A noninvasive method of measuring wave intensity, a new hemodynamic index: application to the carotid artery in patients with mitral regurgitation before and after surgery

Abstract

Wave intensity (WI) is a new hemodynamic index, which is defined as (dP/dt)(dU/dt) at any site of the circulation, where dP/dt and dU/dt are the time derivatives of blood pressure and velocity, respectively. Arterial WI in normal subjects has two positive sharp peaks. The first peak occurs during early systole when a forward-traveling compression wave is generated by the left ventricle. The magnitude of this peak increases markedly with an increase in cardiac contractility. The second peak, which occurs towards the end of systole, is caused by generation of a forward-traveling expansion wave by the ability of the left ventricle to actively stop aortic blood flow. The interval between the R wave of the ECG and the first peak of WI (R-1st peak interval) and the interval between the first and second peaks (1st-2nd interval) are approximately equal to the preejection period and left ventricular ejection time, respectively. Using a combined Doppler and echo-tracking system, we obtained carotid arterial WI noninvasively. We examined the characteristics of WI in 11 patients with mitral regurgitation (MR) before and after surgery, and 24 normal volunteers. In the MR group before surgery, the second peak was decreased and the (1st-2nd interval)/(R-R interval) ratio was reduced, compared with the normal group (140 +/- 130 vs 750 +/- 290mmHg m/s3. P < 0.0083; 20.7% +/- 3.4% vs 26.7% +/- 2.8%, P < 0.083). There were no significant differences in the first peak between the normal group and the MR group before and after surgery. The second peak in the MR group was increased significantly (P < 0.016 vs before surgery) to 1,150 +/- 830mmHg m/s3 in the early period after surgery (stage I), and to 1,090 +/- 580mmHgm/s3 in the late period after surgery (stage II). These values did not differ significantly from that of the normal group. At stage I, the (R-1st peak interval)/ (R-R interval) ratio was increased from 13.4% +/- 2.7% to 20.6% +/- 5.6% (P < 0.016 vs before surgery). At stage II, this ratio decreased to 16.2% +/- 2.8% (P < 0.016 vs stage I). but was still significantly higher than that before surgery. The (1st-2nd interval)/(R-R interval) ratio increased significantly after surgery (P < 0.016 vs before surgery) to values (27.0% +/- 4.5% at stage I and 28.9% +/- 2.6% at stage II) which did not differ significantly from that of the normal group. The recovery of the second peak after surgery suggests that the left ventricle had recovered the ability to actively stop aortic blood flow. Wave intensity is useful for analyzing changes in the working condition of the heart.

Clinical usefulness of wave intensity analysis

“…A close relation between the first peak of aortic WI and LV max. dP/dt was found in dogs [14], and the same relation was predicted from theoretical considerations [21,32]. Here, clinical measurements have confirmed a similar relation between carotid arterial WI and LV max.…”

“…They performed animal experiments and found that the height of the first positive peak of aortic wave intensity, which appeared early in systole and coincided with rising pressure and acceleration, increased with an increase in cardiac contractility. This relation was also derived theoretically [21,32].…”

“…After surgery, the second peak appeared clearly. We studied 11 patients with MR, and compared wave intensity indices before and after surgery [21]. …”

“…However, to obtain DP and DU, segmentation of the large-amplitude waveforms P and U is required, which makes DPDU necessarily depend on the sampling interval. Nevertheless, the time-normalized wave intensity (WI) has the same property as the original wave intensity: if WI [ 0, the changes in pressure and velocity caused by the forward wave are greater than those caused by the backward wave, and vice versa [21]. In addition, defining wave intensity as (dP/dt)(dU/dt) is perhaps more intuitive to cardiovascular physiologists because it represents the product of the familiar parameters of rate of change in pressure and flow acceleration.…”

Changes in Non:Invasive Wave Intensity Parameters with Variations of Savitzky:Golay Filter Settings

“…The original formulation required invasive simultaneous measurements of blood pressure and flow velocity (U), but the parallel development of ultrasound systems enabled performing WIA through ultrasound-measured diameter (D) and U instead [3,4]. D and U are commonly filtered (smoothed) to eliminate high-frequency noise; however the filter settings affect WIA parameters, calculated not directly from D and U, but from their time derivatives (dD, dU) [5].…”