Nitroglycerin and afterload: effects of aortic compliance and capacity of the Windkessel

The effect of nitroglycerin (NTG) is mainly a reduction in preload and afterload. The decrease in afterload may be caused by a fall of total systemic resistance (TSR) or by an increase of arterial compliance (AC). The effects of NTG on TSR and AC were tested in 10 patients given 1.6 mg NTG sublingually. The capacity of the whole Windkessel (C) was calculated as C=τ/TSR (τ = time constant of the diastolic aortic pressure decay). The diameter of the descending thoracic aorta was measured from an aortogram. Aortic stiffness SAO) was calculated SAO = ΔP/ΔD. Since mean aortic pressure decreased by 6% after NTG without any change in cardiac index or heart rate, there had to be a primary reduction of afterload as measured from mean systolic resistance (−9%). This reduction of afterload could not be related to a decrease in TSR and SAO, C, however increased by an average of 27%. These data indicate that NTG decreases the muscular tone of postaortic muscular vessels and, hence, increases the Windkessel capacity, while aortic compliance does not change.