During certain bowel surgeries for cancer (colorectal surgery), individuals frequently have to be placed in a head-down position to enable surgeons have optimal access to the cancer site. This position usually also involves bending the hips and knees while supporting the legs in stirrups. However, lying in this position for long periods of time can lower the flow of blood to the legs, which can potentially cause injury. This rare but severe consequence is called Well-Leg Compartment Syndrome (WLCS). If WLCS is not diagnosed quickly, it can lead to other difficult complications and a significant delay in recovery. Unfortunately, because the individual is under anaesthesia, diagnosis is delayed in many cases. There is very little information in the medical literature about how this damage to the legs progresses over the course of the surgery. To better understand how WLCS, how well blood vessels work during surgery will be assessed. The flow of blood and oxygen in the calf muscle will be assessed in 25 individuals placed in a head-down position during colorectal surgery. Likewise, blood samples will be obtained in order to measure the biological markers that may contribute to the development of WLCS.

The development of lower limb compartment syndrome in the absence of trauma (known as well-leg compartment syndrome, WLCS) is a severe complication reported during prolonged pelvic surgery. WLCS is characterised by muscular ischaemia and secondary necrosis from the increase in hydrostatic pressure in a closed fascial compartment.

The aetiology of WLCS appears to be primarily related to ischaemia of the lower limbs after a prolonged period in the head-down tilt lithotomy (HDTL) position, which is commonly used to access the pelvis and perineum during urological, colorectal, and gynaecological surgery. Elevation of the lower limbs above the heart results in a ~2 mmHg drop in mean arterial pressure at the mid-calf for every 2.5 cm that the limbs are raised. These hemodynamic changes are accompanied by an increase in compartment pressures associated with limb elevation and are further exacerbated by a steep (>15°) head-down tilt. The outcome of these changes ultimately leads to the development of significant and (often unrecognised) intraoperative ischemia in the lower limbs.

Prolonged reductions in lower limb perfusion during HDTL can cause the development of localised tissue oedema, leading to marked intracompartmental hypertension, which is further exacerbated once the limbs are lowered and reperfusion of ischemic muscle occurs. This is a consequence known as ischaemic-reperfusion injury (IRI), a phenomenon where reperfusion per se may result in a local and systemic inflammatory response that may augment tissue injury in excess of that produced by ischaemia alone.

Microvascular injury is considered one of the major determinants of IRI, particularly due to increased permeability of capillaries and arterioles that lead to an increase in diffusion and fluid filtration across the tissues. Subsequently, activated endothelial cells produce more reactive oxygen species and less nitric oxide, and this imbalance results in a corresponding inflammatory response and increased oxidative stress, which if sufficiently severe lead to further lower limb muscle and nerve injury.

The incidence of WLCS is unpredictable and as the individual is anaesthetised, the diagnosis is delayed in many cases, so the residual motor-sensory deficit may be severe, even after fasciotomy. Although there are many published case reports of this condition, it is likely that many cases of WLCS go unreported, and some may be mild and escape clinical detection. However, in cases that have been reported, the incidence of permanent disability after the development of WLCS ranges from ≈30% to ≈100%.

The paucity of detailed observational studies into the effects of HDTL on lower limb haemodynamics and tissue oxygenation has restricted the ability of the clinical and scientific society to construct clinical strategies to minimise this complication. Identifying individuals with risk factors and increasing surveillance during surgery in these people may allow for early detection and decrease the morbidity and potential mortality associated with intraoperative WLCS.

The study aims to assess lower limb haemodynamics by measuring cutaneous blood flow, muscle and brain tissue oxygenation, inflammatory and oxidative stress biomarkers during pelvic surgery in patients placed in HDTL. This study has the potential to provide a better understanding of the pathogenesis of WLCS and add novel knowledge regarding its early diagnosis.

Inclusion Criteria:

• Adults scheduled to undergo colorectal surgery in the HDTL position

Exclusion Criteria:

• History of myocardial infarction or cerebro-vascular events in the last 12 months
• Previous revascularisation procedure in their lower limbs
• BMI > 40 kg/m2
• Inability to give informed consent
• Other serious medical conditions, which in the opinion of study investigators, would interfere with safety or data interpretation

Adult individuals(18 years and above) scheduled to undergo surgery in the HDTL position at the Department of Colorectal Surgery, Queen Alexandra Hospital, Portsmouth Hospitals University NHS Trust.