This study is looking at the effects of high spinal anesthesia (also known as total spinal anesthesia) combined with general anesthesia versus general anesthesia alone on the following:

Stress response: Patients undergoing aortic valve replacement surgery have a large incision and a complex operation where they must be placed on the heart-lung machine. The body reacts to the heart-lung machine, increasing the stress response.

High spinal anesthesia using local anesthetics when combined with general anesthesia has been shown to block some of the stress response to surgery and the response to the heart-lung machine. This study will examine if blood levels of stress hormones and also inflammatory mediators can be lowered with the use of high spinal anesthesia.

Heart function: High spinal anesthesia in combination with general anesthesia may help the heart work better when there is a narrowed valve (aortic stenosis). The heart may also have improved ability to pump blood with this anesthetic technique.

Lung function and post-operative pain control: After surgery, patients often have pain which prevents them from taking deep breaths and coughing. This can lead to pneumonia. This study will also examine if the post-operative pain relief provided by spinal morphine (given together with the spinal anesthetic) can provide any better pain control following surgery. By doing this, we want to see if patients can take bigger breaths after their surgery when spinal morphine is used, and try to prevent the complications that occur if patients are not able to breath deeply after surgery.

It is hypothesized that high spinal anesthesia combined with general anesthesia decreases the intraoperative stress and inflammatory response and improve post-operative pain control and respiratory function in this patient population. It is also hypothesized that the technique will provide stable intraoperative hemodynamics during aortic valve replacement surgery.

Stress response: Levels of hormones such as epinephrine, norepinephrine and cortisol are elevated during cardiac surgery and on the initiation of cardiopulmonary bypass. This stress response has previously been shown to be blunted with the use of high spinal anesthesia when combined with general anesthesia in coronary artery bypass surgery patients (Lee, Grocott, et al).

Inflammatory response: In addition to the stress response there is also an accentuated inflammatory response. With contact of the patient's blood to the artificial bypass circuit, there is activation of various plasma protease pathways that generate multiple proinflammatory mediators. Complement levels and cytokine levels also rise. Clinical organ dysfunction involving the cardiovascular, pulmonary, renal and neurological systems can ultimately result. The effects of high spinal anesthesia on the inflammatory response that occurs with bypass have not been studied.

Hemodynamics: It has previously been shown that high-spinal anesthesia for coronary artery bypass surgery provides stable intra-operative hemodynamics (Kowalewski, MacAdams, et al; Lee, Grocott, et al.). Although the use of spinal anesthesia in patients with aortic stenosis has been considered to be relatively contra-indicated, total spinal anesthesia may actually improve cardiac function by decreasing systemic afterload and increasing myocardial contractility.

Post-operative analgesia and pulmonary function: The spinal administration of opioids, such as morphine, has been shown to improve post-operative pain management in patients having both cardiac and non-cardiac surgery (Jacobsohn, Lee, et al). Total spinal anesthesia with bupivacaine and spinal morphine combined with general anesthesia may also improve post-operative pain management and facilitate improved post-operative lung function.

Inclusion Criteria:

• Undergoing surgery for aortic valve replacement due to aortic stenosis with or without CABG.

Exclusion Criteria:

• INR > 1.4, PTT > 40 seconds
• platelet count < 80, 000 per microlitre
• local infection or deformity at the site of administration of the spinal anesthetic
• raised intracranial pressure or evolving neurological deficit at the time of surgery