A 73-year-old man with history of hypertension and osteoarthritis is evaluated for gradually increasing dyspnea over the preceding 6 weeks. He takes metoprolol for hypertension and naproxen for the arthritis. He has occasionally awakened in the night with mild dyspnea relieved by sitting up but has not noticed edema. Physical examination shows BP of 148/94, HR 96, and RR 16. O2 saturation is 92%. Neck veins show the jugular column 7 cm above the sternal angle. Lung examination reveals mild basilar crackles but no wheezing. Cardiac examination shows sustained apex impulse, S4 gallop, and no murmur. There is no peripheral edema. ECG shows stable left ventricular hypertrophy; no Q waves are seen. Chest x-ray shows increased interstitial markings and some cephalization of flow to the upper lobe vessels. The cardiac silhouette is boot-shaped, but there is no definite cardiomegaly. Echocardiogram shows left ventricular hypertrophy and LV ejection fraction of 55% (normal 50%-70%). What is the likely pathogenesis of this patient's dyspnea?