*2.4. Vascular Resistance*

Arterial vascular Resistance (*aVR*) was calculated as millimeters of mercury per milliliter per minute (mmHg/mL/min) based on a previously reported method in adherence to the Poiseuille's Law, using the following formula: [28,29]

$$R = \Delta P/Q$$

*R* = *aVR* = arterial vascular resistance, ∆*P* = blood pressure gradient = mean arterial blood pressure (*MAP*)–mean venous blood pressure (MVP). The mean venous blood pressure (MVP) is estimated to be close to 0 and, as a result, disregarded in the calculation of vascular resistances. This results in the following formula: [28–31]

$$aVR = MAP/aBF$$

#### *2.5. Data Analysis and Synthesis*

Descriptive analysis was performed for patient demographics. Data are shown as mean ± standard deviation. Changes in blood flow and vascular resistance between the different time points within one group were calculated using the paired Student's *t*-test. Blood flow and vascular resistance between DIEP and ms-TRAM flaps at different time points was analyzed using an unpaired Student's *t*-test. Nonparametric data were analyzed with the Wilcoxon matched-pairs rank test within one group, whereas the Mann–Whitney U test was used for analyses of nonparametric data between DIEP and ms-TRAM flaps. The correlation of data assuming Gaussian distribution was calculated using the Pearson correlation coefficient. The Spearman's rank correlation coefficient was used for data not passing a test for normality. The significance level was set at *p* < 0.05. Three outliers (one arterial and two venous blood flow values at the recipient site (Measurement *R*) before anastomosis) were identified using the ROUT method (*Q* = 1%) and appropriately excluded from statistical analysis. Statistical analyses and graphic illustrations were performed using GraphPad Prism (GraphPad Software, Inc., San Diego, CA, USA).

### **3. Results**

A total of 20 female patients receiving 24 DIEP or ms-TRAM flaps for breast reconstruction were included in this prospective study. Patients' average age was 52 years, ranging from 39 to 68 years. Fourteen flaps were harvested as DIEP (57%), and 10 as ms-TRAM (43%). Seven ms-TRAM flaps were classified as ms1-TRAM flap, and three as ms2-TRAM flap, according to the classification by Nahabedian et al. (Table 1) [32]. The median flap weight was 435 g, ranging from 299 to 1169 g. The median weight of DIEP flaps (390 g) was not significantly different from that of ms-TRAM flaps (491 g). The average flap ischemia time was 46 min. The venous coupler size ranged from 2.5 to 3.5 mm.

#### *3.1. Blood Flow Volume (mL/min)*

The average blood flow of the flap artery isolated as pedicle prior to free tissue transfer (F) was 9 ± 4 mL/min (mean ± SD). Its venous outflow was lower (7.5 ± 3.5 mL/min), resulting in an artery-to-vein (A/V) flow ratio of 1.4 ± 0.7. The mean blood flow of the recipient internal mammary artery and vein prior to flap anastomosis (R) was 16.9 ± 6.3 and 9.4 ± 8 mL/min, respectively. After anastomosis (AA), the arterial and venous blood flow volume was 11.3 ± 5.3 and 7.4 ± 4.1 mL/min, respectively, with an A/V flow ratio of 1.8 ± 1.3. The arterial blood flow of the intact recipient artery (R) significantly decreased after anastomosis with the flap artery (AA) (*p* = 0.002). However, the arterial and venous blood flow of the included flaps did not significantly change after flap transfer. (Figure 3) The blood flow of the intact recipient artery (R) did not alter the blood flow of the flaps after anastomosis. There was a significant positive correlation between the arterial inflow and the venous outflow both before (F) and after anastomosis (AA) (*p* < 0.05). The arterial and venous blood flow rates before and after anastomosis in DIEP flaps were lower than in ms-TRAM flaps (Table 2). There was no correlation between the arterial blood flow volume and the flap weight. The flap ischemia time did not change the blood flow rates of the examined flaps.

#### *3.2. Vascular Resistance (mmHg/mL/min)*

The mean arterial vascular resistance (aVR) of the included flaps prior to tissue transfer (10 ± 4.2 mmHg/mL/min) did not significantly change after anastomosis (9.2 ± 5.2 mmHg/mL/min). The vascular resistance of the recipient artery, however, significantly increased from 5.4 ± 2.6 to 9.2 ± 5.2 mmHg/mL/min after anastomosis to the flap (*p* < 0.001). The average arterial vascular resistance (aVR) of DIEP flaps prior to (F) and after flap transfer (AA) was 12 ± 3.8 and 11.2 ± 5.8 mmHg/mL/min, respectively. By contrast, ms-TRAM flaps had significantly lower arterial vascular resistance values prior to (7.2 ± 3 mmHg/mL/min; *p* = 0.004) and after flap reperfusion (6.5 ± 2.3 mmHg/mL/min; *p* = 0.02) (Figure 4 and Table 3). There was no correlation between the arterial vascular resistance and the weight of the included flaps before or after flap transfer.

**Table 1.** Characteristics of the included patients. Abbreviations: DIEP: deep inferior epigastric perforator; ms-TRAM: muscle-sparing transverse rectus abdominis musculocutaneous.


**Figure 3.** Arterial and venous blood flows (mL/min) at three predefined time points (F, R, AA). The bars represent means ± standard error (\* indicates significant differences).


Bold numbers indicate significant differences.

**Figure 4.** Arterial Vascular Resistance (aVR) at three predefined time points (F, R, AA). The bars represent means ± standard error (\* indicates significant differences).

**Table 3.** Comparison of Arterial Vascular Resistances (mmHg/mL/min) of the flap pedicle in situ (F), of the Recipient Artery (R) and After Anastomosis (AA).


The bold numbers indicate significant differences.
