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In September of 1985, the field of interventional cardiology expanded to include procedures on the adult aortic valve with the first case of balloon aortic valvuloplasty. The excitement was tempered, however, by early valve restenosis within one year in most patients after the procedure. A resurgence of interest in percutaneous aortic valve intervention, however, appeared in April 2002, with the first human implantation of an aortic valve mounted in a balloon expandable stent.
Since then, percutaneous heart valve (PHV) implantation has been attempted in a series of 40 profoundly ill patients with aortic stenosis on a compassionate basis at our institution. The results of the earliest cases are already partially reported and are encouraging.

TECHNIQUE
The PHV is a tri-leaflet bioprosthetic valve sutured to a balloon expandable 23-mm stent and crimped over a 22-mm delivery Z-MED II (NuMED, Inc.) balloon catheter (Figure 1).
* PRE-VALVE PREPARATION:
The entire procedure is performed under local anesthesia and mild sedation. Both common femoral veins and common femoral arteries are cannulated. Baseline hemodynamic parameters are registered. A temporary pacing lead in the right ventricle is placed for rapid ventricular pacing during PHV deployment and valve pre-dilatation. Pre-dilatation of the valve is performed retrogradely with a 23 mm balloon. Supra-aortic angiogram is performed after pre-dilatation to detail the anatomy of the valve and the relation of the coronary arteries. After supra-aortic angiography, PHV implantation can proceed by either the antegrade transseptal or the retrograde approach.
* ANTEGRADE TRANSSEPTAL APPROACH FOR PHV IMPLANTATION:
The antegrade transseptal approach has been the primary mode of valve delivery. Though more technically demanding, the advantages of this approach has been facilitated implantation of the device in heavily diseased valves, and the lower risk of inserting a 24-Fr sheath in the femoral vein rather than the femoral artery. This approach requires puncture of the intra-atrial septum and placement of a stiff guidewire from the right femoral vein to the left femoral artery (using a snare) via the right atrium, left atrium, left ventricle and aorta. Dilatation of the septum (10-mm septostomy balloon) is a critical step to allow unrestricted passage of the PHV. The PHV/balloon assembly is then introduced from the 24-Fr sheath, over the guidewire, and across the interatrial septum. Using the reference image obtained during supra-aortic angiography and fluoroscopy, the horizontal equator of the PHV stent should be transected by the leaflet calcifications. The balloon is then quickly inflated under rapid pacing. After full inflation, the balloon is deflated and withdrawn from the valve orifice. Final trans-valvular gradient is measured and aortography is performed.
* RETROGRADE APPROACH FOR PHV IMPLANTATION:
The retrograde approach was developed as a faster and simpler procedure for valve implantation. The two limitations, however, are cannulating the femoral artery with a 24 Fr sheath and crossing the aortic valve with the PHV. This approach, therefore, is limited to patients with femoral and iliac vessels that are adequate in size (inner diameter > 7 mm), without significant tortuosity or disease, and to patients with mild valvular calcification. Other than pre-closure of the femoral puncture site using two 10 Fr Prostar XL devices, the retrograde technique is exactly the same as the antegrade approach up until the transseptal catheterization.

The 24-Fr sheath is introduced into the femoro-iliac arteries. Through the 24-Fr sheath, this device is advanced over the guidewire, and pushed across the aortic valve. Using the same landmarks and technique, the PHV is implanted under rapid pacing.
* FOLLOW-UP. The patients are closely monitored during follow-up. Clinical evaluation and echocardiographic examination are performed immediately post procedure, at day 1, day 7, one month and every three months thereafter. Post-procedural treatment includes 75 mg of clopidogrel daily for one month and 160 mg of daily aspirin indefinitely. Subcutaneous low molecular weight heparin is administered during the hospitalization stay, and oral antibiotics are continued for 48 hours.

RESULTS OF THE SERIES
Currently, PHV implantation for the treatment of aortic stenosis is performed only by study protocol in compassionate patients contra-indicated for surgical replacement by 2 independent cardiac surgeons. Eligibility for the study requires the presence of severe aortic valve stenosis (= 0.7 cm2) and associated severe symptoms (dyspnea class IV by NYHA classification).

After successful implantation, the valve area increases to 1.7 cm2 with minimal transvalvular gradient (<5 mm Hg invasively). If aortic regurgitation is present, it is always paravalvular in origin. In appropriately sized patients, implantation of a 23 mm PHV should result in minimal to none paravalvular leak. The performance of the valve appears to be lasting without restenosis (>23 months). Today, 12 patients from our initial feasibility trial are alive without symptoms of aortic stenosis.

LIMITATIONS AND PERSPECTIVES
Despite these encouraging initial results, PHV implantation is still in its infancy and will require advances in hardware and operator experience before it can be used routinely in clinical practice. Some of the complications associated with the procedure have been stroke (1 patient), tamponade (2 patients, pacing lead and transseptal puncture induced). Immediate valve migration has occurred in 3 patients as a result of mis-sizing of the prosthesis (1 patient), mis-placing the prosthesis (1 patient), and native valve disruption during pre-dilatation (1 patient). Moderate to severe (grade 3) paravalvular leak occurred in 25% of patients in the first 20 patients attempted and should not occur if smaller patients are implanted (23 mm PHV) or a larger 26 mm valve is used (awaiting approval). Mortality after the procedure remains high and is related to pre-existing comorbid conditions rather than device failure.

Currently, advances in the technique and hardware such as the use of a steerable catheter to facilitate retrograde implantation, a lower profile delivery system, and a 26 mm PHV, should further simplify the procedure, reduce the risk of complications, and expand the scope of patients implanted. Canadian experience with these new technologic improvements is very encouraging (20 patients).

CONCLUSION
Percutaneous implantation of a bioprosthetic stent-valve in the aortic position is becoming a novel method to treat inoperable patients with aortic stenosis. Though surgical aortic valve replacement remains the standard of care, percutaneous valve intervention may become a realistic way for the cardiologist to treat select patients in the near future.