Comparison Between Two Mathematical Methods to Estimate Arterial Occlusion Pressure and Tourniquet Effectiveness in Lower Limb Surgery: Randomized Comparative Study

After ethical committee approval and informed consent from the patients, 80 adult patients 18-40 years old, (ASA I-II), scheduled for knee arthroscopy under general anesthesia with pneumatic tourniquet around the thigh, will be randomly allocated (using closed opaque envelop) into one of two groups:

• Group (A): where the pneumatic tourniquet inflated 20 mmHg above the arterial occlusion pressure which will be estimated using the equation of Unver B. et al., [1]: (AOP=[SBP+10]/KTP)
• Group (B): where the pneumatic tourniquet inflated 20 mmHg above the arterial occlusion pressure which will be estimated using the equation of Hong-yun Liu et al., [2]: (AOP = 17.986 + 3.158X1 + 0.408X2) Patients will be excluded from the study if they are outside the age range, hypertensive, diabetic, complaining of any lower limb claudications, any vascular disease, hemolytic blood disorders or hypercoagulability.

Upon arrival to the operating room, standard monitoring will be attached (5 lead ECG, pulse oximeter and non-invasive blood pressure) followed by a wide-bore canula will be inserted a big vein in the upper limb. General anesthesia will be induced using fentanyl 1 mcg/kg and propofol 2-3 mg/kg laryngeal mask airway device will be inserted after atracurium 0.5 mg/kg and lungs will be mechanically ventilated to maintain end-tidal CO2 at 36-40 mmHg and maintenance of anesthesia will be achieved using isoflurane 1-2 volume %. Hemodynamic stabilization will be assured and systolic blood pressure will be maintained within the pre-induction values ± 20% using intravenous fluid infusion and titration of inhalational anesthetics.

In all patients, the thigh circumference will be measured 20 cm proximal to the superior pole of the patella then, pneumatic the tourniquet cuff will be placed (deflated) around the thigh with the distal edge 15 cm proximal to the proximal pole of the patella. The tissue padding coefficients based on limb circumferences will be calculated in all patients of group (A) according to table (1) [3].

After three successive stable systolic blood pressure reading (5 minutes apart), an initial systolic blood pressure [initial SBP] reading will take and estimated arterial occlusion pressure (AOP) will be quickly calculated according to one of the previously mentioned equation followed limb exsanguination using an Esmarch bandage, then the tourniquet cuff will be inflated to a safety margin pressure 20 mmHg above the calculated (AOP) [3] which will be the [initial tourniquet pressure]. The time between the initial SBP taking and tourniquet inflation will be measured [initial SBP-to- tourniquet inflation time]. The absence of arterial flow in the lower limb is assured using Doppler probe (HD, Philips) positioned, by an independent anesthetist, at either the dorsalis pedis artery or the posterior tibial artery on the foot to monitor arterial flow distal to the tourniquet. If any arterial flow sound will be heard, the estimated tourniquet pressure will be considered insufficient and the patient will excluded from the study. When no arterial blood flow is secured, the surgeon (blinded to the groups) will allowed to start and will be asked to rate the bloodless surgical field using a 4-point scale [1 (Excellent) = No blood in the surgical field, 2 (Good) = Some blood in the surgical field but no interference with surgery, 3 (Fair) = Blood in the surgical field but no significant interference with surgery, 4 (Poor) = Blood in the surgical field obscures the view] [3] at the beginning, in the middle, and at the end of the surgical procedure.