High Frequency Ventilation During Off Pump Coronary Artery Bypass Graft (CABG)

This study is to test the safety of high frequency-low volume ventilation during off pump coronary artery bypass as reflected in the near infrared spectroscopy values for cerebral oxygenation.

Ventilatory protocol: Patients will be randomized to either the apnea group or the HFV group. In the HFV group the patients will be ventilated using a tidal volume of 1.5 ml per kg and a respiratory rate of 100 per minute for as long as the surgeon needs to perform the distal anastomosis of the coronaries.The same surgeon will perform all surgeries. In the Control group, ventilation will be stopped for the duration of the anastomosis as it's done currently. At all other times during the surgery, the ventilatory parameters will be adjusted to maintain a PaCO2 between 40 and 45 mmHg.

The anesthesia protocol for the off pump coronary artery bypass will be as follows: premedication will be left at the discretion of the anesthesiologist. In the operating room, the patient will be instrumented with a radial arterial line before induction. Induction will consist of a bolus of 1.0 mcg per Kg of Sufentanil. Anesthesia will be maintained with 1mcg per Kg per hour of Sufentanil. One per cent Isoflurane will be used as hemodynamic parameters allow. Rocuronium 0.7 mg per Kg will be used to facilitate endotracheal intubation. Following endotracheal intubation the patients will monitored with a pulmonary artery catheter and a femoral arterial line. A perfusionist will be on standby at all times for possible conversion to extracorporeal circulation.

Measures to effect:

Routine baseline laboratory investigations will be performed at admission of the patient to the ward.

Hemodynamic parameters:

Arterial blood pressure, heart rate, pulmonary artery pressure, right ventricular pressure will be recorded five minutes intervals throughout the surgery.

Ventilatory parameters and blood gases:

End tidal CO2 and pulse oxymetry will be recorded at five minutes intervals throughout the surgery.

Arterial blood gases will be performed after induction, before and after each distal coronary anastomosis and when judged necessary. The type and time for each anastomosis will be noted.

C-reactive protein and brain natriuretic peptide:

Measurements of CRP and BNP will be perform at admission, after induction of anesthesia, in the intensive care unit after the surgery and 24 hrs after admission to the intensive care unit.

We will use the serum for measure of CRP.CRP will be measured using the Dade Behring N High Sensitivity CRP assay (Dade Behring Diagnostics,Marburg,Germany) on the BN ProSpec Nephelometer (Dade Behring Diagnostics).The assay will use monoclonal anti-CRP antibodies coated to polystyrene particles and a human calibrator traceable to the WHO reference material. We will use the Heparinised Plasma for measure of NT Pro-BNP.NT Pro-BNP will be measure by electrochemiluminescence immunoassay using the Roche Pro BNP assay (Roche Diagnostics , Mannheim,Germany) on the ElecSys 2010 analyzer (Roche Diagnostics).This assay will use two polyclonal antibodies in a sandwich format.

Cerebral Oximetry Monitoring:

Cerebral oxymetry (rSO2) using near infrared spectroscopy (NIRS, Invos 5100; Somanetics Corporation, Troy, MI) will be performed on all patients. After cleaning adjacent skin area with alcohol, an adhesive optode pad will be placed over each front-temporal area. Resting baseline rSO2 values will be obtained after waiting at least 1 min after placement of sensors once values had stabilized, with patient resting quietly and receiving 3-5 L of O2/min by nasal cannula. Continuous rSO2 values will be store on a floppy disk with a 15 s update for the duration of the intraoperative period. With application of the chest dressing, and before leaving the OR, monitoring will be discontinued and optodes will be removed.

Transesophageal echocardiographic (TEE) measurements:

A comprehensive TEE examination will be performed for all patients as it's done routinely(25). Specific TEE measurements as recommended be the ASE will be perform before and after distal coronary anastomosis(26). RVMPI will be defined as the sum of isovolumic contraction time and isovolumic relaxation time divided by the ejection time and the normal value is 0.28±0.04). RV MPI will be measured using both pulse wave and tissue Doppler imaging from a deep transgastric view and upper esophageal view. Right ventricular FAC % will be measure in 4 chambers view and calculated as RV change in area in diastole minus RV change in area in Systole divided by the calculated RV area in Diastole (37).

Analysis of HR and BP variability:

Baseline HR and BP variability will be analysed using wavelet transformation (27). The extraction of characteristic frequencies, or specific oscillations, of a signal that is composed of the consecutive R-R intervals for HR variability analysis, or consecutive systolic blood pressures for BP variability analysis, is used. The analysis will be made using MATLAB® and the dedicated toolbox software Wavelab. High frequency power of the HR variability is indicative of changes in parasympathetic nervous system. Total power of the BP variability signal is used as a measure of sympathetic outflow.

Eligibility Criteria:
Inclusion Criteria:

* Age \> 18 years.
* Patients able to give consent with knowledge of french or english language.

Exclusion Criteria:

* Patients undergoing emergency surgery.
* Patients with an intra aortic balloon pump.