Vasodilators in Acute Heart Failure: Review of the Latest Studies

Vasodilators play an important role in the management of acute heart failure, particularly when increased afterload is the precipitating cause of decompensation. The time-honored approach to afterload reduction has been largely focused on use of intravenous nitrovasodilators and, when properly dosed, this class of agents does provide substantial symptom relief for patients with acute hypertensive heart failure. Despite this, nitrovasodilators have never been shown to diminish mortality or provide any post-discharge outcome benefit leading to an on-going search for viable and more effective alternatives. While no new vasodilators have been approved for use in acute heart failure since nesiritide more than a decade ago, a number of novel agents have been developed, with some showing significant promise in recent clinical trials. In this review, we summarize the latest study data as it relates to vasodilator therapy and provide a glimpse into the not too distant future state of acute heart failure care.

Acute Heart Failure Treatment

Dyspnea is the predominant symptom for patients with acute heart failure and initial treatment is largely directed towards the alleviation of this. Contrary to conventional belief, not all patients present with fluid overload and the approach to management is rapidly evolving from a solitary focus on diuresis to one that more accurately reflects the complex interplay of underlying cardiac dysfunction and acute precipitant. Effective treatment thus requires an understanding of divergent patient profiles and an appreciation of various therapeutic options for targeted patient stabilization. The key principle within this paradigm is directed management that aims to diminish the work of breathing through situation appropriate ventillatory support, volume reduction and hemodynamic improvement. With such an approach, clinicians can more efficiently address respiratory discomfort while reducing the likelihood of avoidable harm.

Management of systemic and pulmonary hypertension

The major therapeutic approach to systemic and pulmonary hypertension and vasospasm in cardiac surgery patients involves the use of parenteral agents that reverse systemic vasoconstriction and produce vasodilation. Potential pharmacologic approaches include 1) alpha1-adrenergic receptor blockers, ganglionic blockers, and calcium channel blockers; 2) central alpha2-adrenergic receptor agonists, dopamine1-adrenergic receptor agonists, potassium channel modulators, and vascular cyclic nucleotide stimulators; 3) phosphodiesterase enzyme inhibitors, and 4) angiotensin-converting enzyme inhibitors. Of the currently available intravenous vasoactive therapies, the mainstay agents are the nitrovasodilators and the dihydropyridine-type calcium channel blockers. The nitrovasodilators, a diverse group of drugs that achieve vascular relaxation by stimulating cyclic nucleotides and thereby releasing nitric oxide, include nitroglycerin and sodium nitroprusside. Although these drugs are useful, rapid development of tolerance is a drawback to nitroglycerin, while nitroprusside can cause coronary steal and increase intracranial pressure. Intravenous dihydropyridine-type calcium channel blockers inhibit mechanical responses of cardiac muscle and vascular smooth muscle by blocking inward calcium currents. Nicardipine is an arterial specific vasodilator. Treatment for vasospasm is usually empiric; pharmacologic options include nitroglycerin, but dihydropyridine calcium channel blockers and phosphodiesterase inhibitors should also be considered.

Mechanisms of nitric oxide generation from nitroglycerin and endogenous sources during hypoxia in vivo

Nitroglycerin (GTN), often used in conditions of cardiovascular ischaemia, acts through the liberation of nitric oxide (NO) and the local concentration of NO in the tissue is responsible for any biological effect. However, little is known about the way in which the concentration of NO from GTN and other NO-donors is influenced by low oxygen tension in the target tissues. To evaluate the impact of changes in oxygen tension in the metabolism of NO-donors we measured exhaled NO in anaesthetized rabbits in vivo and expired NO and perfusate nitrite (NO(2)(-)) in buffer-perfused lungs in situ. The impact of acute hypoxia on NO formation from GTN, isosorbide-5-mononitrate (ISMN), dissolved authentic NO, NO(2)(-) and NO generated from endogenous NO-synthase (NOS) was studied in either model. Acute hypoxia drastically increased exhaled NO concentrations from all NO-donors studied, both in vivo and in the perfused lung. During similar conditions endogenous NO generation from NOS was strongly inhibited. The effects were most pronounced at less than 3% inspired oxygen. The mechanisms for the increased NO-formation during hypoxia seems to differ between GTN- and NO(2)(-)-derived NO. The former phenomenon is likely due to diminished breakdown of NO. In conclusion, hypoxic conditions preserve very high local NO concentrations generated from organic nitrates in vivo and we suggest that this might benefit preferential vasodilation in ischaemic tissue regions. Our findings point out the necessity to consider the influence of oxygen tension when studying the action of NO-donors.