recipient Vessel Physiology: Intraoperative Evidence of Flow Coupling in Free Flaps

Background: The role of the recipient artery in free flap hemodynamics remains debated. Prior work has supported both a coupled system, in which recipient vessel inflow influences flap pedicle flow, and a decoupled system, in which pedicle flow is determined primarily by intrinsic flap resistance. This study aimed to characterize the relationship between recipient artery flow and pedicle arterial flow using intraoperative transit-time flowmetry, and to evaluate waveform propagation using harmonic analysis. Methods: A retrospective cohort of patients undergoing deep inferior epigastric perforator flap reconstruction was identified. Time-resolved flow and pulsatility index (PI) were obtained intraoperatively in recipient and pedicle arteries. A short-time Fourier transform was applied to the flow signal to quantify harmonic power and derive harmonic distortion indices. Results: Thirty flaps in 21 patients were identified. Mean arterial flow was significantly higher in recipient vessels than in flap pedicles (31.5 vs. 9.5 mL/min; P<0.001), while PI did not differ significantly (P=0.64). Recipient artery flow demonstrated a moderate positive correlation with pedicle flow (r=0.46, P=0.01). PI was also positively correlated between recipient and pedicle vessels (r=0.37, P=0.047). In contrast, harmonic distortion metrics did not demonstrate significant associations between recipient and pedicle arteries. Conclusions: Recipient artery inflow is moderately associated with flap pedicle hemodynamics, supporting a partially coupled vascular system rather than complete independence. Pulsatile characteristics appear to propagate across the anastomosis, whereas waveform distortion reflects properties of the distal flap microcirculation. These findings suggest that recipient vessel physiology contributes to intraoperative flow dynamics and that waveform-based metrics may provide complementary insight into flap-specific hemodynamic features.