Two points of measurement are required; one in the left ventricle and one in the aorta.
The ventricular and aortic pressures can be measured as follows:
1.
A catheter placed retrograde into the ventricle. This may be either:
(a)
A dual lumen Langston catheter with one “pigtail” in the left ventricle and the other “pigtail” in the aorta (Fig. 5.1).
Fig. 5.1
Simultaneous left ventricular and aortic pressure using a dual lumen Langston catheter revealing severe aortic stenosis: Peak Gradient: 89 mmHg, Mean Gradient: 70 mmHg, Cardiac Output: 6.3 l/min, Aortic valve area: 0.65 cm2
(b)
Single catheter in the ventricle that is pulled back into the aorta (Fig. 5.2).
Fig. 5.2
Simultaneous left ventricular and aortic pressure using a dual lumen Langston catheter followed by a pull back gradient revealing severe aortic stenosis
(c)
Catheter or pressure wire in the ventricle and
(i)
A side arm of the arterial sheath in the common femoral / iliac artery .
(ii)
A side arm of the arterial sheath placed in the descending aorta .
(iii)
A catheter placed in into another access site and positioned in the aorta.
(d)
Micro manometer catheter in the ventricle and one in the aorta
2.
A catheter placed via direct apical puncture and a catheter in the aorta.
3.
A catheter placed via trans–septal puncture in the left ventricle and a catheter in the aorta.
General Considerations in Invasive Pressure Gradient Assessment
Certain physiological phenomena and measurement errors may lead to over- or under- estimation of the invasively measured gradient.
1.
Utilizing the peripheral circulation (iliac arteries), as compared to the central aortic pressure, in the presence of peripheral arterial disease as distal aortic or iliac artery stenosis, will lead to an overestimation of the aortic valve gradient.
2.
Utilizing the peripheral circulation (iliac arteries), as compared to the central aortic pressure leads to a time delay of 80–120 ms compared to the peak aortic pressure waveform. In a classic study by Folland et al. in 26 patients, utilizing the ventricular and femoral artery pressure lead to an overestimation of the ventricular-aortic pressure gradient by 9 mmHg. On the other hand, phase – shift of the femoral arterial pressure tracing to align with the ventricular pressure lead to an underestimation of the ventricular-aortic pressure gradient by an average of 10 mmHg. Averaging of the unaligned and aligned ventricular-arterial pressure gradient may be used as a closer value to the true trans-aortic gradient [4] (Fig. 5.3).
Fig. 5.3
A case from the study by Folland et al. demonstrating a mean LV-aortic gradient in a patient of 31 mmHg (A), an unaligned with time delay LV-arterial mean gradient of 37 mmHg (B), an aligned LV-arterial mean gradient of 22 mmHg (C)
3.
Additionally, the peak arterial pressure obtained using the femoral artery may be higher than the aortic pressure due to amplification. This is pressure amplification occurs mainly in elderly patients with calcified blood vessels and results from pressure wave form moving into small diameter conduits with faster velocity and decrease arterial compliance. Amplification leads to an underestimation of the pressure gradient (Fig. 5.4) [3].
Fig. 5.4
The pressure gradient in the same patient using the ventricular-aortic pressure waveform (ΔPPPG 68 mmHg & ΔPmean 43 mmHg) (Panel a) is higher than the pressure gradient obtained using the ventricular arterial pressure waveform (ΔPPPG 53 mmHg & ΔPmean 34 mmHg) (Panel b). This occurs due to amplification noted in the peripheral circulation leading to underestimation of the pressure gradient
4.
In severely stenotic aortic valves, the mere presence of a catheter across the aortic valve may decrease the peripheral arterial pressure by 10 mmHg that is recovered after the catheter is withdrawn from the ventricle (Carabello ’ s sign). This overestimates the severity of aortic valve stenosis, but is highly specific for the presence of severe artic valve stenosis. Carabello noticed an increase in peripheral arterial pressure of 10 mmHg when the catheter was withdrawn from the left ventricle across a severely stenotic aortic valve (<0.6 cm2). This was shown to be 75 % sensitive and 100 % specific for severe aortic stenosis in one study [3]
5.
Pull back gradients may be affected by catheter whip, bounce effects, ectopic beats, and respiratory variations
6.
Catheter site – placement in either the ventricular cavity or the ascending aorta can alter the measured pressure gradients. As such, placing the ventricular catheter in the ventricular outflow tract as opposed to the LV apex may underestimate the gradient by as much as 30 mmHg. Conversely, placing the aortic catheter too close to the aortic valve may overestimate the aortic valve gradient, while placing it more distally (after pressure recovery) provides a lower gradient that is more reflective of the true hemodynamic burden (ΔPnet) [3].
7.
Proper transducer calibration, ensuring the absence of bubbles within the system, and utilizing dual catheters or a dual lumen catheter placed in the ventricle and ascending aorta provide the most accurate assessment of trans aortic valve gradients.
8.
Common artifacts in pressure gradient may include
(a)
An underestimated gradient may occur when one or more of the side holes of the pigtail is in the aorta and the others are in the ventricle
(b)
When two catheters are used and with pull back there is disparity of diastolic pressures, sheath damping is suggested
(c)
Ringing refers to artifacts from the transducer chambers, catheters, pressure tubing, or manifolds that confound waveform analysis and appear as an overshoot of the pressure waveform and also know as under-damping . Over–damping may occur when pressure waveforms are obtained after the pigtail is filled with contrast [3]
(d)
Utilizing the side arm of a sheath with the same caliber of the catheter (six french catheter in a six french sheath) placed in the iliac arteries or abdominal aorta will lead to a dampened pressure waveform.
Invasive Measures of Aortic Valve Areas
In their original article in 1951, Richard Gorlin (a cardiologist) and his father, an engineer, published an equation to invasively derive the mitral valve area. This equation has been extended to derive areas of other cardiac valves (including the aortic valve area) as well as cardiac shunts [5].
Since its inception, the equation has undergone rigorous review and critique and suggested modifications and/or simplifications, the most famous of which is the simplified equation proposed by Hakki et al. in 1981 [6].
Gorlin Equation
The Gorlin equation was derived from the “rounded edge” orifice or short tube hydraulic system model. This model was selected since it approximates flow across stenotic orifices where kinetic energy losses are very high with pressure losses are mainly and rapidly dissipated in conversion to velocity as opposed to pressure losses that occur mainly due to friction forces (as occurs with the large wetted perimeter hydraulic model). As such, the Gorlins deducted that resistance calculated by Poiseuille’s law would not be an accurate gauge of stenosis severity [5].
Two equations were the basis of the derived Gorlin equation; the first was the relationship of flow to area and velocity; Torricelli’s law and the second was the conservation of energy law where in a closed circuit, the kinetic energy equals the pressure or potential energy.
These equations are highlighted as follows:
1.
Torricelli’s law:
Where F = Flow, A = Area, V = Velocity, CO = coefficient of orifice contraction dictating the ratio of the stream contraction area to that of the orifice area.
2.
Law of Conservation of Energy: Since energy can only be transferred from one form to the other, kinetic energy = potential energy. Hence,
m = mass
CV = coefficient of velocity (only a portion of pressure is converted to velocity and the rest is lost as friction and turbulence).
g = gravitational acceleration constant = 980 cm/s/s.
h = pressure in height of a given fluid above orifice.
If we rearrange the equations, we come up with the equation
3.
A = Area
F = blood flow rate through the orifice when it is open.
C is an empirical constant (discharge coefficient) that accounts for both CO and CV and also incorporates the conversion of pressure from mmHg to cm of H2O (and a correction for the equation used to calculate the diastolic filling period in the mitral valve cases).
h = P1–P2 = pressure gradient across the orifice.
The Gorlin equation for calculating the aortic valve area is:
CO = Cardiac Output.
C = Empirical constant (discharge coefficient) and has a value of 1 in calculating aortic valve area
HR = Heart rate
SEP = systolic ejection period
P1–P2 = mean pressure gradient across the aortic valve
The Gorlin equation was initially created to provide an estimate of the GOA and as such, included a constant for the coefficient of contraction (C o ) that relates the jet stream area at the vena contracta (EOA) to the GOA. In addition to the C o , the Gorlin equation included a velocity conversion factor or coefficient of velocity (C v ) whereby only a certain fraction of pressure is converted to velocity and the rest is dissipated as losses due to viscous friction losses, turbulence, and so forth. It relates the space average velocity of the formed jet stream to its velocity profile.
If there is a relatively flat velocity profile through the stenosis, the maximal velocity measured = space average velocity and Cv = 1. C0 and Cv are both included in the empiric constant C (discharge coefficient ) that also empirically incorporates the conversion factor of pressure units from mmHg to cm H 2 O, the conversion factor for blood density. The final constant C was estimated at 0.7 and later adjusted to 0.85 for the mitral valve area calculation (it also included a correction for the method used to estimate the diastolic filling period for patients with mitral stenosis). The empiric constant C was further assumed to have a value of 1 for aortic valve area calculation [7, 8].
In reality, the continuity equation and the Gorlin equation are based on similar hydrodynamic principles and as such should both provide a measure of the EOA and not the GOA. Although the EOACath was initially set up as a measure of the GOA, inherent errors in the Gorlin equation render the catheter-derived area more a reflection of the EOA rather than the GOA. Dumesnil and Yoganthan described two small errors in the Gorlin equation that explain why the equation measures an estimate of the EOA rather than that of the GOA.
1.
The first error is the use of mean flow instead of square root of the mean flow squared (the square root of the average of the squared instantaneous flow) as the mean flow is slightly greater than the other value and thus leads to an inherent small underestimation of the AVA.
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