Noninvasive haemodynamic evaluation of aortic valve prostheses : Doppler echocardiographic studies at rest and during exercise

The present series of investigations was designed to supply information on in vivo pressure-flow relationships and to analyse the incidence of regurgitation in different heart valve prostheses in the aortic position.

Patients and methods: The study population comprised 185 patients with mechanical Björk-Shiley Monostrut prostheses, 54 patients with Biocor stented porcine bioprostheses, 50 patients with extended Biocor stentless bioprostheses and 59 patients with cryopreserved aortic homografts. Transvalvular pressure differences and volume flow estimates were derived noninvasively from Doppler measurement of blood velocity across the prosthesis and in the left ventricular outflow tract. Measurements were performed at rest and during supine symptom-limited bicycle exercise. Aortic regurgitation was evaluated semi-quantitatively using colour flow Doppler and continuous-wave Doppler. Patients were investigated after surgery, prior to hospital discharge (baseline) and approximately every year postoperatively.

Results: Reference values for baseline Doppler echocardiographic variables for normally functioning prostheses in the aortic position were established. The newly-developed extended Biocor stentless bioprosthesis showed low pressure differences associated with a significant regression of left ventricular hypertrophy during the first six months after aortic valve replacement, and low incidence of aortic regurgitation. A decrease of 40% in pressure differences from baseline to six-month examination was observed in patients with stentless prostheses, partly depending on different haemodynamic states of patients early after the operation. Subcoronarily implanted cryopreserved aortic homografts. demonstrated excellent haemodynamic performance with low pressure differences and large effective orifice areas. Timerelated increasing risk of progression of aortic regurgitation observed during follow-up justifies more frequent echocardiographic monitoring in patients with homograft valves, especially when aortic regurgitation is detected on baseline examination. Different types of prosthesis of similar size were compared at rest and during exercise. The lowest pressure differences and the largest effective orifice areas were demonstrated in homograft valves, followed by extended Biocor stentless bioprostheses. With a 65-70% increase in cardiac output induced by exercise, pressure differences and valve resistance increased significantly while effective orifice area remained unchanged in all four prosthesis types studied. Doppler-derived haemodynamic variables can be obtained at rest and during supine symptom-limited exercise with low variability. The exercise performed in this study was interrupted at a high level of subjective exertion; thus the exercise pressure differences obtained are likely to represent maximal values for pressure differences occurring in the patients' daily activities.

Conclusions: In patients undergoing aortic valve replacement, Doppler echocardiography at rest provides basic information on prosthetic valve function. In addition, exercise Doppler echocardiography provides further valuable insight into valve haemodynamics during physiologically increased volume flow. These results can be obtained with low variability. The stentless porcine bioprostheses and cryopreserved aortic homograft valves exhibit low valve resistance to flow at rest and exercise, reflecting the low haemodynamic load imposed by these valves on the left ventricle.