Utilization of translesional hemodynamics: comparison of pressure and flow methods in stenosis assessment in patients with coronary artery disease

Comprehensive physiological evaluation of epicardial and microvascular coronary domains using vascular conductance and zero flow pressure

AIMS

Assessment of the coronary circulation has been based largely on pressure ratios (epicardial) and resistance (micro-vessels). Simultaneous assessment of epicardial (CEPI) and microvascular conductance (CMICRO) provides an intuitive approach using the same units for both coronary domains and expressing the actual deliverability of blood. The aim of this study was to develop a novel integral method for assessing the functional severity of epicardial and microvascular disease.

METHODS AND RESULTS

We performed intracoronary pressure and Doppler flow velocity measurements in 403 vessels in 261 patients with stable coronary artery disease. Hyperaemic mid-to-late diastolic pressure and flow velocity (PV) relationships were calculated. The slope of the aortic PV indicates the overall conductance and the slope of the distal PV relationship represents CMICRO. The intercept with the x-axis represents zero-flow pressure (Pzf). CEPI was derived from microvascular and overall conductance. Median CEPI was higher compared to CMICRO (4.2 [2.1-8.0] versus 1.3 [1.0-1.7] cm/s/mmHg, p<0.001). CMICRO was independent of stenosis severity (1.3 [1.0-1.7] in FFR ≤0.80 versus 1.4 [1.0-1.8] in FFR >0.8, p=0.797). ROC curves (using FFR and HSR concordant vessels as standard) demonstrated an excellent ability of CEPI to characterise significant stenoses (AUC 0.93). When CEPI<CMICRO, a decrease in flow velocity and coronary pressure (optimal cut-off value 0.97, AUC 0.90) was demonstrated.

CONCLUSIONS

A comprehensive assessment of separate CEPI and CMICRO was feasible. CEPI has a remarkable diagnostic efficiency to detect a clinically relevant stenosis. When CEPI<CMICRO, distal flow and pressure decrease steeply, indicating myocardial ischaemia. CMICRO can be used to explore the severity of microvascular disease.

Coronary pressure and flow relationships in humans: phasic analysis of normal and pathological vessels and the implications for stenosis assessment: a report from the Iberian-Dutch-English (IDEAL) collaborators

Background

Our understanding of human coronary physiological behaviour is derived from animal models. We sought to describe physiological behaviour across a large collection of invasive pressure and flow velocity measurements, to provide a better understanding of the relationships between these physiological parameters and to evaluate the rationale for resting stenosis assessment.

Methods and results

Five hundred and sixty-seven simultaneous intracoronary pressure and flow velocity assessments from 301 patients were analysed for coronary flow velocity, trans-stenotic pressure gradient (TG), and microvascular resistance (MVR). Measurements were made during baseline and hyperaemic conditions. The whole cardiac cycle and the diastolic wave-free period were assessed. Stenoses were assessed according to fractional flow reserve (FFR) and quantitative coronary angiography DS%. With progressive worsening of stenoses, from unobstructed angiographic normal vessels to those with FFR ≤ 0.50, hyperaemic flow falls significantly from 45 to 19 cm/s, P_trend < 0.001 in a curvilinear pattern. Resting flow was unaffected by stenosis severity and was consistent across all strata of stenosis ( P_trend > 0.05 for all). Trans-stenotic pressure gradient rose with stenosis severity for both rest and hyperaemic measures ( P_trend < 0.001 for both). Microvascular resistance declines with stenosis severity under resting conditions ( P_trend < 0.001), but was unchanged at hyperaemia (2.3 ± 1.1 mmHg/cm/s; P_trend = 0.19).

Conclusions

With progressive stenosis severity, TG rises. However, while hyperaemic flow falls significantly, resting coronary flow is maintained by compensatory reduction of MVR, demonstrating coronary auto-regulation. These data support the translation of coronary physiological concepts derived from animals to patients with coronary artery disease and furthermore, suggest that resting pressure indices can be used to detect the haemodynamic significance of coronary artery stenoses.

Effect of simultaneous intracoronary guidewires on the predictive accuracy of functional parameters of coronary lesion severity

The aim of this study was to assess the influence of a second guidewire on the diagnostic accuracy of functional parameters of coronary lesion severity. Sixty-five patients with intermediate coronary lesions underwent myocardial perfusion scintigraphy. Fractional flow reserve (FFR), coronary flow velocity reserve (CFVR), and hyperemic stenosis resistance (HSR) index (HSR = stenosis pressure gradient ÷ velocity) were determined in 77 lesions. Distal pressure and velocity were acquired simultaneously (dual wire) and sequentially (single wire) with two sensor-equipped guidewires. Overall, functional parameters deteriorated from single- to dual-wire assessment. In patients without ischemia, the good diagnostic performance of FFR, CFVR, and HSR deteriorated significantly ( P < 0.001) when assessed by dual wires, with an increase in the number of false-positive results. This trend was more pronounced for HSR, since the presence of a second wire reduced maximal velocity and increased the pressure gradient. The presence of two guidewires, especially across a myocardial perfusion scintigraphy-induced nonsignificant lesion, is associated with overestimation of the hemodynamically assessed lesion severity and, therefore, is likely to have a major impact on clinical decision making. This underscores the advantage of a dual-sensor-equipped guidewire for the evaluation of stenosis severity by combined pressure and velocity measurements.

Correlation of Hemodynamic Events with Clinical and Pathological Observations

The correlation of hemodynamic events with clinical or pathological observations is represented by a variety of applications reflecting the broad range of this theme. The position paper describes several cases in which benefits of combining imaging information with transport models of contrast material, can cause a gain in hemodynamic information. What appears to be lack of cohesiveness among the cases illustrates the variety in the application of hemodynamic research to the practice of medicine. Some of the contributions presented at the symposium do not directly apply to clinical medicine, but instead described mathematical models or applications to animal physiology or technical advancements in measuring blood rheology. Although related to this theme topic they fall somewhat outside the main scope. The topics summarized below demonstrate four examples in which translation of the research to the clinical arena can be realized in a short period of time. Overall recommendations for priority objectives related to this topic are provided at the end of this position paper.

Renal fractional flow reserve: A hemodynamic evaluation of moderate renal artery stenoses

The objective of this study was to perform a hemodynamic evaluation of moderate (50-90%) renal artery stenosis (RAS) under conditions of rest and maximum hyperemia. Identifying patients with RAS who have hemodynamically significant stenoses and are most likely to benefit from revascularization is clinically important. Current methods used to evaluate RAS, including angiography, have limitations. Physiologic evaluation of RAS may have a role in identifying patients with hemodynamically significant stenosis. Patients with suspected renovascular hypertension due to aorto-ostial RAS were included in the study. Hyperemia was induced by administration of intrarenal papavarine. Translesional pressure gradients were measured and renal fractional flow reserve (FFR) was calculated using a 0.014'' pressure guidewire. Thirteen patients and 14 arteries with moderately severe (50-90%) RAS were studied. The mean translesional pressure gradient rose from a baseline of 6.3 +/- 3.9 to 17.5 +/- 10.8 mm Hg with maximal hyperemia. The renal FFR ranged from 0.58 to 0.95. There was a poor correlation between angiographic stenosis measurement and the renal FFR (r = -0.18; P = 0.54) and the hyperemic translesional mean pressure gradient (r = 0.22; P = 0.44). There was an excellent correlation between renal FFR and the resting mean translesional pressure gradient (r = -0.76; P = 0.0016) and the hyperemic mean translesional pressure gradient (r = -0.94; P < 0.0001). Selective renal arterial papavarine administration induces maximum hyperemia, permitting the calculation of renal FFR in renal arteries with aorto-ostial stenoses. The renal FFR correlates well with other hemodynamic parameters of lesion severity, but poorly with angiographic measures of lesion severity.

Single-wire pressure and flow velocity measurement to quantify coronary stenosis hemodynamics and effects of percutaneous interventions

BACKGROUND

Lack of high-fidelity simultaneous measurements of pressure and flow velocity distal to a coronary artery stenosis has hampered the study of stenosis pressure drop-velocity (DeltaP-v) relationships in patients.

METHODS AND RESULTS

A novel 0.014-inch dual-sensor (pressure and Doppler velocity) guidewire was used in 15 coronary lesions to obtain per-beat averages of pressure drop and velocity after an intracoronary bolus of adenosine. DeltaP-v relations from resting to maximal hyperemic velocity were constructed before and after stepwise executed percutaneous coronary intervention (PCI). Before PCI, half of the DeltaP-v relations revealed the presence of a compliant stenosis, which was stabilized by angioplasty. Fractional flow reserve (FFR), coronary flow reserve (CFVR), and velocity-based indices of stenosis resistance (h-SRv) and microvascular resistance (h-MRv) at maximal hyperemia were compared. Stepwise PCI significantly lowered h-SRv, with an initial marked reduction in hyperemic pressure drop followed by further gains in velocity. A concomitant significant reduction of h-MRv accounted for half of the gain in velocity after PCI. The average magnitude of absolute incremental hemodynamic changes was highest for h-SRv (56.8+/-39.2%) compared with CFVR (35.3+/-34.5%, P<0.005) or FFR (19.5+/-25.2%, P<0.0001).

CONCLUSIONS

DeltaP-v relations comprehensively visualize improvements in coronary hemodynamics after PCI. h-SRv is a powerful and sensitive descriptor of the functional gain achieved by PCI, combining information about both pressure gradient and velocity, which are oppositely affected by PCI. Simultaneous assessment of stenosis and microvascular resistance may provide a valuable tool for guidance of PCI.

Numerical modeling of the flow in stenosed coronary artery. The relationship between main hemodynamic parameters

The severity of coronary arterial stenosis is usually measured by either simple geometrical parameters, such as percent diameter stenosis, or hemodynamically based parameters, such as the fractional flow reserve (FFR) or coronary flow reserve (CFR). The present study aimed to establish a relationship between actual hemodynamic conditions and the parameters that define stenosis severity in the clinical setting. We used a computational model of the blood flow in a vessel with a blunt stenosis and an autoregulated vascular bed to simulate a stenosed blood vessel. A key point in creating realistic simulations is to properly model arterial autoregulation. A constant flow regulation mechanism resulted in CFR and FFR values that were within the physiological range, while a constant wall-shear stress model yielded unrealistic values. The simulation tools developed in the present study may be useful in the clinical assessment of single and multiple stenoses by means of minimally invasive methods.

The diastolic flow-pressure gradient relation in coronary stenoses in humans

OBJECTIVES

We assessed the feasibility and reproducibility of the instantaneous diastolic coronary flow velocity-pressure gradient relation to characterize different degrees of coronary stenoses.

BACKGROUND

Assessment of the hemodynamic significance of coronary stenoses can be difficult. Using sensor-tipped guidewires, various physiologic indexes can be determined in the catheterization laboratory. Each of the current methods, however, has limitations.

METHODS

After positioning a Doppler flow wire and a pressure wire distal of a coronary stenosis, the flow velocity signals and the proximal and distal pressure were sampled simultaneously, at baseline and after intracoronary administration of adenosine. The instantaneous diastolic flow velocity and pressure gradient of single cardiac cycles at baseline, at maximal and intermediate hyperemia were plotted. Data were fitted with a regression line using the equation: Delta P = 0 +kv+Sv(2). Measurements were performed in 11 normal coronary arteries, 20 intermediate stenoses and in 7 severe stenoses before and after percutaneous transluminal coronary angioplasty plus stenting.

RESULTS

We found significant differences between normal coronary arteries, intermediate and severe stenoses. Percutaneous transluminal coronary angioplasty nearly normalized the highly abnormal flow-pressure gradient relation in the severe stenoses. A high degree of reproducibility was observed. In 3% of the measurements, analysis was not possible due to the occurrence of pressure drift or bad flow velocity signals.

CONCLUSIONS

It is feasible to assess the diastolic flow velocity-pressure gradient relation over a wide range of stenoses. It characterizes the hemodynamics of epicardial coronary stenoses and allows discrimination between normal coronary arteries, intermediate and severe stenoses.

Pressure-based simultaneous CFR and FFR measurements: understanding the physiology of a stenosed vessel

Arterial stenosis is known to be one of the most serious cardiovascular diseases. Angiographical estimation of arterial stenosis provides limited information on the severity of the occlusion and the flow of blood through it. Hemodynamical assessment of the flow and pressure behaviour, is known to be clinically important. Hemodynamically based parameters, such as pressure based myocardial fractional flow reserve (FFR) and the flow based coronary flow reserved (CFR) were introduced to provide a much better tool for treating arterial diseases. We have developed a new method for simultaneous measurement of pressure-derived CFR and FFR. The advantage of pressure derived hemodynamic parameters is very substantial, and its relatively straightforward application in clinical setting is solid. The method has been validated by means of a computational fluid dynamics (CFD) model of the arterial stenosis and in vitro bench studies.

Prediction of the physiologic severity of coronary lesions using 3D IVUS: validation by direct coronary pressure measurements

This study was performed to determine whether three-dimensional intravascular ultrasound (3D IVUS) could predict the physiologic significance of coronary lesions. Seventeen lesions were evaluated by means of 3D IVUS, pressure measurements, and quantitative coronary angiography. Physiologic parameters were calculated from the 3D IVUS measures using established equations and compared to values measured by pressure guidewire. IVUS minimum lumen area (MLA) correlated with fractional flow reserve (FFR; R2 = 0.55, P = 0.003) and pressure gradient (R2 = 0.52, P = 0.003). Lesion length (L) had a positive correlation with pressure gradient (R2 = 0.45, P = 0.007). By multivariate analysis, the only significant independent determinant of FFR was MLA/L measured by IVUS. The IVUS-predicted pressure gradient and FFR were well correlated with values measured directly (R2) = 0.88, P < 0.001; R2 = 0.90, P < 0.001, respectively). The physiologic severity of coronary lesions is primarily influenced by lumen area and lesion length and can be established by 3D IVUS.

Intracoronary Doppler- and quantitative coronary angiography-derived predictors of major adverse cardiac events after stent implantation

BACKGROUND

Distal coronary flow velocity reserve (CVR) is significantly improved after a successful balloon angioplasty (PTCA). Furthermore, a postinterventional CVR >2.5 and a percent diameter stenosis (%DS) </=35% are predictive for a low incidence of major adverse cardiac events (MACE) at 6 months of 16%. Similar results are lacking for coronary stenting.

METHODS AND RESULTS

In 150 patients, baseline and hyperemic coronary flow velocities were recorded with a Doppler guidewire distal to the target lesion and in an unobstructed reference artery before and after PTCA, after stenting, and at 6 months. Distal CVR and relative CVR (CVR(rel)) were calculated. Logistic regression and receiver operating characteristic analyses were applied to determine prognostic cutoff values of CVR, CVR(rel), %DS, and minimal lumen diameter separately and in combination to predict MACE at 6 months. After stenting, CVR (2.96+/-0.87 versus 2.40+/-0.7; P:=0.001), CVR(rel) (1.02+/-0.24 versus 0.81+/-0.24; P:=0.001), and minimal lumen diameter (2.98+/-0.56 versus 2.11+/-0.74 mm; P:=0.001) were significantly higher than after PTCA. Thirty-three patients developed MACE. A postinterventional CVR(rel)>0.88 was the best single predictor of MACE, with an incidence of 6.8%, whereas the combination of a CVR(rel)>0.88 and a %DS </=11.2% predicted an incidence of MACE of 1.5%.

CONCLUSIONS

Measurement of CVR(rel) and %DS after stent implantation are best suitable to predict MACE at 6 months.

Development of an MR-compatible, rotation-insensitive, annular pressure sensor

There is a growing interest in performing intravascular interventions guided by MR imaging--a technique which offers the possibility of flow measurements during the intervention. For a reliable assessment of the haemodynamic significance of a stenosis, the flow and the pressure decay within the stenosis should both be measured. We have developed an optical, MR-compatible, pressure sensor (Annupres) that uses a novel annular element. Existing optical pressure sensors measure pressures unilaterally, thus giving rise to artefacts because of the dependence of the measurement on the angular orientation of the aperture. The annular element, however, measures blood pressure on all sides, and we show that by using circularly polarized light this pressure measurement is intrinsically insensitive to rotation of the sensor around its long axis. The Annupres sensor has been tested in an experimental set-up, and was able to measure pressures from 50 mmHg to 180 mmHg reliably with an accuracy of 1.5%.

Evaluation of a pressure-recording guidewire in patients with coronary arterial disease

The accuracy and feasibility of coronary arterial pressure measurements with a 0.018-in. pressure-recording guidewire (PRGW) was evaluated in patients. Transstenotic pressure gradients were measured with the PRGW and a guiding catheter, at baseline and during coronary vasodilatation. Proximal intracoronary pressure was measured with both systems before and after gradient measurements. Zero pressure was measured with the PRGW before and after intracoronary use. The average of all proximal intracoronary PRGW readings were close to guiding catheter values, but there were substantial individual deviations. Average change in proximal deviation before and after gradient measurements was -1 mm Hg, standard deviation (S.D.) 7.6, range -16 to 15. Errors in zero pressure measurements after intracoronary use (average 2.8 mm Hg, S.D. 8.8, range -9 to 35) were much greater than before use (average 0.1 mm Hg, S.D. 1.4, range -4 to 3, P < 0.001). The PRGW was successfully introduced through an 8F guiding catheter and positioned across the stenosis in 21 of 26 attempts (81%). Intracoronary advancement of the PRGW through a double-lumen multifunctional probing catheter was successful in all nine attempts. In conclusion, errors in PRGW-measurements caused uncertainty in gradient interpretation. However, we found the wire useful in several cases, particularly for exclusion of hemodynamically significant lesions. The steerability of the wire is inferior to ordinary guidewires, but it can be advanced to a distal intracoronary position through an over-the-wire catheter.