ABSTRACT: Background. Simultaneously measured pressure and flow distal to coronary stenoses can be combined, in conjunction with anatomical measurements, to assess the status of both the epicardial and microvascular circulations. Methods and Results. Assessments of coronary hemodynamics were performed using fundamental fluid dynamics principles. We hypothesized that the pressure-drop coefficient (CDPe; trans-stenotic pressure drop divided by the dynamic pressure in the distal vessel) correlates linearly with epicardial and microcirculatory resistances concurrently. In 14 pigs, simultaneous measurements of distal coronary arterial pressure and flow were performed using a dual sensor-tipped guidewire in the setting of both normal and disrupted microcirculation, with the presence of epicardial coronary lesions of 50% AS. The CDPe progressively increased from lesions of 50% AS and had a higher resolving power (45 ± 22 to 193 ± 140 in normal microcirculation; 248 ± 137 to 351 ± 140 in disrupted microcirculation) as compared to fractional flow reserve (FFR) and coronary flow reserve (CFR). Strong multiple linear correlation was observed for CDPe with combined FFR and CFR (r = 0.72; p 50% area stenosis (AS), as well as to compare CDPe to existing clinical parameters such as FFR and CFR.
Methods
Animal preparation. The preclinical study protocol was approved by the Institutional Animal Care and Use Committee at the University of Cincinnati and at Cincinnati Children’s Hospital Medical Center. The in-vivo study was performed on 14 Yorkshire pigs (46 ± 3 kg), two groups of 7 pigs each assigned to normal18 and abnormal coronary microcirculation, respectively. The animals were fasted for 24 hours before the procedure and were premedicated with intramuscular ketamine (20 mg/kg) or telazol (2–7 mg/kg), atropine (0.4 mg/kg), xylazine (2 mg/kg) and buprenorphine (0.005 mg/kg). General anesthesia was maintained with 2% of isoflurane and endotracheal oxygen supply as per the surgical procedural standards.4,19 Heart rate, oxygen saturation and end-tidal CO2 level were monitored every 15 minutes, and ventilator changes made as needed to maintain these values in the normal range. In a closed chest pig heart model, an arterial sheath was placed by surgical cut-down in the carotid artery followed by the insertion of a 6 French (Fr) guide catheter, which was advanced under fluoroscopic guidance to the left main coronary ostium. An intravenous dose of heparin (300 U/kg) was injected immediately. Angiographic images were used to select a segment of left anterior descending (LAD) artery with no significant side branches.18
Anatomical measurements. After engaging the guide catheter at the coronary ostium, the native vessel lumen area was measured by motorized pullback (1 mm/sec) of a 2 Fr In-Vision Gold intravascular ultrasound (IVUS) catheter (Volcano Corp., Rancho Cordova, California). Before IVUS measurements, a bolus dosage (0.1–1.0 µg/kg/min) of intracoronary nitroglycerin was injected to eliminate spasm that can be caused by insertion of the IVUS catheter. Based on the IVUS and angiographic images, a portion of the LAD was selected for creating epicardial flow obstruction. Epicardial flow obstruction was induced by inflating a Voyager coronary angioplasty balloon (Guidant Corp., Indianapolis, Indiana), with the outer diameter determined from the inflation pressure table provided by the manufacturer. Sample balloons were tested for variation of diameter with respect to recommended inflation pressure within an in-vitro flow-loop system pressurized to physiologic pressure. The influence of the physiologic pressure on the balloon diameter is expected to have negligible influence since inflation pressure within the balloon was maintained at a much higher pressure (at least 3 times the physiologic pressure). The balloon was mounted on the dual sensor-tipped guidewire used for pressure and flow measurements. The configurations of sensor-tipped guidewires and balloon placement used to calculate the % AS are summarized in Figure 1.
Functional measurements. In the first group of 7 pigs (normal microcirculation group), the phasic distal coronary pressure (pd) and APV were measured simultaneously with a dual sensor-tipped guidewire (Combowire, Volcano Therapeutics) as shown in Figure 1. The mean proximal aortic pressure (pa) was continuously recorded at the tip of the guide catheter. The hemodynamic measurements were performed at baseline flow and at maximal hyperemic flow after injecting intracoronary papaverine (10 mg). The FFR and CFR were recorded simultaneously using the ComboMap System (Volcano Corp.). The procedure was repeated after varying the balloon diameter to simulate various degrees of epicardial stenoses.
In the other group of 7 pigs (abnormal microcirculation group), the diseased microcirculation was simulated by microvasculature disruption via injection of ~12,000 microspheres of 90 µm diameter (Polysciences, Inc., Warrington, Pennsylvania).14 The severity of epicardial stenosis was again varied by inflating the balloon. Distal pressure and flow were recorded as explained earlier.
Calculations of indices. CDPe, developed from fundamental fluid dynamics principles, is defined as the ratio of trans-stenotic pressure drop and distal dynamic pressure (0.5 × ρ × APV2) measured at peak hyperemia where blood density ρ does not change significantly, as reported in several past studies,16,18,20 and thus can be assumed constant, with a value of 1.05 gm/cm3.
See equation above (last image).
Statistical analysis. A two-way ANOVA model21 was used to distinguish between the severity of epicardial stenosis and microvascular dysfunction simultaneously. The severity of epicardial stenosis was categorized as lesions of 50% AS. Considering the literature,22,23 typically 60–65% area blockage. Here, we used an angioplasty balloon catheter of relatively greater length (range: 10–18 mm) to create the internal blockage in place of a focal lesion observed in clinical setting. In addition, it is expected that the 0.79 mm diameter shaft of the balloon catheter will also add extra resistance to the blood flow. Hence, to account for the combined resistances offered by greater balloon length and the shaft diameter of the balloon catheter, we estimated 50% AS16–18 to be a better combined resistance for the two groups ( 50%) of stenosis severity. The severity of microvascular dysfunction was categorized as normal or abnormal (disrupted microvasculature by injecting microspheres) microcirculation. The pair-wise simultaneous comparison of treatment level means and contrast estimations were calculated by Scheffe’s and Bonferroni’s methods, respectively. Multiple linear regression analysis was also applied to compare the simultaneous correlations among hemodynamic parameters. Data analysis was performed using SAS version 9.1.3 (SAS Institute, Inc., North Carolina), with p Results
A total of 316 simultaneous pressure-flow readings were recorded in 14 pigs (45 readings for 50% AS stenosis for the normal microcirculation group; 88 readings for lesions 50% AS for the abnormal microcirculation group). The FFR, CFR, and CDPe, were calculated from the above pressure-flow data. Mean native LAD diameter was 3.01 ± 0.53 mm. The mean % AS varied from 0.37 ± 0.12 to 0.64 ± 0.1 for lesions of 50% AS, respectively, with no statistically significant difference between the normal and abnormal microcirculation groups. The hyperemic mean proximal aortic pressure after microsphere injection was 48 ± 13 mmHg, similar to the range observed by others.4,19
The hyperemic mean pressure difference was significantly reduced from normal (21 ± 8 mmHg) to abnormal microcirculation (15 ± 6 mmHg), with p 50% AS (23 ± 11 cm/s), with p 50% AS (12 ± 2.4 cm/s). The difference between the hyperemic mean APV for the normal and abnormal microcirculation groups (i.e., 26 - 12 = 14 cm/s) is significantly higher than that of the difference between lesions in the 50% AS groups (i.e., 18 - 16 = 2 cm/s; p 90% would not need further physiological assessment. For such patients, angioplasty is usually performed based on angiographic evidence only. For significant area stenosis (e.g., > 90%) it is expected that the epicardial resistance will be higher and comparable to the microvascular resistance.
CDPe correlations. The CDPe for microvascular impairment correlated moderately with FFR (r = 0.65, p 50% AS with normal microcirculation (0.71 vs. 0.59; p = 0.0004) or with abnormal microcirculation (0.66 vs. 0.59; p = 0.0017). FFR could not differentiate between normal and abnormal microcirculation in cases of lesions of 50% AS (0.59 vs. 0.59; p = 0.94). In contrast, CFR could significantly differentiate between normal and abnormal microcirculation irrespective of degree of epicardial stenosis, e.g., 50% AS (1.55 vs. 1.36; p = 0.00014). However, CFR could not differentiate between lesions of 50% AS with normal or abnormal microcirculation using the number of samples included in this study. The total group means of epicardial or microvascular disease show the same results. Thus, FFR and CFR are complementary for determining the severity of epicardial stenosis and microvascular impairments.
The CDPe are summarized for all disease combinations in Table 2, where number of readings is shown within parenthesis. The CDPe increased from lesions of 50% AS (45 vs. 193; p 50% AS, yet retaining sufficient statistical significance (248 vs. 351; p 50% AS (i.e., 289–180 = 109) using Bonferroni’s method with p 50% AS, while CFR could not distinguish between the severities of epicardial stenosis under normal or abnormal microcirculation. Most importantly, there was a significant stepwise increase in CDPe values in different disease states as shown in Figure 3C, and hence, CDPe could differentiate between all disease-state combinations. The summary of these results is further tabulated in Table 4 for easy reference.
Further, the ratio of maximum CDP evaluated at the site of stenosis and its theoretical limiting value of the minimum cross-sectional area has also been evaluated. Since such a ratio needs area (anatomic) measurement in addition to the pressure drop-flow (functional) measurements, it is considered a prospective parameter from the perspective of clinical use; thus, its results are discussed in Appendix 1.
Discussion
In the present in-vivo study, we further validated CDPe for simultaneous evaluation of the severity of epicardial stenosis and microvascular dysfunction. This diagnostic parameter was evaluated by simultaneous measurements of trans-stenotic pressure drop and velocity using a single guidewire under a similar measurement setup used to evaluate FFR and CFR. Furthermore, the index was compared with the information provided by the currently used indices for FFR and CFR.
CDPe provides a non-dimensional flow resistance parameter that is unique and ranges from zero to infinity. It has the advantage of higher resolving power for separating normal and diseased conditions of epicardial vessels and microvasculature simultaneously (Table 2). A lower value of CDPe indicates a healthier epicardial artery as well as microvascular status, whereas mild, moderate and severely elevated values of CDPe indicate abnormal epicardial stenosis, abnormal microvascular function and a combination of both, respectively. Thus, CDPe may be useful to delineate severity of microvascular disease under epicardial lesions of 50% AS. Importantly, CDPe combines the advantage of FFR and CFR for simultaneous evaluation of epicardial and microvascular status and correlates well with FFR and CFR combined (p 50% AS with normal microvasculature and lesions of Study Limitations
Flow measurements. The epicardial arterial blockage was introduced internally by inflating the angioplasty balloon. Errors in flow measurement could occur if the downstream placement of the tip of the Doppler flow sensor relative to the angioplasty balloon18 is inaccurate. While placing the tip of the sensor tip downstream of the balloon, we avoided branch arteries between the tip and the balloon, while maintaining sufficient distance between the two in order to avoid instabilities in flow measurement.
External vs. internal occlusion. The internal balloon occlusion represents a different hemodynamic condition than the external stenosis17,18 in terms of spatial velocity profiles, eccentricity effect and additional flow resistance offered by the balloon shaft (diameter: 0.79 mm; Figure 1). This could result in a difference in overall magnitude of CDPe between external and internal obstruction. However, it is expected that this parameter will follow a similar trend if the resistance offered by internal balloon obstruction and balloon shaft are combined and compared with external stenotic resistance.
Collateral flow. The diagnostic indices were not affected by collateral flow since pig hearts are not known to have significant coronary collaterals. The effect of collaterals on CDPe needs further evaluation.
Microvascular disease model. The method of disrupting microcirculation by injecting microspheres may represent only a certain variant of structural microvascular diseases.14 Though we injected the same size and amount of microspheres in the group of pigs with abnormal microcirculation, these microspheres might affect different arterioles and capillaries of the heart, creating variable levels of microcirculations and microvascular resistances. It may be noted that two degrees of diseased microcirculation were simulated in the abnormal microcirculation group of 7 pigs by microvasculature disruption via injection of 2 sequential dosages (1st and 2nd) of ~12,000 microspheres of 90 µm diameter (Polysciences, Inc). Pressure-drop flow data were acquired for each sequential dosage (both 1st and 2nd) of microsphere injection. It was observed that the flow impairment, measured by the reduced APV values, and pressure drop were nearly unchanged after the initial (1st) injection of microsphere, indicating that the maximum flow disruption was achieved following the initial dosage of microsphere injection. In view of this, only the data for initial dosage of microvascular injection are reported. It is possible that a lower-diameter (e.g., 50 µm) microsphere would have targeted smaller microvessels, and this could have provided a graded microvascular disruption following each sequential injection of microspheres.
Conclusion
CDPe is a functional index developed from fundamental fluid dynamics principles. This index is able to simultaneously assess and distinguish between the epicardial coronary stenosis and microcirculatory resistance. Future clinical trials should be performed using a variable degree of disease severity for epicardial disease and microvascular status simultaneously, so that cut-off values of CDPe can be determined for all disease combinations. This process may be helpful in improving diagnostic accuracy and guide decision-making in the cardiac catheterization laboratory. Further clinical trials are also needed to study the role of collateral vessels and their effect on CDPe.
From the aDepartment of Mechanical Engineering; the bDepartment of Biomedical Engineering; the cDepartment of Internal Medicine and Cardiology; the dDepartment of Cardiothoracic Surgery, University of Cincinnati, Cincinnati, Ohio; and the eDepartment of Cardiology, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio; and fJet Propulsion Laboratory, California Institute of Technology, Pasadena, California.
The authors report no conflicts of interest regarding the content herein.
Acknowledgments. This work is supported by Grant-In-Aid of Great Rivers Affiliate and National-Scientific Development Grant of American Heart Association (Grant reference #s: 0755236B and 0335270N).
Manuscript submitted March 9, 2009, provisional acceptance given April 23, 2009, final version accepted May 28, 2009.
Address for correspondence: Rupak K. Banerjee, PhD, PE, Department of Mechanical (primary) and Biomedical (secondary) Engineering, 598 Rhodes Hall, P.O. Box 210072, Cincinnati, OH 45221-0072. E-mail: rupak.banerjee@uc.edu
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