Mode of action of nitrglycerin in angina pectoris. Correlation between haemodynamic effects during exercise and prevention of pain.

Nitroglycerin isgenerallyagreed tobethemost effective drugavailable forthetreatment ofangina pectoris, butitsmodeofaction remains uncertain. Two mainexplanations havebeenputforwird. Thefirst suggests that itactsbylowering thearterial pressureandsoreducing cardiac work;thesecond thatitactsbydilating thecoronaryarteries andso improving themyocardial bloodsupply. Formany yearsthecoronarydilator action ofthedrughas beenwidely accepted astheprobable mechanism for itstherapeutic effect, anditsaction inreducing blood pressurehasbeenregarded asofsecondary importance, ifnotactually harmful. Attempts to demonstrate a beneficial effect on coronaryflowin patients withischaemic heartdisease, however, havemetwith no success,anditappearsthatsublingual nitroglycerin isunable toincrease themyocardial bloodsupply whenthecoronaryarteries are diseased (Gorlin etal., 1959; Bernstein etal., 1966). Thisfailure toconfirm thecoronarydilator theory hasledtorenewed interest inthepossibility that the drugactsbyreducing cardiac work,buttheevidencetosupportthisviewisinconclusive. Most studies oftheeffect ofnitroglycerin on thecirculatory responsetoexercise haveshownthat thedrug reduces arterial pressurebothinnormalsubjects and in patients withischaemic heartdisease (Eldridge etal., 1955;MullerandR0rvik, 1958; Christensson, Karlefors, and Westling, 1965; Najmi etal., 1967).Innoneofthese investigations, however, havethechanges inthecirculatory response beencorrelated withthealteration in capacity forexercise. Inconsequence,though itis generallyaccepted thatnitroglycerin doescause some attenuation ofthecirculatory responseto exercise, ithasbeenimpossible toassesstheextent