*3.3. Intrinsic Transit Time*

The mean Intrinsic Transit Time (ITT) after flap reperfusion was 44 ± 18 s (s), ranging from 14 to 77 s. The average ITT of DIEP flaps (52 ± 18 s) was significantly higher than the average ITT of ms-TRAM flaps (33 ± 11 s) (*p* = 0.005). The average vascular resistance at the time of ITT measurements was 9 ± 4.7 mmHg/mL/min. There was a significant negative correlation between the arterial blood flow and the ITT after anastomosis (*p* = 0.001)

(Figure 5). By contrast, a significant positive correlation was seen between the arterial vascular resistance (aVR) and the ITT after anastomosis (*p* = 0.0006) (Figure 6). There was no correlation between the ITT and flap ischemia time, flap weight, or mean arterial pressure (MAP).

**Figure 5.** Arterial blood flow (mL/min) versus Intrinsic Transit Time (ITT, seconds); y = −0.182x + 19.33; *p* = 0.001; r2 = 0.3875; red dots = DIEP flaps; blue dots = ms-TRAM flaps.

**Figure 6.** Arterial Vascular Resistance (aVR, mmHg/mL/min) versus Intrinsic Transit Time (ITT, seconds); y = 0.1744x + 1.308; *p* = 0.0006; r2 = 0.4196; red dots = DIEP flaps; blue dots = ms-TRAM flaps.

Of all included free flaps, one DIEP flap required surgical revision due to a thrombotic event occurring on the fourth day after autologous breast reconstruction. The ITT of this flap was 77 s. After emergency thrombectomy, no further complication occurred. The overall flap survival rate was 100%.

#### **4. Discussion**

Numerous recently developed technologies enable the illustration and measurement of vascularity and perfusion in free flaps at a pre-, intra, or postoperative stage, with the ultimate goal to increase their safety and efficacy [7,11,12,18,33–36]. The combination of Transit-Time Flowmetry (TTFM) and microvascular Indocyanine Green Angiography (mICG-A) is

considered a unique approach aiming to meticulously evaluate and compare the intraoperative blood flow characteristics of DIEP and ms-TRAM flaps. A recent study successfully established this combination for the detection of early venous congestion in an animal flap model [37]. However, no study so far assessed the combined potential of these techniques in a clinical setting for autologous breast reconstructions. Our results show that the overall arterial blood flow of both DIEP and ms-TRAM flaps did not significantly increase after anastomosis with the recipient internal mammary vessel. The blood flow of the intact recipient artery did not influence the arterial blood flow of the included flaps. In fact, it seemed to be the opposite. In this study, both DIEP and ms-TRAM flaps downregulated the recipient artery flow towards blood flow values of the in situ flap prior to tissue transfer. These observations are supported by other studies showing that the flow of the recipient artery can either be down- or upregulated after flap anastomosis, approximating blood flow values of the flap isolated on its pedicle before tissue transfer [19,38–40]. Lorenzetti et al. measured the blood flow of the thoracodorsal artery before and after anastomosis with ms-TRAM flaps and reported an upregulation of the recipient artery. Before anastomosis, the thoracodorsal artery had relatively low blood flow values (4.9 ± 3 mL/min) in situ. However, after anastomosis with the ms-TRAM flap, the blood flow increased (13.7 ± 5 mL/min) towards the original blood flow rate of the isolated flap pedicle in situ before tissue transfer [38]. This phenomenon was observed not only in fasciocutaneous but also in musculocutaneous and muscle free flaps and therefore seems to be irrespective of the tissue composition [40]. Previous studies reported generally different blood flow rates and vascular resistances in fasciocutaneous, musculocutaneous, muscle, and intraperitoneal flaps [31,38]. These findings support the notion that both blood flow and vascular resistance depend on the type of tissue and its relative proportion. The tissue composition determines the vascularity of each flap, which at the same time, reflects the vascular resistance. Free flaps mainly composed of muscle tissue contain a rich vascular network connected by resistance vessels, resulting in a lower vascular resistance than fasciocutaneous flaps with a rather sparse network of much smaller vessels [30,31]. In our study, ms-TRAM flaps had an average arterial blood flow of 13.5 mL/min and a vascular resistance of 6.4 mmHg/mL/min after anastomosis. By contrast, DIEP flaps showed significantly lower blood flow values and consequently a higher vascular resistance. Although both flaps were, apart from a small segment of the rectus abdominis muscle in ms-TRAM flaps, grossly composed of the same tissue, the difference in vascular resistance seemed to be a matter of vascularity. We theorize that a larger number of perforators in ms-TRAM flaps was the main reason for a lower vascular resistance, hence providing a higher overall and weight-adjusted arterial blood flow in comparison to DIEP flaps. The overall arterial inflow of the included flaps was about 1.4 to 1.8 times greater than the venous outflow. Selber et al. reported similar results in ms-TRAM and other fasciocutaneous flaps [41]. We theorize that the peripheral leakage of small blood vessels at the flap edges caused the disparity between arterial inflow and venous outflow. An A/V ratio of more than 1 seems to a certain level inevitable and needs to be considered by surgeons during free flap flow measurements.

Microvascular Indocyanine Green Angiography (mICG-A) combined with the microscopeintegrated software FLOW800 provides valuable information on vascular patency and enables the real-time visualization of arterial in- and venous outflow in free flaps [42–44]. It is a matter of common pathophysiological knowledge that the alteration in blood flow, as part of the Virchow's triad, is a main contributor to thrombus formation in blood vessels [45,46]. Previous studies have already theorized that a prolonged ITT might be an indicator of low blood flow velocities, hence accounting for increased vascular resistances [27]. The combination of TTFM with mICG-A enabled to measure and detect a positive correlation of ITT with vascular resistance in free flaps. ITT, similar to blood flow, seems to depend on the flap tissue composition. In our study, the DIEP flap, which was composed of fasciocutaneous tissue, had a significantly higher average ITT (52 s) than the ms-TRAM flap (33 s), classified as musculocutaneous flap. These observations are supported by a previous study measuring a shorter ITT in muscle flaps (27.7 s) than in fasciocutaneous flaps (47.5 s) [26]. Holm et al. reported that an ITT of more than 50 s was associated with an increased risk for vascular compromise and surgical revision in free

tissue transfer procedures [27]. The study, however, showed essential methodological flaws such as a heterogenous study population with varying free flap entities. In our study, eight flaps surpassed the threshold of 50 s without any hemodynamic postoperative complication. Only one DIEP flap with an ITT of 77 s required surgical revision due to a thrombotic event several days after flap transplantation. Although this was the highest ITT of all included flaps, the scarce occurrence of just a single hemodynamic complication several days after autologous breast reconstruction did not allow drawing any correlation between a prolonged ITT and the increased risk of postoperative hemodynamic complications. To the best of our knowledge, this is the first study that measures, compares, and detects hemodynamic differences between DIEP and ms-TRAM flaps. The clinical relevance of this study is the establishment of standard values of intraoperative hemodynamic and perfusion properties of DIEP and ms-TRAM flaps. We could detect significant differences in hemodynamics properties between DIEP and ms-TRAM flaps. Flaps with abnormally high or low blood flow values, according to our newly established standard hemodynamic characteristics, made a closer intraoperative assessment of anastomotic patency necessary. We are aware that these techniques do not replace a clinical examination but rather help to improve our intraoperative decision-making process. The results of this study, however, do not provide any recommendation in terms of favoring one or the other free flap type for autologous breast reconstruction. Historically, the choice between DIEP and ms-TRAM flap has been a far more extensive topic that needs to take numerous other variables into account. Our meticulous assessment of arterial and venous blood flow, arterial vascular resistance, and ITT at crucial intraoperative time points enables the establishment of normative values. This should help to assess vascular patency especially in cases where the surgeon or devices such as a regular hand-held Doppler fail to detect a more subtle vascular compromise.

#### **5. Conclusions**

In this study, we evaluated the hemodynamic characteristics of free DIEP and ms-TRAM flaps. The combination of Transit-Time Flowmetry and microvascular Indocyanine Green Angiography enabled the qualitative and quantitative intraoperative assessment of anastomotic patency. Our study serves as fundamental work for the determination of predictive values for postoperative thrombotic events and of cut-off values that will ease intraoperative decision making in the future.

**Author Contributions:** Conceptualization: A.G., R.E.H. and A.A.; Investigation: A.G., R.E.H., I.L. and A.A.; Methodology: A.G., I.L. and A.A.; Project administration: A.G., R.E.H. and A.A.; Resources: R.E.H.; Supervision: R.E.H. and A.A.; Validation: A.G. and A.A.; Visualization: A.G.; Writing original draft: A.G.; Writing—review & editing: A.G., R.E.H., I.L. and A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** We acknowledge financial support by Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität Erlangen-Nürnberg within the funding programme "Open Access Publication Funding".

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of Friedrich-Alexander-University of Erlangen-Nuremberg, Germany (registration number: 447\_19B).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The datasets generated during the current study are available from the corresponding author on reasonable request.

**Acknowledgments:** The present work was performed in fulfillment of the requirements for obtaining the degree "Dr. med." for the author A.G.

**Conflicts of Interest:** The authors declare no conflict of interest.
