Lower leg blood pressure decreases while calf external pressure increases with the angulation of the Trendelenburg position in the lithotomy position with calf- and foot-supported leg holders

Patients who underwent lower abdominopelvic surgeries in the lithotomy position (LP) and the Trendelenburg position (TP) with the leg holder are at risk of developing well leg compartment syndrome (WLCS). However, contributing factors related to the LP with TP associated with WLCS are unknown. This study aimed to investigate the associations between external pressure at the calf in the LPs at different angulations of the TP and physiological characteristics. Eighty-four university students (age, 21.7 ± 0.9; 42 men and 42 women) voluntarily participated in the study. The awake participants were placed in the LPs using the calf- and foot-supported leg holder at 0° (horizontal level), 5°, 10°, and 20° head-down tilts by moving the electric operating table. The peak contact pressure (pCP) was measured at the calf as a representative external pressure using the pressure distribution measurement system BIG-MAT®. Lower leg blood pressure significantly decreased with TP angulation, while calf pCP significantly increased with it at 0°, 5°, 10°, and 20° head-down tilts (39.4 ± 15.2, 46.5 ± 17.7, 47.2 ± 16.9, and 50.3 ± 17.6 mmHg, respectively). The calf pCP with a 10° head-down tilt was correlated positively with the calf total force (P < 0.001) and negatively with the calf contact area (P < 0.001). Blood hypoperfusion due to low lower leg blood pressure secondary to lower leg elevation and head-down tilt, and high calf external pressure due to direct external compression from the leg holder where it is loaded may contribute to WLCS.


Introduction
The lithotomy position (LP) with the Trendelenburg position (TP), which is the Lloyd Davies position [16], has been developed to facilitate access to the pelvis for urological, gynecological, and colorectal procedures. Recently, laparoscopic and robot-assisted lower abdominal and pelvic surgeries, the majority of which require the patients to be in the LP with TP, have become more common than conventional surgeries because of their reduced invasiveness.
Several factors, such as direct occlusion of arterial blood flow to the lower leg, obstruction of venous drainage from the lower leg, and general hypoperfusion of the lower leg in the LP [18], are likely to contribute to the development of WLCS. Interference with arterial and venous blood flow may be related to patient positioning. LP is assumed to decrease blood perfusion pressure (PP) in the lower legs by reducing arterial blood pressure (BP) due to vascular insufficiency [19]. The other cause of WLCS is direct external compression from the leg holders. WLCS occurs when intra-compartmental pressure (ICP) pathologically increases within the neuromuscular spaces through each neurovascular structure enclosed by strong non-expansible fascial layers or bone in the four compartments of the calf. There is an increase in calf ICP or venous pressure, which further compromises calf local blood perfusion. However, contributing factors related to the LP with TP associated with WLCS remain unknown.
Given these recent surgical trends on the LP with TP, there is a need to clarify the relationship between external pressure at the calf and physical characteristics to prevent WLCS. This study aimed to identify the contributing factors to the occurrence of WLCS and suggest preventive strategies. We investigated the relationships between external pressure at the calf in the LP at different angulations of the TP and physical characteristics, such as body size, lower leg size, and foot size, and physiological characteristics, such as the calf force, calf contact area, and hemodynamics associated with the LP with TP.

Study participants
The study was approved by the ethics committee of Okayama Prefectural University (approval numbers 453, 17-1, and 18-1) and registered in the UMIN Clinical Trials Registry (UMIN-CTR ID 000030416). The study recruited 84 university student volunteers [42 men and 42 women]. We obtained written informed consent for the publication of images and data from the participants before beginning. The study excluded participants with motor or sensory disturbances in the lower extremities. Height, body weight, lower leg length, tibial length (TL), peroneal head circumference, maximum calf circumference (maxCC), minimum calf circumference (minCC), foot length (FL), and heel width in the left lower extremity were measured before the study.

Lithotomy position (LP) with head-down tilts
The awake participants laid down in the supine position on a tilting electric operating table class IB ® (DR-3000-A; Takara Belmont Corp, Osaka, Japan) in a quiet room. The leg holder (Bel Flex ® ; L 356 mm × W 200 mm; Takara Belmont Corp), which supports the posterior aspect of the distal part of the lower leg, including the calf and foot, was connected to the class IB ® tilting operating table. The participants were placed in the LP characterized by the following: the hip joints flexed at 10° from the trunk, abducted at 45°, and minimally externally rotated from the midline; and the knee joints flexed at 60° using an angle gauge (Fig. 2a) [15]. Beginning with the participants in the horizontal position, which is the level table 0°, they were placed at 5°, 10°, and 20° head-down tilts by moving the class IB ® (Fig. 2a-d). Each position was maintained for 5 min.

Arterial blood pressure (BP)
The non-invasive arterial systolic BP (sBP) and diastolic BP (dBP) in the right lower leg were measured to determine the BP from the right anterior tibial artery, posterior tibial artery, and peroneal artery using the automated digital BP monitor COLIN BP-508 typeS ® (Colin Medical Instruments Corp., Aichi, Japan) in the LP with 0°, 5°, 10°, and 20° head-down tilts. sBP and dBP in the left upper arm were measured to determine the BP from the left brachial artery using the automated digital BP monitor HEM-7430 ® (Omron Healthcare Co., Ltd, Kyoto, Japan), and the heart rate was measured.

External pressure
The pressure distribution measurement system BIG-MAT ® (Nitta Corp., Osaka, Japan) is a non-invasive evaluation material developed to measure external pressure for industrial applications [34]. The BIG-MAT ® system comprises a pressure distribution measurement sheet with a 10-mm pitch and 2,112 (44 × 48) sensors (L 440 mm × W 480 mm × D 0.4 mm; BIG-MAT2000P3BS ® , Nitta Corp.), a sensor connector, and a personal computer with built-in BIG-MAT ® software.
The pressure distribution measurement using the BIG-MAT ® system has been described previously [20][21][22][23][24][25]. The BIG-MAT ® system was used to measure the external pressure at the calf to investigate the contributing factors of WLCS [22,24,25], at the fibular head for common peroneal nerve paralysis [20,21], at the lateral region of the distal part of the fibula for superficial peroneal nerve paralysis [20], and at the sacral region for decubitus ulcers [23] in the LP.
The BIG-MAT ® system was calibrated by careful placement of a 25-kg concrete block. The digitally measured values were converted to pressure information using the software, which displayed two-dimensional visually understandable squares for each of the 2,112 sensor cells. Outputs from all sensor cells were also displayed as numbers ranging from 0 to 255. Changes in pressure values were recorded consecutively, and the chronological changes were saved as movie files on a personal computer.
The left lower extremity (i.e., opposite side of measuring lower leg BP) was placed on the BIG-MAT2000P3BS ® sheet spread over left Bel Flex ® (Fig. 1a). One hundred pressure distribution views at the left calf and foot obtained using the BIG-MAT2000P3BS ® sheet were recorded in the LPs with 0°, 5°, 10°, and 20° head-down tilts. Figure 1b displays a representative external pressure distribution view for the contact of the left calf and foot in the LP.
A square area, displayed with a green box corresponding to the left calf, was selected. The calf total force (total loading value on the sensor cells within the green box), calf contact area, and two external pressure measurements within the green box, namely, calf contact pressure (CP) and peak CP (pCP), were evaluated. The calf CP represents the mean external pressure on the loaded sensor cells within the green box, which is equal to the calf total force divided by the contact area covered by the loaded sensor cell. The calf pCP represents the mean external pressure on the 2 × 2 loaded sensor cells, which corresponds to the highest pressure within the green box representing the peak area. It is equal to the calf total force in the four squares divided by the loaded sensor cell area within the peak area. The left calf total force, contact area, CP, and pCP were measured in the LP at 0°, 5°, 10°, and 20° head-down tilts.

Statistical analysis
The data were expressed as mean ± standard deviation. The body mass index (BMI) was calculated as the participant's body weight divided by the square of their height. All statistical analyses were performed using R ® statistical software version 4.0.0 (The R Foundation for Statistical Computing, Vienna, Austria). The unpaired Student's t test was used for statistical comparisons of men and women. The Bonferroni method, after the Kruskal-Wallis chi-squared test, was Representative left calf and foot external pressure distribution views obtained using the pressure distribution measurement system BIG-MAT ® in the LP are shown. The total force, contact area, contact pressure (CP), and peak CP (pCP) using the BIG-MAT ® system are measured at the calf and foot in contact with Bel Flex ® . High-and low-pressure areas are indicated by red and blue squares, respectively. The green square box represents the calf. The accumulation of red squares on the sole side of the green square box corresponds to the heel head-down tilts. Multiple regression analysis was used to determine the associations between the calf pCP at 0° or 10° head-down tilts and independent variables: sex, height, body weight, TL, maxCC, minCC, FL, calf total force, calf contact area, lower leg sBP and dBP, upper arm sBP and dBP, and heart rate. Statistical significance was set at P < 0.05. Table 1 shows the physical characteristics of the 84 participants, including age, body size, left lower leg size, and foot size. There was no significant bias in the physical characteristics of the participants. Table 2 shows the hemodynamics associated with the LPs at 0°, 5°, 10°, and 20° head-down tilts. The right lower leg sBP and dBP significantly decreased with the angulation of the TP. The left upper arm sBP or dBP did not significantly change with TP angulation. The heart rate did not significantly change with TP angulation. The lower leg and upper arm sBP and dBP at 0°, 5°, 10°, and 20° head-down tilts were significantly higher in the men than in the women. There was no significant difference between the heart rates of the men and women.

Hemodynamics
Calf total force, contact area, contact pressure (CP), and peak contact pressure (pCP) Table 3 shows the left calf total forces, contact areas, CPs, and pCPs in the LPs at 0°, 5°, 10°, and 20° head-down tilts. The calf pCP significantly increased with TP angulation. The calf CPs and pCPs at 10° and 20° head-down tilts were significantly higher in the men than in the women. Table 4 shows the relationships between the left calf pCP in the LP at 0° or 10° head-down tilt and independent variables, including sex, body size, left lower leg size, foot size, left calf total force, calf contact area, and hemodynamics for all the 84 participants. The calf pCP at 0° was significantly higher in the men, correlated positively with body weight and the calf total force, and correlated negatively with the calf contact area on multiple regression analysis. The calf pCP at a 10° head-down tilt was correlated positively with the calf total force and negatively with the calf contact area. In addition to the TL and minCC, which are independent variables, the multiple regression analysis results of the relationships between the calf pCP at 0° or 10° head-down tilt and the independent variables for 60 participants were similar to those for all the 84 participants.

Discussion
The lower leg sBP and dBP decreased while the calf pCP increased with TP angulation in the LP with the calfand foot-supported leg holder. The calf pCP at the 10° 6.0 ± 0.6 (n = 24) 6.3 ± 0.4* (n = 12) 5.8 ± 0.6 (n = 12) 0.03 head-down tilt correlated positively with the calf total force and negatively with the calf contact area.

Lithotomy position (LP) and Trendelenburg position (TP)
The combination of decreased blood PP in the lower legs and increased calf ICP results in a significant decrease in driving pressure on the lower legs, which exaggerates by a head-down tilt [7,26]. The LP, especially that with a head-down tilt, resulted in a decrease in static blood PP to the lower legs because their positions were above the heart [7,8,26]. Only LP during pelvic surgery was not associated with any demonstrable decrease in lower leg blood PP [11]. The addition of a 15° head-down tilt during pelvic procedures led to an immediate and significant decrease in lower leg blood PP. Thus,  the addition of TP caused profound ischemia in the lower leg. By observing the dorsalis pedis arterial BP, the lower leg blood perfusion is compromised only by the combined adoption of the LP and TP. General hypoperfusion of the lower legs may exacerbate when the lower legs are elevated in the LP, particularly on addition of a head-down tilt. During the RARP procedure, the patient's lower legs are above the heart level in the LP with steep TP, and the BP of the lower legs may decrease. Lower leg handling, such as tight compressive wrappings on elevated lower legs [18], may affect blood flow. The mean arterial BP (mBP) at the heart level ranged from 55 to 63 mmHg, and that at the lower legs level in the LP with TP was approximately 20 mmHg. It can be assumed that there was a decrease in the mBP by 2 mmHg for every vertical inch of elevation of the lower legs above the heart. The mBP to this level has been associated with the development of WLCS.
The result studied in awake volunteers demonstrated a decrease in the lower leg blood PP on the adoption of the LP, and there was a further decrease in lower leg BP when a 15° head-down tilt was added [8]. It is suggested that a lower leg sBP of 75 mmHg in the TP is unlikely to compromise oxygen delivery to the calf compartments.
The addition of TP consistently increased the lower leg ICP. The lower leg ICP rose when the lower legs were in the LP [2]. The increase in lower leg ICP increased with the addition of TP. The lower leg ICP in the LP with TP was within the range known to compromise local blood perfusion. The results demonstrated that calf ICP increases as soon as the lower legs are in the LP with TP. Placing Table 4 Relationships between peak contact pressure at the left calf in the lithotomy position with 0° or 10° head-down tilt and independent variables P value by multiple regression analysis maxCC maximum calf circumference, FL foot length, sBP systolic blood pressure, dBP diastolic blood pressure, SE standard error, t t value, P P value, VIF variance inflation factor the lower legs in the supine position allows the calf ICP to return to normal. The ankle mBP in the supine position was 130.5 mmHg, and it decreased to 77.2 mmHg in the LP with a 10° headdown tilt [29]. Reversing the table tilt to bring the ankle elevation to 0° restored the ankle mBP to 114.3 mmHg. Placing the lower leg with a calf-supported leg holder increased the calf mean ICP from 3.0 to 11.6 mmHg. Reversing the table head-down tilt significantly restored lower leg blood perfusion in patients undergoing pelvic surgery with elevated lower legs and may protect against subsequent WLCS.
The LP with TP compromises the local blood perfusion to the muscular compartments of the calf through a combination of decreased arterial BP and increased ICP. The LP led to a significant decrease in the arterial BP in the lower leg from 87 to 67.9 mmHg and a significant rise in lower leg ICP from 13 to 31 mmHg [37]. The LP, particularly in combination with TP, is likely to lead to ischemia of the calf muscular compartments [38]. These two effects compromise blood perfusion to the calf muscle compartments in the lower leg during surgical procedures where the LP and TP are required.
These studies on the effect of lower leg elevation on the arterial BP in the calf concluded that the LP does not compromise the blood perfusion to the lower leg. However, a combination of lower leg elevation and TP compromises blood perfusion to the lower leg does [8,11,29].

Capillary blood pressure (BP)
The capillary BP using microinjection was 32 mmHg in the human skin [14]. Capillary vessel occlusion is induced if the external pressure exceeds 32 mmHg, resulting in ischemic injury. External pressure loading to the skin surface below 32 mmHg, and as low as possible, has been recommended [13].
In the canine lower leg anterolateral muscular compartment, the capillary hydrostatic pressure was 25 mmHg [9]. The results pressurized by infusion of autologous plasma suggest that the risk of muscle damage is significant at ICP greater than 30 mmHg. It has been demonstrated that ICP of > 30 mmHg tends to impair capillary blood flow. The compliance of the tissue ICP at 30 mmHg was severely impaired. Furthermore, muscle ischemia significantly increases the permeability of capillary membranes to plasma proteins. Transudation of plasma proteins at this critical ICP of 30 mmHg is expected to produce prodigious increases in ICP.
In the in vivo fluorescence microscopic study of the hamster striated muscle, an increase in the external pressure was associated with a diameter reduction ranging from 5 to 25%, with cessation of the blood flow at the mean external pressure between 27 and 33 mmHg [39]. Blood flow ceased in 50% of the muscle capillaries at an external pressure of 12 mmHg. At distinct external pressure levels, venous and capillary blood flow ceases, but the arterioles are still capable of carrying blood flow. This study complies with the hypothesis that reduced arteriovenous pressure gradients cause blood perfusion cessation in WLCS because of the constriction-induced increase in venular resistance.
Therefore, capillary hydrostatic pressure ranging from 20 to 40 mmHg is generally accepted, with a mean of 32 mmHg in healthy individuals [17]. In the present study, the mean calf pCPs 39.4, 46.5, 47.2, and 50.3 mmHg at 0°, 5°, 10°, and 20° head-down tilts, respectively, were significantly higher than the 32 mmHg threshold (P < 0.001, paired Student's t test). Therefore, high calf external pressure can occlude the capillary vessels in the compartments with the angulation of the TP. This finding means that direct external compression from the leg holder to the calf in the LP with TP may lead to capillary occlusion in the calf and the development of WLCS. There are risks associated with performing the necessary steps to prevent patients from developing WLCS as an iatrogenic complication during lower abdominopelvic surgical procedures performed in the LP with TP.

Relationship between pressure, force, and area
In the present study, the calf pCP in the LP at 0° was higher in men than in women and positively correlated with body weight and the calf total force. Muscle density in men at 79.9 mg/cm 3 with peripheral quantitative computed tomography was higher than in women at 78.6 mg/cm 3 [35]. The risk factors for the development of WLCS include muscular calves and obesity [36]. Furthermore, the calf pCP in the LP at a 10° head-down tilt correlated positively with the calf total force and negatively with the calf contact area. The contact pressure of the lower leg increased after RARP in the LP with TP, and this increase was correlated with BMI [40].
Pressure is defined as the force per unit area and is calculated using the following formula: P = F/A; symbols P = pressure, F = force, and A = area. The large body weight exerts a large pressure due to the large force and the small body weight exerts a small pressure due to the small force. The sharp body exerts a large pressure due to the small area of contact and the dull body exerts a small pressure due to the large area of contact. Therefore, for a given force, if the area decreases by a factor such as the contact area with the calf-and foot-supported leg holder, the pressure would increase.

Study limitations
This study has some limitations.
First, this study was performed for only a short period of time. Prolonged operative duration is a risk factor for the development of WLCS [36]. The ICP shows minor elevations after initial LP and gradually increases with levels rising to 30 mmHg over an average time period of 5 h [2]. Canine skeletal muscle necrosis associated with impending WLCS occurs at a threshold ICP of 30 mmHg after 8 h (10). Therefore, measuring continuous CP and pCP in participants in the LP with TP for a long time period may be physically difficult.
Second, the participants in this study were young, healthy university students with relatively homogenous body habitus. The external pressure and variables measured in the participants should be applied to more patients with a wide age and body weight range. Therefore, CP and pCP in patients under general anesthesia in the operating rooms should be measured.
Third, clear evidence on the correlation between CP or pCP and invasive pressure measurements such as the PP and ICP was not able to be gathered. Measurements of PP and ICP in patients would make this study even more conclusive, although they may be ethically hard. Alternatively, near-infrared spectroscopy in the calf region could have been used along with CP and pCP.

Conclusion
Arterial BP in the lower leg decreases while external pressure from the calf-and foot-supported leg holder to the calf increases with the angulation of the TP in the LP. Calf external pressure correlates positively with the calf total force and negatively with the calf area in contact with the leg holder in the LP with TP. Blood hypoperfusion due to low lower leg BP secondary to lower leg elevation and head-down tilt, and high calf external pressure due to direct external compression from the leg holder on the part of the body weight where it is loaded may be the causes of WLCS. Therefore, the contributing factors to the occurrence of WLCS in the LP with TP are the calf force and calf contact area, and the angulation of the TP in the LP may contribute to the development of WLCS.