Coronary Blood Flow Measurements



Fig. 29.1
Angiography (upper left) shows geometry of lesion responsible for pressure (P) and flow (Q) reductions associated with ischemia. Two lesions of the same dimension can have markedly different P-Q relationships (lower left). FFR is calculated from distal pressure/proximal pressure ratio at maximal hyperemia (upper right). Coronary flow reserve is calculated from ratio of maximal hyperemic average velocity to baseline velocity (lower right)



The hemodynamic significance of a given stenosis, as determined by the pressure-flow relationship, can be measured and incorporated into clinical discussion using sensor angioplasty guidewires. A hemodynamically significant coronary lesion is associated with one or more of the following parameters which have been correlated to provokable myocardial ischemia:


  1. 1.


    For Coronary Doppler,


    1. (a)


      Poststenotic absolute coronary flow reserve (CVR) <2.0

       

    2. (b)


      Relative coronary flow reserve (rCVR) <0.8

       

    3. (c)


      Proximal to distal flow velocity ratio (P/D) <1.7

       

    4. (d)


      Diastolic to systolic velocity ratio (DSVR) <1.8

       

     

  2. 2.


    For Coronary pressure, the hyperemic translesional pressure ratio, also known as the FFR has a treat/no treat threshold of <0.80 to treat for best outcomes. An ischemic threshold of <0.75 has a 90 % sensitivity and 90 % specificity for correspondence to stress testing [1]

     



Indications


Indications for performing coronary physiologic measurements include those lesion subsets whose clinical relevance is uncertain following angiography. These include intermediate severity stenosis, assessment of culprit lesions in patients with multivessel coronary artery disease and guiding therapy in patients with tandem lesions or diffuse disease in an epicardial vessel. Coronary physiologic studies are useful in areas poorly studied angiographically eg. ostia and bifurcations, and can also be used to assess prognosis following stent implantation. Coronary physiologic testing is useful in assessing ischemic potential of patients with prior myocardial infarction (MI), in patients with treated unstable coronary syndromes and to assess the collateral circulation. Table 29.1 summarizes the indications for using coronary physiologic measurements.


Table 29.1
Indications for physiologic measurements in the catheterization laboratory









































Coronary pressure measurements

1. Assessment of Intermediate stenosis in one or more coronary arteries (including left main)

2. In patients with multivessel disease, determination of one or more target stenoses (either serially or in separate vessels)

3. Evaluation of ostial or distal left main and ostial right lesions, especially when these regions can not be well visualized by angiography

4. Guidance of treatment of serial stenoses in a coronary artery

5. Determination of significance of focal treatable region in vessel with diffuse coronary artery disease

6. Determination of prognosis after stent deployment

7. Assessment of stenosis in patients with previous (non-acute) myocardial infarction

8. Assessment of lesions in patient with treated unstable angina pectoris

9. Assessment of the collateral circulation

(NOTE: For STEMI, not useful for acute IRA lesion assessment <6 days after MI)

Coronary Doppler flow

1. Assessment of Microcirculation

2. Endothelial function testing

3. Myocardial viability in acute myocardial infarction

Combined coronary pressure and Doppler flow velocity

1. Assessment of intermediate stenosis

2. Assessment of the microcirculation


Contraindications


The contraindications to physiologic measurements are few. According to current guidelines, no physiologic measurement (FFR/CVR) is needed for clinical decisions when the clinical, angiographic and objective ischemia markers are concordant for the diagnosis. Other contraindications include the inability to use anticoagulation for angioplasty sensor wire placement, unstable clinical syndromes (relative contraindication) and unsatisfactory hemodynamic recording equipment [2].


Equipment


Intracoronary physiologic measurements are made with standard angioplasty equipment and techniques. Doppler flow velocity can be measured using a Doppler-tipped angioplasty guidewire. This guidewire is a 175 cm long, 0.014 in. diameter, flexible, steerable wire with a piezoelectric ultrasound transducer integrated into the tip (FloWire; Volcano Therapeutics, Del Mar, CA)

Several companies make pressure wire/catheter products. Unique handling characteristics arise from special construction of the device. Current pressure wire sensors are either piezo-electric or optical. Pressure wires also differ from regular workhorse wires having to incorporate the thin wires or optical fibers that transmit the pressure signals (Fig. 29.2).

A111453_1_En_29_Fig2_HTML.jpg


Fig. 29.2
Comparisons of available pressure wires. Sensor Wire/Catheter Construction and special features of current pressure wire and microcatheters systems for FFR. Left standard wire core surrounded by thin transmission and ground wires for piezo-resistive transducer signal. Center shape of Rxi microcatheter. Right nitinol or cobalt chromium wire core around central optical fiber to transmit pressure signal. Increase in core dimension and concentricity produces increase torque. Piezo-electric wires have core wire (Steel, Thin, low torque) compared to optical wires with hollow wire (nitinol or Cobalt Chromium, larger, high torque. From Kern MJ. Cath Lab Digest, May 2016)


Technique


After diagnostic angiography or during angioplasty, the sensor guidewire is passed through an angioplasty Y-connector attached to a diagnostic or guiding catheter. (Note: Side holes in large diameter guiding catheters have been used to alleviate catheter related partial obstruction of the coronary ostium. Side hole guiding catheters required an approximate doubling of the intracoronary (IC) adenosine dose due to loss of drug from the side holes during instillation.) Intravenous (IV) heparin 40–70 units/kg and IC nitroglycerin (100–200 μg) are given several minutes before the guidewire is advanced into the artery.

For flow velocity, the sensor tip is advanced at least 5–10 artery-diameter lengths (>2 cm) beyond the stenosis to measure velocity in a region of re-established laminar flow. Resting flow velocity data are recorded. Induction of coronary hyperemia by IC or IV adenosine is performed, continuously recording through peak hyperemic flow velocity. Coronary flow velocity reserve, CFVR is computed as maximal hyperemic to basal average peak velocity (APV). Poor Doppler signal acquisition may occur in 10–15 % of patients even within normal arteries. Like transthoracic echo Doppler studies, the operator must adjust the guidewire position (sample volume) to optimize the velocity signal [3].

For translesional pressure (FFR) measurements, the wire pressure is first matched to the guide catheter pressure in the central aortic location, and then the wire is advanced into the artery beyond the stenosis. Baseline pressure is recorded, followed by induction of coronary hyperemia with IC or IV adenosine, continuously recording both guide catheter and sensor-wire pressures. FFR is computed Pressuredistal/Pressureaorta at maximal hyperemia. Pressuredistal is recorded from the pressure wire, Pressureaorta is recorded from the guide catheter that delivers the pressure wire. Pressure signal artifacts may be reduced by careful attention to technique.

Stenosis severity should always be assessed using measurements obtained during maximal hyperemia. Adenosine is the most common agent for hyperemia. It has a short half-life, with a return to basal flow within 30–60 s after cessation of infusion. IV and IC adenosine are very well tolerated with ~10 % drop in mean arterial pressure but may be accompanied by short lived symptoms of dyspnea or chest burning. Although transient, AV block may rarely occur at higher IC doses in the RCA. IV adenosine uses weight-adjusted dosing (140 mcg/kg/min), and is required for the evaluation of ostial lesions or for the assessment of diffuse disease during pullback recordings. Compared to IC, IV administration has a higher incidence of side effects such as flushing, chest tightness, bronchospasm, nausea, and transient AV block or bradycardia. IC adenosine doses that produced maximal hyperemia equivalent to IV adenosine are 50–100 mcg for the right coronary artery, 100–200 mcg for the left coronary artery produces [4].

Alternative to adenosine includes Regadenson, an α2A adenosine receptor agonist that induces coronary vasodilatation and increased myocardial blood flow in a manner reportedly equivalent to adenosine with fewer adverse effects and IC nitroprusside. Regadenoson has a half-life of 2–3 min in the initial phase, 30 min in the intermediate phase and 2 h in the terminal phase. It is administered as single intravenous bolus (0.4 mg), and thus may be easier to use, but its cost and prolonged effect may complicate the measurement of multiple lesions or arteries [5].

IC nitroprusside can be an alternative to IC adenosine. Serial doses of IC nitroprusside (boluses of 0.3, 0.6, and 0.9 mcg/kg) [6] produced equivalent coronary hyperemia with a longer duration (about 25 %) compared with IC adenosine. IC nitroprusside (0.9 mcg/kg) decreased systolic blood pressure by 20 % with minimal change in heart rate, whereas IC adenosine had no effect on these parameters. IC nitroprusside, in doses commonly used for the treatment of the no-reflow phenomenon, can produce coronary hyperemia suitable to measure FFR without detrimental systemic hemodynamics.

A summary of pharmacologic hyperemic agents for cath lab measurements is provided in Table 29.2.


Table 29.2
Hyperemic agents used for coronary physiology assessment































Agent

Route

Dose

Comments

Adenosine

IV infusion

140 μg/kg per min

Reference standard. Side effects include dyspnea and chest pain. Prolonged hyperemia allows pressure wire pullback

Adenosine

IC bolus

>100 μg

Easy to use, inexpensive, no significant side effects. Transient heart block at high doses. Hyperemia lasts only 10–15 s

Adenosine

IC infusion

240–360 μg/min

Inconvenient set-up. Fewer side effects compared with IV infusion. Prolonged hyperemia allows pullback. Not well-validated

Regadenoson

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Nov 3, 2017 | Posted by in CARDIOLOGY | Comments Off on Coronary Blood Flow Measurements

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