Aortic valve area

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Lakshmi Gopalakrishnan, M.B.B.S. [2]; Usama Talib, BSc, MD [3]

Overview

The aortic valve area is the size of the orifice for blood to flow from the left ventricle to the aorta. The aortic valve area is reduced in aortic stenosis, and the aortic valve area is the metric that is used to gauge the need for aortic valve replacement surgery. The pressure gradient across a narrowed aortic valve cannot be used to gauge the need for valve replacement as the gradient may be low in patients with impaired left ventricular function.

Pathophysiology

The normal aortic valve offers little or no resistance to the blood flow across the valve despite the high flow velocities.[1] With progressive narrowing of the aortic valve (aortic stenosis), the aortic valve orifice offers progressively greater resistance to the blood flow through the valve with a consequent in the pressure gradient between the left ventricle and the aorta. The latter can be calculated using echocardiographic flow velocities while the trans-valvular pressure gradient can be calculated using the following equation:

Pressure Gradient = 4 x (velocity of blood through the valve)2 mm Hg

However, the absence of a large gradient across the aortic valve does not exclude the presence of critical aortic stenosis. Patients with left ventricular dysfunction can no longer generate a pressure gradient, and they are referred to as having low pressure, low flow, low ejection fraction aortic stenosis. Therefore, it is for this reason that the best measure of the severity of aortic stenosis is the aortic valve area and not the aortic valve gradient.

Cardiac Catheterization

  • Simultaneous measurement of left ventricular output (measures the flow through the aortic valve) and the pressure gradient across the aortic valve provides the variables that are required to calculate the aortic valve area and resistance.[2][3]
  • Fluid dynamic mediated subvalvular pressure gradients are often present in patients with severe aortic stenosis in the absence of an anatomic subvalvular obstruction and constitute ~50% of the total measured transvalvular gradient. The extent of increase in cardiac output during exercise is inversely related to the magnitude of subvalvular gradient.[1]

Aortic Valve Area

  • According to the current recommendations, following dobutamine infusion, if the aortic valve area increases to >1.2 cm2, and the mean pressure gradient rises above 30 mmHg, such patients may benefit from aortic valve replacement. Failure to achieve these improvements has shown to be associated with higher early surgical mortality in comparison to patients who can augment their contractility and gradient: 32-33% versus 5-7%, respectively. Additionally, 5-year survival was lower in patients who could not augment their contractility in comparison to those who could: 10–25% versus 88%, respectively.
  • Aortic valve area can be calculated by the following two equations:[4]

Aortic Valve Area (cms2) = (Stroke volume (mL/beat) ÷ Systolic ejection period (secs/beat)) ÷ (44.3 x square root of mean systolic pressure gradient between the left ventricle and aorta (mm Hg))

Aortic Valve Area (cms2) = (Cardiac output (liters/minute)) ÷ (Square root of mean systolic pressure gradient between the left ventricle and aorta (mm Hg))

Aortic Valve Resistance

  • Furthermore, aortic valve resistance is less flow-dependent than aortic valve area which is of particular benefit in patients with low output aortic stenosis.[5]
  • Aortic valve resistance can be calculated using the equation:[6][7]

Aortic Valve Resistance (dyne seconds per cms5) = { (Mean Pressure Gradient between the left ventricle and aorta (mm Hg) x Heart Rate (beats/min) x Systolic ejection period (secs/beat) ) ÷ Cardiac output } x 1.33

Echocardiography

  • The above two equations derived, aid in calculating the maximum pressure gradient that is obtained using the instantaneous aortic jet velocity that is assessed with doppler echocardiography; however, cardiac catheterization is required to calculate peak pressure gradient across the valve.

Aortic Valve Area

  • The continuity principle states that the flow in one area must equal the flow in a second area if there are no shunts in between the two areas.
  • Using doppler velocities, aortic valve area can be calculated using the following continuity pinciple:[11]

Aortic Valve Area (cms2) = { (Cross-sectional Area of LVOT x Time Velocity Integral across the LVOT) ÷ (Time Velocity Integral across Aortic Valve) }

  • The weakest aspect of this calculation is the variability in measurement of cross-sectional area of LVOT, because it involves squaring the LVOT dimension. Such variations in the aortic valve area derived using doppler velocities may be observed during exercise[9] or in conditions that increase the blood flow across the valve.[12][13]
  • Based on a study that simultaneously determined Gorlin formula and transesophageal echocardiography planimetry valve areas, demonstrated that acute changes in trans-valvular blood flow substantially altered valve area as calculated by the Gorlin formula but did not result in significant alterations of the anatomic valve area in aortic stenosis. This suggests that the flow-related variation in the Gorlin aortic valve area may be due to a disproportionate blood flow dependence of the formula itself and not a true change in valve area.[14] Therefore, the advantage of continuity equation over Gorlin formula is that the former is less susceptible to blood flow across the valve.

Aortic Valve Resistance

  • Doppler derived aortic valve resistance correlates well with catheterization derived aortic valve resistance and hence may provide an additional non-invasive parameter for the assessment of aortic stenosis severity.[15]

References

  1. 1.0 1.1 Laskey WK, Kussmaul WG (2001). "Subvalvular gradients in patients with valvular aortic stenosis: prevalence, magnitude, and physiological importance". Circulation. 104 (9): 1019–22. PMID 11524395. Retrieved 2012-04-12. Unknown parameter |month= ignored (help)
  2. Hirshfeld JW, Kolansky DM. Valve function: Stenosis and regurgitation. In: Diagnostic and Therapeutic Cardiac Catheterization, 2nd ed, Pepine CJ, Hill JA, Lambert CR (Eds), Williams & Wilkins, Baltimore 1994. p.443
  3. Carabello BA, Grossman W. Calculation of stenotic valve orifice area. In: Cardiac Catheterization and Angiography, 3rd ed, Grossman W (Ed), Lea and Febiger, Philadelphia 1986. p.143.
  4. James D. Thomas & Nicholas Furiasse (2016). "Exercise Testing in Paradoxical Low-Flow Aortic Stenosis: Where Is the Truth?". JACC. Cardiovascular imaging. doi:10.1016/j.jcmg.2016.05.012. PMID 27568123. Unknown parameter |month= ignored (help)
  5. 5.0 5.1 Cannon JD, Zile MR, Crawford FA, Carabello BA (1992). "Aortic valve resistance as an adjunct to the Gorlin formula in assessing the severity of aortic stenosis in symptomatic patients". Journal of the American College of Cardiology. 20 (7): 1517–23. PMID 1452925. Retrieved 2012-04-12. Unknown parameter |month= ignored (help)
  6. Badano L, Cassottano P, Bertoli D, Carratino L, Lucatti A, Spirito P (1996). "Changes in effective aortic valve area during ejection in adults with aortic stenosis". The American Journal of Cardiology. 78 (9): 1023–8. PMID 8916482. Retrieved 2012-04-12. Unknown parameter |month= ignored (help)
  7. Ford LE, Feldman T, Chiu YC, Carroll JD (1990). "Hemodynamic resistance as a measure of functional impairment in aortic valvular stenosis". Circulation Research. 66 (1): 1–7. PMID 2295132. Retrieved 2012-04-12. Unknown parameter |month= ignored (help)
  8. Bermejo J, Antoranz JC, Burwash IG, Alvarez JL, Moreno M, García-Fernández MA, Otto CM (2002). "In-vivo analysis of the instantaneous transvalvular pressure difference-flow relationship in aortic valve stenosis: implications of unsteady fluid-dynamics for the clinical assessment of disease severity". The Journal of Heart Valve Disease. 11 (4): 557–66. PMID 12150306. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  9. 9.0 9.1 Kadem L, Rieu R, Dumesnil JG, Durand LG, Pibarot P (2006). "Flow-dependent changes in Doppler-derived aortic valve effective orifice area are real and not due to artifact". Journal of the American College of Cardiology. 47 (1): 131–7. doi:10.1016/j.jacc.2005.05.100. PMID 16386676. Retrieved 2012-04-12. Unknown parameter |month= ignored (help)
  10. Otto CM (2006). "Valvular aortic stenosis: disease severity and timing of intervention". Journal of the American College of Cardiology. 47 (11): 2141–51. doi:10.1016/j.jacc.2006.03.002. PMID 16750677. Retrieved 2012-04-12. Unknown parameter |month= ignored (help)
  11. Baumgartner H, Hung J, Bermejo J, Chambers JB, Evangelista A, Griffin BP, Iung B, Otto CM, Pellikka PA, Quiñones M (2009). "Echocardiographic assessment of valve stenosis: EAE/ASE recommendations for clinical practice". Journal of the American Society of Echocardiography : Official Publication of the American Society of Echocardiography. 22 (1): 1–23, quiz 101–2. doi:10.1016/j.echo.2008.11.029. PMID 19130998. Retrieved 2012-04-13. Unknown parameter |month= ignored (help)
  12. Arsenault M, Masani N, Magni G, Yao J, Deras L, Pandian N (1998). "Variation of anatomic valve area during ejection in patients with valvular aortic stenosis evaluated by two-dimensional echocardiographic planimetry: comparison with traditional Doppler data". Journal of the American College of Cardiology. 32 (7): 1931–7. PMID 9857874. Retrieved 2012-04-13. Unknown parameter |month= ignored (help)
  13. Lester SJ, McElhinney DB, Miller JP, Lutz JT, Otto CM, Redberg RF (2000). "Rate of change in aortic valve area during a cardiac cycle can predict the rate of hemodynamic progression of aortic stenosis". Circulation. 101 (16): 1947–52. PMID 10779461. Retrieved 2012-04-13. Unknown parameter |month= ignored (help)
  14. Tardif JC, Rodrigues AG, Hardy JF, Leclerc Y, Petitclerc R, Mongrain R, Mercier LA (1997). "Simultaneous determination of aortic valve area by the Gorlin formula and by transesophageal echocardiography under different transvalvular flow conditions. Evidence that anatomic aortic valve area does not change with variations in flow in aortic stenosis". Journal of the American College of Cardiology. 29 (6): 1296–302. PMID 9137227. Retrieved 2012-04-13. Unknown parameter |month= ignored (help)
  15. Ho PP, Pauls GL, Lamberton DF, Portnoff JS, Pai RG, Shah PM (1994). "Doppler derived aortic valve resistance in aortic stenosis: its hemodynamic validation". The Journal of Heart Valve Disease. 3 (3): 283–7. PMID 8087265. Unknown parameter |month= ignored (help); |access-date= requires |url= (help)
  16. Bermejo J, García-Fernández MA, Torrecilla EG, Bueno H, Moreno MM, San Román D, Delcán JL (1996). "Effects of dobutamine on Doppler echocardiographic indexes of aortic stenosis". Journal of the American College of Cardiology. 28 (5): 1206–13. doi:10.1016/S0735-1097(96)00287-2. PMID 8890817. Retrieved 2012-04-13. Unknown parameter |month= ignored (help)


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