Vascular Disease
Implications of Pulmonary Artery Wave Reflection in Patients With Pulmonary Hypertension
2022 Volume 86 Issue 6 Pages 956-957
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2022 Volume 86 Issue 6 Pages 956-957
Pulmonary hypertension (PH) is characterized by increased pulmonary vascular resistance (PVR) and pulmonary artery pressure, leading to right ventricular failure. Although PVR is used as a definitive parameter in patients with PH, it does not fully explain the pulmonary circulation, because blood flow is pulsatile rather than continuous.
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Recently, several groups have reported the significance of reflective wave in the pulmonary artery.1–3 López-Candales et al reported that the severity of PH could be diagnosed by visual assessment of the timing of right ventricle outflow tract (RVOT) spectral Doppler signals.1 Patients with PH showed a shorter time to peak RVOT signal and a shorter overall RVOT signal duration compared with subjects without PH. In addition, “notching” was detected mid-systole in the Doppler signals in their patients with estimated pulmonary artery systolic pressure between 69 and 94 mmHg by echocardiography. Afonso et al discussed the presence of similar notching in the RVOT signals of patients with massive or submassive acute pulmonary embolism, which can lead to abrupt increases in right ventricular afterload.2 The notching in their patients was detected during early systole when the pulmonary valve is opened, and this finding was confirmed by M-mode echocardiography. This phenomenon in patients with acute pulmonary thrombi can be explained not only by a shorter reflection distance but also by reduced pulmonary arterial elasticity, faster reflected wave velocity, and a greater reflection coefficient compared with normal subjects. Bech-Hanssen et al have reported that augmented pressure values obtained by echocardiography can be used to assess pulmonary pressure reflection in patients with PH.3
Wave intensity analysis (WIA) is a time-domain-based pulse-wave analysis approach originally developed by Parker and Jones.4 Recently, WIA has been used to explain the pulmonary circulation.5–7 By using WIA and the Joukowsky equation (aka the waterhammer equation), the direction of the wave travel can be separated into forward and backward waves, and the effects on arterial pressure can be separated into compression and decompression waves (Figure).5 Among these 4 types of waves, the forward compression wave, which increases flow and pressure, originates from contraction of the right ventricle in the early stage of ventricular systole. In contrast, the backward compression wave, which decreases the flow but increases the pressure mid-systole, originates from the distal reflected wave due to vascular impedance mismatch.
Flowchart of the outcomes of wave intensity analysis and wave separation. P, pressure; U, velocity.
Usually, pressure and velocity data for WIA are collected invasively using a combined dual-tipped pressure and Doppler flow sensor wire.6 Recently, Yoshida et al reported a non-invasive approach by echocardiography for assessing pulmonary artery wave reflection (PAWR) in dogs using WIA.8 In this issue of the Journal, Hayama et al9 report using the same approach in human subjects. In the approach used by both groups, the velocity of the RVOT flow is measured by pulsed-wave Doppler, and an estimate of the pressure is obtained by using the simplified Bernoulli equation together with continuous-wave Doppler detection of the velocity of tricuspid valve regurgitation. The total pressure is then separated into forward pressure (Pf) and backward pressure (Pb) by means of WIA and the Joukowsky equation. Although Hayama et al did not compare PAWR parameters measured noninvasively and invasively, Yoshida et al confirmed in their canine study that the ratio of peak Pf to peak Pb obtained noninvasively by echocardiography strongly correlated with that obtained invasively by intravascular catheter.8 The backward compression wave (and Pb) is considered the main component of the notching observed in RVOT signals in previous studies.1–3 Hayama et al also report that Pb strongly correlated with both pulmonary artery pressure and PVR, and that Pb showed good sensitivity and specificity for predicting PVR >3 Wood units, a cutoff value for diagnosing precapillary or combined pre- and post-capillary PH,8 suggesting that Pb may be useful for early diagnosis of PH. Changes in Pb during nitric oxide inhalation in a patient with idiopathic pulmonary arterial hypertension were reported by Su et al;7 Pb, as well as Pf and the Pb/Pf ratio, decreased in response to inhalation of NO, suggesting that the effect of vasodilators may also be evaluable by Pb.7
These previous studies examining WIA and wave separation were performed using the MATLAB commercial computing environment (MathWorks, Natick, MA, USA). The original matlab script for the study of WIA and wave separation was developed and released by Parker.10 The new means of determining PAWR by echocardiography discussed here involves only a novel interpretation of echocardiographic data that can be easily obtained in clinical practice. At this point, there is a possibility that PAWR by echocardiography could be an important parameter widely used in clinical practice, and further study is needed to evaluate the usefulness of PAWR by echocardiography in PH.
The authors declare that they have no conflicts of interest to disclose.
None.
