The lower leg sBP and dBP decreased while the calf pCP increased with TP angulation in the LP with the calf- and foot-supported leg holder. The calf pCP at the 10° 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 (Mumtaz FH 02)(Gill M 19).
The LP, especially that with a head-down tilt, results in a decrease in static blood PP to the lower legs because their positions were above the heart (Halliwill JR 98)(Mumtaz FH 02)(Gill M 19). Only LP during pelvic surgery was not associated with any demonstrable decrease in lower leg blood PP (Horgan AF 99). 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 (Martin JT 92), 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 (Halliwill JR 98). 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 (Chase J 00). 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 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° head-down tilt (Peters P 94). 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 (Turnbull D 02). The LP, particularly in combination with TP, is likely to lead to ischemia of the calf muscular compartments (Turnbull D 01). 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 (Peters P 94)(Halliwill JR 98)(Horgan AF 99).
Capillary blood pressure (BP)
The capillary BP using microinjection was 32 mmHg in the human skin (Landis EM 30). 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 (Krouskop TA 90).
In the canine lower leg anterolateral muscular compartment, the capillary hydrostatic pressure was 25 mmHg (Hargens AR 78). 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 (Vollmar B 99). 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 causes 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 (Lyder CH 03). 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/cm3 with peripheral quantitative computed tomography was higher than in women at 78.6 mg/cm3 (Sherk VD 14). The risk factors for the development of WLCS include muscular calves and obesity (Simms MS 05). 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 (Yamada Y 16).
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 (Simms MS 2005). The ICP shows minor elevations after initial LP and gradually increases with levels rising to 30 mmHg over an average time period of 5 hours (Chase J 2000). Canine skeletal muscle necrosis associated with impending WLCS occurs at a threshold ICP of 30 mmHg after 8 hours (Hargens AR 1981). 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