During many surgeries, increased muscle tension makes it harder for the surgeon to expose the site of surgery and work within the incision. Neuromuscular blockade (NMB) drugs such as Vecuronium bind to neurotransmitter (acetyl choline) receptors at the neuromuscular junction, blocking their action and producing muscle relaxation. This muscle relaxation allows easier retraction of muscle tissues and manipulation of structures in the wound. Improved surgical conditions are likely to result in improved patient outcomes. While increased depths of NMB have been shown to optimize surgical conditions during intra-abdominal and retroperitoneal procedures, the impact of NMB depth has not been reported for orthopedic surgeries.1 To address this, we propose to study the effect of NMB depth on surgical conditions during total hip replacement (THR).

Specific Aims

1. Assess difference in surgical conditions between moderate and deep NMB groups. Enrolled patients will be randomized to receive moderate (n=58) or deep (n=58) NMB. Difference in surgical conditions will be evaluated by:

   1. The number of requests from the surgeon for additional relaxation (NMB) during the procedure. At any time during the operation if the surgeon feels the muscle tension is interfering with ease of operation he will ask for additional muscle relaxation. If the patient is moderately relaxed they will be converted to deep relaxation with additional muscle relaxants. If they are already deeply relaxed no additional relaxants will be administered (as is our current practice). All requests will be recorded.
   2. Rating by the surgeon after each surgery using an internally developed satisfaction scale. The scale was developed by modifying a scale used in a previous study of muscle relaxation in intra-abdominal surgery1 to specify two key elements identified by our surgeon: ease of muscle retraction and femur manipulation.

2. Assess the impact of deep vs moderate NMB on time of surgery, measured from the time of incision to joint reduction.

SIGNIFICANCE If we identify improved surgical conditions with deeper relaxation we will incorporate deep NMB into our routine anesthesia practice for THR.

Vecuronium will be used as NMB drug in all study patients; this agent is currently used in over 90% of THR cases at Maine Medical Center (MMC). As is currently routine Vecuronium will be given after initiation of general anesthesia with propofol to facilitate intubation and further doses of Vecuronium will be given throughout the case as noted below to maintain NMB at the desired depth until the femoral implant is reduced. After the intubating dose of Vecuronium, NMB depth will be monitored every 5 minutes and dosing will be adjusted as needed to maintain a constant depth of NMB according to our current routine practice.

Group 1: Moderate NMB: Intubating dose of Vecuronium of 0.1 mg/kg (IBW) and re-dosing with 0.0125 to 0.05 mg/kg as needed to achieve and maintain 1 to 2 train of four (TOF) contractions. Redosing in this manner is a current clinical practice.

Group 2: Deep NMB: Intubating dose of Vecuronium of 0.2 mg/kg (IBW) and re-dosing with 0.025 to 0.1 mg/kg to achieve and maintain zero twitches in the TOF, and post tetanic count (PTC) of 1 to 2 contractions. This level of blockade is new to the practice since approval of the drug for use at MMC but is in common use since the advent of Sugammadex.

The surgeon may request additional relaxation at anytime for inadequate surgical conditions thought to be related to muscle tension. All requests will be recorded. Patients in the moderate NMB group will receive additional doses of vecuronium to achieve deep NMB (PTC of 1- 2). In the deep NMB group with PTC of 1-2, a saline dose without NMB will be given.

NMB reversal Sugammadex will be given for reversal of NMB after the prosthesis has been reduced, using routine dosing of 2 mg/kg for the moderate group and 4 mg/kg for the deep group, per package insert by Merck.

Inclusion Criteria:

• American Society of Anesthesiologists (ASA) Physical status 1-3
• age 50-75
• English speaking
• able to provide informed consent
• BMI equal to less than 30
• non-emergent THR by anterolateral minimally invasive non-cemented total hip arthroplasty

Exclusion Criteria:

• Revision surgery
• Bilateral THR
• ASA 4+
• age less than 50 or greater than 75
• BMI greater than 30
• unable to provide informed consent
• women taking oral contraceptives (Sugammadex used for reversal interferes with their efficacy
• contraindications to general inhalation anesthesia (such as malignant hyperthermia)
• contraindications to NMB (known allergy to NMB)
• chronic kidney disease

Patients were monitored for the duration of their surgical case and time in the post-anesthesia care unit until discharge from the hospital, usually 24 hours, and had a post-surgical follow-up several weeks after discharge.

The anesthesiologist will NOT be blinded, but the orthopedic surgeon performing the hip replacement surgery will be.