Aortic stenosis on the pressure-volume loop: what changes and why
Physiology — The Cardiac Cycle and Pressure-Volume Relationships
The question
A 76-year-old man has severe calcific aortic stenosis with a peak transvalvular gradient of 70 mmHg and a well-preserved ejection fraction. A left ventricular pressure-volume loop is recorded before surgery. Compared with a normal loop, which set of changes is most characteristic?
- A reduced peak systolic pressure with an unchanged stroke volume
- A wider loop with a markedly increased end-diastolic volume and little change in peak systolic pressure
- Loss of the isovolumetric contraction phase, so the loop has no vertical left-hand limb
- A taller loop, no wider than normal, with a raised end-diastolic pressure and a prolonged isovolumetric contraction phase
- A loop displaced to the right with an increased end-systolic volume and a flatter end-systolic pressure-volume relationship
The answer
A taller loop, no wider than normal, with a raised end-diastolic pressure and a prolonged isovolumetric contraction phase
A reduced peak systolic pressure with an unchanged stroke volume — The ventricle must generate a higher pressure to eject blood through the stenotic valve, not a lower one. The gradient given in the stem represents the difference between ventricular and aortic systolic pressure. That is the defining feature of the lesion.
A wider loop with a markedly increased end-diastolic volume and little change in peak systolic pressure — This describes volume overload, as seen in chronic aortic or mitral regurgitation. In aortic stenosis the haemodynamic burden is one of pressure rather than volume, and stroke volume remains preserved until the ventricle eventually decompensates.
Loss of the isovolumetric contraction phase, so the loop has no vertical left-hand limb — This is the characteristic signature of mitral regurgitation. The ventricle ejects backwards into the atrium from the moment contraction begins, eliminating the closed-chamber phase during which pressure would otherwise build. In aortic stenosis, isovolumetric contraction is not lost; it is prolonged.
A taller loop, no wider than normal, with a raised end-diastolic pressure and a prolonged isovolumetric contraction phase — Correct. The ventricle must generate a systolic pressure well above aortic pressure to eject through the narrowed valve, which raises the top of the loop and stretches the isovolumetric contraction limb. Concurrently, the hypertrophied, stiff ventricle requires a higher filling pressure for any given volume, elevating the end-diastolic pressure point.
A loop displaced to the right with an increased end-systolic volume and a flatter end-systolic pressure-volume relationship — A rightward shift with a flatter end-systolic pressure-volume relationship (ESPVR) describes impaired contractility, as seen in dilated cardiomyopathy. The stem specifies a preserved ejection fraction. Compensated concentric hypertrophy produces a steeper ESPVR, not a flatter one.
Explanation
The pressure-volume loop is read anticlockwise from end-diastole. The left vertical limb represents isovolumetric contraction (the mitral valve has closed and the aortic valve has not yet opened). The top of the loop represents ejection, ending when the aortic valve closes. The right vertical limb is isovolumetric relaxation. The bottom represents diastolic filling. The width of the loop corresponds to stroke volume. Its area represents stroke work. The two boundary relationships are the end-systolic pressure-volume relationship (whose slope reflects contractility) and the end-diastolic pressure-volume relationship (whose slope reflects diastolic stiffness).
Consider what aortic stenosis does to this loop. The aortic valve opens only when ventricular pressure exceeds aortic pressure, so a severe gradient means the ventricle must generate a systolic pressure far above what would be measured at the radial artery. The loop becomes taller: peak systolic pressure rises, the isovolumetric contraction limb lengthens, and because pressure work is metabolically expensive, the area of the loop (reflecting myocardial oxygen demand) increases substantially for the same cardiac output.
The ventricle adapts through concentric hypertrophy. Sarcomeres are added in parallel to normalise wall stress according to the Law of Laplace. This adaptation preserves ejection fraction and stroke volume for years, so the loop remains no wider than normal. However, there is a cost: diastolic compliance falls. The end-diastolic pressure-volume relationship steepens, and the ventricle requires a higher filling pressure to achieve an adequate end-diastolic volume. This is why atrial contraction contributes considerably more than the usual 15-20% of ventricular filling, and why loss of sinus rhythm can be haemodynamically catastrophic.
These two features together define the anaesthetic priorities, and the pressure-volume loop serves as a more effective aide-memoire than any list. A thick, stiff ventricle depends on adequate preload, so hypovolaemia and venodilatation are tolerated poorly. It depends on coronary perfusion pressure, because elevated systolic wall stress raises subendocardial oxygen demand while a fall in systemic vascular resistance reduces the diastolic pressure that supplies it. It depends on an appropriate heart rate, because tachycardia shortens both diastolic filling time and coronary perfusion time. And it depends on sinus rhythm for the atrial contribution to filling.
Tips for the candidate
The pressure-volume loop is most effectively learnt as four boundaries, with each valvular or myocardial lesion analysed by asking which boundary it displaces. Pressure overload pushes the top of the loop upward. Volume overload pushes the right side outward. Contractile failure flattens the ESPVR and shifts the loop to the right. Diastolic disease steepens the EDPVR and elevates the bottom. Most valve questions can be answered from this framework alone.
The most decisive discriminators are those that abolish an entire phase. Mitral regurgitation eliminates isovolumetric contraction (there is no closed chamber to contract against). Aortic regurgitation eliminates isovolumetric relaxation (the valve never forms a seal). If an option states that a vertical limb is absent, the candidate should identify which valve is incompetent. If the stem describes a stenotic valve, a missing-limb option is a distractor.
Acute perturbations should be distinguished from chronic disease. Questions about a vasopressor bolus or a fluid challenge ask the candidate to move one variable on an otherwise normal loop: preload widens it, afterload raises and narrows it. Questions about chronic disease concern remodelling, in which the boundary relationships themselves have shifted. Confusing the two is the most common source of error in this topic.
If the stem provides an ejection fraction, it should be used. A preserved ejection fraction with a large gradient indicates a compensated, tall, narrow loop. A reduced ejection fraction in the same patient indicates that decompensation has begun and the loop is widening and shifting to the right. That single datum often eliminates two options immediately.