Pulmonary vascular effects of verapamil

Calcium channel blockers and pulmonary hypertension

In vascular smooth muscle from the pulmonary circulation, potassium evoked contractions are abolished in calcium-free medium and by calcium channel blockers (CCB). Noradrenaline, histamine, and serotonin induced contractions are partly resistant. Pulmonary arterial hypertension may occur both as a primary disorder and secondary to cardiac and pulmonary diseases; in both types there may be a component of pulmonary arterial vasoconstriction. In animal models, hypoxic pulmonary hypertension is counteracted by CCBs, nifedipine being particular effective. In patients with this disorder, CCBs also seem able to lower pulmonary arterial pressure and vascular resistance, but no controlled trials documenting their clinical efficacy have been performed. This is valid also for primary pulmonary hypertension, where some of the CCBs may have clinical value.

Some extracardiac effects of diltiazem and other calcium entry blockers

Calcium entry blockers have a well documented relaxing effect of smooth muscle, vascular as well as non-vascular. Mainly as a consequence of this action, the drugs have been used for treatment of several non-cardiac disorders where hyperactivity of smooth muscle is considered to have an important role in the pathogenesis. In this short review some of these extracardiac effects of calcium entry blockers are discussed and also their clinical application.

Hypoxia and pulmonary vascular endothelium

The steady development of pulmonary hypertension in the rat between Days 1 and 14 of exposure to hypoxia is based largely on structural remodelling of the pulmonary arterial circulation. The changes induced by hypobaric hypoxia (380 Torr; PaO2 of 40 mmHg) are different in the pre-acinar muscular arteries from the intra-acinar (alveolar wall) arteries, including the effect on the endothelial cell. Quantitative analysis of normal lungs has revealed structural features peculiar to the pulmonary microcirculation. The precapillary arterial unit that lies within the alveolar region includes (i) a non-muscular region and (ii) a partially muscular region as well as (iii) a muscular region. The endothelial cells in these segments or 'compartments' appear different from each other in the normal animal and behave differently in disease. In the various segments the endothelial cell has different neighbours and there are differences in its boundary markers (the basement membrane and elastic lamina). In the normal precapillary arterial unit the neighbour may be a pericyte, intermediate cell or smooth muscle cell. The pericyte and intermediate cell are precursor smooth muscle cells that under the influence of hypoxia develop into mature muscle.

Calcium channel blockers and cardiac surgery

Calcium channel blockers have an important role in the pharmacotherapy of cardiovascular disorders. These agents act by inhibiting the slow inward current into excitable cells, exert direct negative inotropic, chronotropic, and dromotropic activity, and are potent vasodilators. These direct effects are modified by reflex autonomic stimulation and by pathologic states. Serious adverse effects of the calcium channel blockers are most frequently observed in patients with ventricular dysfunction, conduction system disease, or concomitant beta blockade. Calcium channel blockers are indicated in the treatment of angina pectoris, supraventricular arrhythmias, and hypertension. The use of these agents in patients with hypertrophic cardiomyopathy, congestive heart failure, and pulmonary hypertension is investigational. The calcium channel blockers are gaining increased importance in the management of patients undergoing cardiac surgery. Verapamil is indicated for the treatment of post-cardiac-surgical atrial flutter and fibrillation; however, the calcium antagonists are not effective as prophylaxis against postoperative supraventricular arrhythmias. Laboratory studies have shown that drug interactions exist between calcium channel blockers and inhalational anesthetics and nondepolarizing neuromuscular blocking agents; clinical studies have demonstrated that these interactions are rarely significant. Perioperative coronary spasm can be effectively treated with the calcium channel blockers. The timing of calcium antagonist withdrawal prior to surgery is controversial, but continuation of therapy until surgery is usually safe. The clinical significance of platelet function inhibition by the calcium antagonists is unknown. Protection of ischemic myocardium by calcium channel blockers has been demonstrated. Important interactions between the calcium antagonists, hypothermia, and the ionic constituents of cardioplegia require further study before the role of these agents as adjuncts to clinical cardioplegia is defined. Expanded indications and the introduction of new calcium channel blockers will result in increased use of these agents in the future.

Calcium channel blockers in hypoxic pulmonary hypertension

Hypoxia is the major cause of pulmonary hypertension and right ventricular hypertrophy in chronic obstructive pulmonary disease, cystic fibrosis, kyphoscoliosis, chronic mountain sickness, and the obesity-hypoventilation and sleep apnea syndromes. Pulmonary hypertension develops in these patients because the long-standing vasoconstriction produced by hypoxia causes muscular hypertrophy of the pulmonary arteries and arterioles. These pathologic changes may regress if alveolar hypoxia is corrected and hypoxic pulmonary vasoconstriction is continuously inhibited. Intermittent inhibition of hypoxic pulmonary vasoconstriction does not reverse these pathologic changes. Since patient noncompliance with oxygen therapy makes it difficult to achieve continual relief of alveolar hypoxia, a drug that inhibits hypoxic vasoconstriction may be useful. Experimental findings indicate that hypoxic pulmonary vasoconstriction requires calcium influx and can be inhibited by certain slow-channel calcium blockers. Studies also demonstrate that slow-channel calcium antagonists can attenuate the pulmonary hypertension and right ventricular hypertrophy produced in rats by chronic hypoxia. Recently, two studies have shown that nifedipine inhibits hypoxic pulmonary vasoconstriction in patients with chronic obstructive pulmonary disease. If further studies demonstrate that these short-term effects are sustained, certain slow-channel calcium blockers may become a useful adjuvant to low-flow oxygen therapy in the treatment of hypoxic pulmonary hypertension.

Therapeutic application of calcium-channel antagonists for pulmonary hypertension

Calcium-channel antagonists may provide an effective approach to the treatment of pulmonary hypertensive disorders. Biochemical evidence suggests that pulmonary vasoconstriction results from the transmembrane flux of calcium into vascular smooth muscle; accordingly, the pulmonary pressor responses in experimental hypoxic pulmonary hypertension can be attenuated by verapamil and nifedipine. In patients with chronic obstructive lung disease, nifedipine decreases pulmonary artery pressures and pulmonary vascular resistance in proportion to the severity of hypoxemia before treatment. However, little pulmonary vasodilator effect is seen when hypoxemia is corrected by inhalation of oxygen, and systemic arterial oxygen desaturation can occur after nifedipine in patients breathing room air; most importantly, long-term studies in patients with chronic lung disease are lacking. In selected patients with primary pulmonary hypertension and other obliterative diseases of the pulmonary vasculature, nifedipine produces short- and long-term hemodynamic improvement at rest and during exercise, and these benefits are frequently paralleled by amelioration of dyspnea and fatigue. However, in patients in whom right ventricular function has been severely compromised by chronic pressure overload, both verapamil and nifedipine may exert notable depressant effects on right ventricular performance, despite the decrease in right ventricular afterload that would be expected to accompany a decrease in pulmonary vascular resistance. These negative inotropic actions may result in serious deleterious clinical reactions. Although calcium-channel antagonists represent a promising approach to the management of patients with pulmonary hypertension, the long-term efficacy and safety of these drugs in this disorder remain to be established.

Adverse hemodynamic and clinical effects of calcium channel blockade in pulmonary hypertension secondary to obliterative pulmonary vascular disease

The hemodynamic and clinical responses to calcium channel blockade with verapamil and nifedipine were compared with those of hydralazine in 12 patients with pulmonary hypertension secondary to obliterative pulmonary vascular disease. All three drugs produced a marked and similar decrease in pulmonary vascular resistance; however, this was accompanied by a significant increase in cardiac index with hydralazine (+0.71 liter/min per m2, p less than 0.01), no change in cardiac index with nifedipine and a significant decrease in cardiac index with verapamil (-0.25 liter/min per m2, p less than 0.05). Mean pulmonary artery pressure decreased markedly with both calcium channel blocking drugs (-16.0 mm Hg with verapamil and -14.5 mm Hg with nifedipine, both p less than 0.01), but this was associated with a concomitant increase in mean right atrial pressure (+6.2 mm Hg with verapamil and +4.4 mm Hg with nifedipine, both p less than 0.01); neither variable changed after hydralazine. Hence, right ventricular performance (as reflected by right ventricular stroke work index) deteriorated during treatment with both calcium channel blocking drugs, despite the decrease in resistance to right ventricular ejection; in contrast, right ventricular stroke work index increased after hydralazine. The unfavorable hemodynamic effects of calcium channel blockade were accompanied by severe adverse clinical events, including profound hypotension and cardiogenic shock during acute drug administration and the exacerbation of right heart failure during long-term treatment. These deleterious responses to verapamil and nifedipine are likely the result of a direct depressant effect exerted by these drugs on right ventricular function independent of their pulmonary vasodilatory actions.

Detrimental effects of verapamil in patients with primary pulmonary hypertension

Calcium channel blockade provides a logical approach to the treatment of pulmonary hypertension because these drugs exert direct vasodilator effects in the highly constricted pulmonary circulation. To determine the effectiveness of verapamil in the treatment of primary pulmonary hypertension the haemodynamic effects of the drug were evaluated in seven patients with this disorder; 10 mg was given intravenously to six patients and 120 mg orally to one patient. Verapamil produced a 20% decline in pulmonary vascular resistance and a 27% decrease in mean pulmonary arterial pressure without significant changes in systemic vascular resistance. One patient who received verapamil 480 mg orally daily for three months showed sustained haemodynamic and clinical improvement. Concomitant with its beneficial effects on the pulmonary circulation, however, verapamil produced a pronounced decrease in right ventricular stroke work index (42%) and increase in right ventricular filling pressure (50%), indicating a direct depressant effect of the drug on right ventricular function. In one patient these cardiodepressant effects were sufficiently pronounced to produce severe hypotension and cardiac arrest. In conclusion, although verapamil appears to exert preferential vasodilator effects on the pulmonary circulation, its negative inotropic effects may be particularly detrimental to patients with primary pulmonary hypertension who have pre-existing right ventricular dysfunction; hence, treatment with verapamil is not recommended in such cases.

Effects of verapamil on pulmonary haemodynamics during hypoxaemia, at rest, and during exercise in patients with chronic obstructive pulmonary disease

The haemodynamic effects of intravenous verapamil at rest, during hypoxaemia, and during progressive exercise were evaluated in 10 patients with chronic obstructive lung disease. Verapamil produced significant decreases in the peak heart rate and systemic blood pressure during exercise but exercise capacity and pulmonary gas exchange at exhaustion were unaffected. There were no significant changes in pulmonary artery pressure or total pulmonary vascular resistance during exercise or during the breathing of either air or a hypoxic gas mixture at rest. No clinically useful benefit was found with verapamil in the dosage used in this group of patients and the value of calcium antagonists in the treatment of patients with chronic obstructive lung disease requires further clarification.

Hypoxia-induced cardiac hypertrophy in rabbits treated with verapamil and nifedipine

Young rabbits were exposed, eight at a time, to 310 h of hypoxia (O2 at 70-80 torr), at atmospheric pressure. The animals were injected with 1 mg kg-1 nifedipine (F) or 5 mg kg-1 verapamil (V) or an equivalent volume of the vehicle (H) (Cremophor EL), i.p. twice a day. A fourth group (N), also injected with vehicle, was not made hypoxic. The animals were from 6 litters, 6 rabbits in each litter, and were distributed so that every group had litter mates in the other groups. Right ventricular hypertrophy was induced in all the hypoxic groups (H, +39%; V, +46%; F, +44%). Differences between groups were not statistically significant, but all were significantly hypertrophied relative to their normoxic litter mates (N). The right atria were less hypertrophied (H, +3.6%; V, +20%; F, +21.6%), but there was no left ventricular or left atrial hypertrophy. There was also a small increase in haematocrit in the hypoxic groups (H, +20.6%; V, +17.5%; F, +28.8%). The doses administered were equivalent to the highest used clinically producing blood levels of verapamil and nifedipine within or above the clinical range and had no effect on the development of cardiac hypertrophy.