Spontaneous ventilation and respiratory motor output during carbachol-induced atonia of REM sleep in the decerebrate cat

Microinjections of carbachol into the pons induce a state that resembles rapid eye movement (REM) sleep in intact cats and, in decerebrate, artificially ventilated cats, produce postural atonia accompanied by a powerful depression of the respiratory motor output. In this study, pontine carbachol was used in decerebrate, spontaneously breathing cats to assess the effects of mechanical and chemical respiratory reflexes on the magnitude and pattern of the carbachol-induced depression of breathing, and to determine whether the depression is altered in those animals in which rapid eye movements are present. Phrenic nerve activity and tidal volume were only transiently depressed at the onset of the carbachol-induced postural atonia, whereas the decrease in respiratory rate and the depressions of hypoglossal and intercostal activities persisted until the response was reversed by a pontine microinjection of atropine 15-101 minutes after the onset of carbachol response. Ventilation was reduced to 70% of control during the steady-state conditions. The irregularity of breathing, characterized by the inter-quartile ranges of the distributions of the peak phrenic nerve activity and respiratory timing, did not increase following pontine carbachol. Neither vagotomy nor vigorous eye movements were associated with increased breathing irregularity. This contrasts with the irregular breathing (with minor average changes in ventilation) typical of natural REM sleep. We propose that the carbachol-injected decerebrate cat provides a useful model of the depressant effects that neural events associated with REM sleep may have on breathing.

Serotonergic excitatory drive to hypoglossal motoneurons in the decerebrate cat

In decerebrate, paralyzed, vagotomized and artificially ventilated cats, serotonin (5-HT) and its analogues, microinjected into the hypoglossal (XII) motor nucleus, altered the activity of the genioglossal branch of XII nerve. 5-HT, carboxamidotryptamine maleate (5-CT) and DOI (1-5 mM) increased the activity by over 200%. Methysergide reversed this increase. Methysergide, mianserin, or ketanserin (100-250 nl, 1 mM) reduced the spontaneous hypoglossal activity by 20-50%. Buspirone, 8-OH-DPAT and (-)-propranolol were without effect. Thus, 5-HT provides a substantial tonic excitatory drive to XII motoneurons. The 5-HT receptors involved are likely to be type 1C or 2, but uncertainty regarding the affinity profiles of the drugs used in in vivo conditions in the cat precludes a definite identification.

The medullary projections of afferent bronchopulmonary C fibres in the cat as shown by antidromic mapping

1. The activity of eighty-seven bronchopulmonary vagal afferent neurones with unmyelinated axons (C fibres) was recorded extracellularly in the nodose ganglia of decerebrate, paralysed and artificially ventilated cats. On the basis of their response latencies following the right atrial injection of capsaicin or phenyldiguanide, the cells were classified as having their receptor endings within the reach of pulmonary (latency less than 3.5 s) or bronchial (latency above 3.5 s) circulation. 2. Pulmonary and bronchial receptor cells differed only slightly in their response characteristics (firing rate, burst duration) and the conduction velocity of their peripheral axons. Bronchial C fibres represented about 70% of the population studied. 3. The medullary distributions of the central branches of six pulmonary and six bronchial C fibres were determined by means of the antidromic mapping technique. The two receptor subtypes did not differ in their central projection patterns. 4. Rostral to the obex, the central branches of the bronchopulmonary C fibres were localized within the medial portions of the nucleus tractus solitarii (NTS) and area postrema, and were most densely distributed along the borders of the parvicellular subnucleus of the NTS. Caudal to the obex, the most dense branching was found in the dorsal portion of the commissural subnucleus. Projections to the contralateral NTS were found, but these were of a much lower density. 5. The central distribution of bronchopulmonary C fibres is compared to the projection patterns of vagal and glossopharyngeal afferents of other modalities that are involved in respiratory and cardiovascular control. This is discussed in relation to the concept of a modality-specific organization of the NTS.

Cholinergic stimulation of the pons depresses respiration in decerebrate cats

The injection of carbachol into the pontine tegmentum of decerebrate cats evokes a postural motor atonia that has many of the characteristics of the atonia of natural rapid-eye-movement (REM) sleep (Morales et al. J. Neurophysiol. 57: 1118-1129, 1987). We have used the carbachol-injected decerebrate cat to study the changes in respiratory neuronal activity that accompany the atonia. The activities of representative respiratory motor nerves--phrenic, intercostal, and hypoglossal--and that of a motor branch of C4 were recorded in decerebrate, vagotomized, paralyzed, and artificially ventilated cats. After the microinjection of carbachol, there was a profound suppression of activity in all the nerves and a decrease in respiratory rate. This was a consistent stereotyped response in which the magnitude of the suppression of respiratory-related activity was phrenic (to approximately 65% of control) less than inspiratory intercostal (approximately 50%) less than hypoglossal (approximately 10%) less than expiratory intercostal (approximately 5%). The decrease in respiratory rate (to approximately 70% of control) was caused by a prolongation of both inspiratory and expiratory durations. Complete reversal of the carbachol effect was elicited by the microinjection of atropine into the same site as the carbachol injection. This allowed us to produce a second episode of atonia by the injection of carbachol into the contralateral pons. Thus we have demonstrated the existence of neural pathways originating in the cholinoceptive cells of the pons that have the potential to powerfully and differentially depress various respiratory motoneuronal pools and to reduce the respiratory rate. These pathways are likely to be activated along with the atonia of REM sleep.

Effect of dorsolateral pontine lesions on diaphragmatic activity during REMS

Muscle atonia is a feature of normal rapid-eye-movement sleep (REMS). The suppression of accessory respiratory muscle activity has been investigated and a role for sleep-disordered breathing hypothesized, but the suppression of diaphragmatic activity has rarely been considered. We hypothesized that the activity of the diaphragm was suppressed by an area of the dorsolateral pons during REMS. Lesions in this region have previously been shown to abolish the atonia of REMS. The diaphragmatic electromyogram (EMG) activity was analyzed in five naturally sleeping cats before and after pontine lesions leading to REMS without atonia. Although respiratory timing parameters were not altered by the lesion, the inspiratory rate of rise was significantly increased in all cats, and the brief pauses (40-100 ms) in the diaphragmatic EMG normally seen in REMS were virtually abolished. We conclude that the dorsolateral pons has a role in suppressing diaphragmatic activation during REMS. This suppression affects the average rate of rise of diaphragmatic activity and also leads to brief intermittent complete cessation of ongoing muscle activity. These decrements in diaphragm activity could jeopardize ventilation during REMS.

Startle-evoked changes in diaphragmatic activity during wakefulness and sleep

Tonic inhibition of some respiratory muscles occurs as part of the generalized muscle atonia of rapid-eye-movement sleep (REMS). A second type of inhibition of the diaphragm during REMS, fractionations, consists of brief pauses in the diaphragmatic electromyogram (DIA EMG) in association with phasic events. Because motor inhibition can occur as part of the startle response, and the brain is highly activated during REMS, we hypothesized that the neural basis of the fractionations might be activation of a startle network. To test this hypothesis, tone bursts (100 dB, 20-ms duration at 15-s intervals) were applied to cats at a fixed inspiratory level in the DIA moving average during REMS, non-rapid-eye-movement sleep (NREMS), and wakefulness. Parallel sham studies (no tone applied) were obtained for each state. The response of the DIA EMG was averaged over 100 ms by using the tone pulse as a trigger, and the following parameters of the DIA EMG were measured: latency to peak and/or nadir, increment or decrement in activity, and duration of peak and/or nadir. After a tone, all five animals studied displayed a profound suppression of DIA activity during REMS (latency to nadir 42.4 +/- 10.0 ms, duration of suppression 35.9 +/- 17.6 ms). Similarly, DIA activity was suppressed in all cats during NREMS (latency to nadir 40.9 +/- 13.3 ms, duration 23.9 +/- 13.4 ms). An excitatory response was observed in only two cats during NREMS and wakefulness. The similarity of startle-induced DIA EMG pauses to spontaneous fractionations of DIA activity during REMS suggests that the latter result from activation of a central startle system.

Sites of termination and relay of pulmonary rapidly adapting receptors as studied by spike-triggered averaging

The sites of termination and relay of pulmonary rapidly-adapting receptors (RARs) were determined by averaging the extracellular field potentials produced in the nucleus tractus solitarii of the cat by individual RAR neurons. Action potentials of individual RARs were recorded extracellularly in the nodose ganglion during mechanical stimulation of the receptive field in the lung and used as triggers for the averaging. The averaged records (2000-4000 sweeps) revealed the presence of terminal and focal synaptic potentials. These potentials, indicating the presence of pre- and postsynaptic elements at the recording site, were found ipsilaterally in the caudal medial and commissural subnuclei and contralaterally in the commissural subnucleus.

Role of medullary inspiratory neurones in the control of the diaphragm during oesophageal stimulation in cats

1. The effect of oesophageal distension and swallowing on the activity of medullary respiratory neurones was recorded in decerebrate, spontaneously breathing cats. The distension, produced by inflating a balloon in the thoracic portion of the oesophagus, was of sufficient magnitude to induce inhibition of the peri-oesophageal part of the crural diaphragm, with little effect on the respiratory function of the diaphragm as measured by the activity in the C5 branch of the phrenic nerve. 2. 424 neurones were tested. They were located bilaterally, in the region of the nucleus tractus solitarius (dorsal respiratory group) or the ambiguous complex (ventral respiratory group). No cell exhibited a change in activity during periods of strong inhibition of crural electrical activity induced by distension or swallowing. The activity of all cells paralleled that of the C5 phrenic neurogram, which was unaffected by the tests. 3. We conclude that the reflex inhibition of the crural diaphragm during oesophageal distension does not result from an inhibition of medullary premotor inspiratory neurones of the dorsal and ventral groups. Additional central pathways must exist that inhibit motoneurones to the crural diaphragm during gastrointestinal reflexes.

Biotransformation of sulindac in end-stage renal disease

In normal humans sulindac, a prodrug, undergoes two major biotransformations: irreversible oxidation to the inactive sulfone metabolite and reversible reduction to the pharmacologically active sulfide metabolite. To assess any effect of end-stage renal failure on sulindac biotransformation, six patients were given 200 mg sulindac orally. Plasma was sampled over 24 hours. Protein binding of sulindac and metabolites was determined by equilibrium dialysis. Results were compared with historic controls. AUC(0-12) for sulindac and the sulfone were similar to controls. AUC(0-12) for the sulfide was significantly reduced to 4.85 micrograms X hr/ml from 13.1 micrograms X hr/ml (P less than 0.02). Protein binding of all three compounds was significantly reduced by renal failure. When corrected for protein binding, the AUC(0-12) for sulindac and the sulfone was twice that of controls whereas that of the sulfide was 42 ng X hr/ml compared with 83 ng X hr/ml in normal individuals (P less than 0.001). This suggests that end-stage renal failure impairs the reduction of sulindac to the active sulfide whereas oxidation to the sulfone is intact.

Bilateral convergence of pulmonary stretch receptor inputs on I beta-neurons in the cat

Extracellular recordings were made from inspiratory beta- (I beta) neurons in the nucleus of the tractus solitarius in decerebrate cats. A reversible direct current block of myelinated fibers in the ipsilateral vagus nerve was used to evaluate the input from pulmonary stretch receptor afferents (PSR) of the contralateral vagus to individual I beta-neurons. This block served to remove all ipsilateral (which includes all monosynaptic) inputs from PSR to I beta-cells. The effect of withholding inflation on the firing rate and the time of onset of firing of I beta-neurons was determined before, during and after application of the direct current block. There was considerable variation in the strengths of the inputs from the ipsilateral and contralateral nerves; some cells received PSR inputs from only the ipsilateral vagus, but the majority were excited with varying magnitude from both vagi. Several neurons had powerful excitatory inputs from PSR of the contralateral vagus, with the ipsilateral (monosynaptic) contribution being of minor importance.

Pulmonary stretch receptor relay neurones of the cat: location and contralateral medullary projections

1. The activity of pump (p.) cells, second-order neurones in the pulmonary stretch receptor pathway, was recorded extracellularly in the nucleus of the tractus solitarius (n.t.s.) of the decerebrate cat. Their firing was proportional to changes in lung volume but unrelated to the centrally determined respiratory rhythm. A systematic search of the n.t.s. for the location of p. cells was made and an assessment of their efferent projection to the contralateral n.t.s. was determined electrophysiologically by the antidromic mapping technique. 2. P. cells were located around, and in close proximity to, the solitary tract. The two sites of greatest density were ventromedial and dorsolateral to the tract, with lower concentrations found laterally and ventrolaterally. 3. For twelve of the thirty p. cells tested, evidence of a projection to the contralateral n.t.s. was obtained; in seven of these cells, axonal arborizations within the projection area were identified. Almost all the cells that sent axons to the contralateral n.t.s. were located dorsolateral to the tract; there was no evidence that cells in the ventromedial region had contralateral projections. 4. No evidence that R beta neurones project to the contralateral commissural and ventrolateral subnuclei was found. 5. No p. cells projected to the contralateral ventrolateral n.t.s. The site of projection and branching was consistently localized just caudal to the obex and medial to the solitary tract, in the caudal medial, and commissural subnuclei of the n.t.s. This same region has been shown to receive a dense, direct projection from pulmonary rapidly adapting receptors.

Monitoring, anesthesia equipment, and space requirements

General anesthesia is provided in the dental office primarily to reduce fear, block pain, produce amnesia, and provide a more comfortable surgical environment. Because the perception of pain is a major obstacle to the obtainment of dental health in the United States, general anesthesia has become an essential part of the practice of dentistry. It benefits both the patient and the surgeon to make dentistry a pleasant, painless experience.

Control of activity of the diaphragm in rapid-eye-movement sleep

Respiration in rapid-eye-movement sleep (REMS) is known to be highly variable. The purpose of this study was to investigate the source of this variability and to determine which ordering principles remained operative in REM sleep. In unrestrained, naturally sleeping cats we recorded the electroencephalogram, electrooculogram, neck electromyogram, and diaphragmatic electromyogram (EMG) and computed its moving average (MAdi). As a reference, we first examined MAdi during "tonic" REMS, since breathing is fairly regular in this state. "Control" ranges for peak amplitude (PEMG), inspiratory time (TI), duration of postinspiratory inspiratory activity, expiratory time, and the calculated inspiratory slope (PEMG/TI) were determined by overlaying individual breath traces of the time course of MAdi during tonic REMS to form a composite tracing. Next, the time course of the EMG during individual breaths in slow-wave sleep (SWS) and a complete period of consecutive breaths in REMS (both tonic and phasic) were compared with this tonic REMS composite. The number of eye movements per breath was tabulated as an index of phasic activity. The inspiratory slopes during SWS and tonic REMS were similar. However, during phasic REMS, many breaths displayed either increases (excitation) or decreases (inhibition) in slope compared with the "typical" breaths seen in tonic REMS. The occurrence of these altered slopes increased with the frequency of phasic events. TI was inversely related to the slope of the EMG, which tended to minimize changes in PEMG.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of pulmonary stretch receptors during ramp inflations of the lung

Studies were conducted in anesthetized paralyzed dogs to determine how the dynamic and proportional sensitivity of pulmonary stretch receptors change during lung inflation. The firing of each receptor was examined at multiple levels of static transpulmonary pressure and during multiple identical inflations at each of several rates. The averaged response of the receptor was computed and receptor activity related to transpulmonary pressure. On the basis of a quantitative criterion, employed to distinguish type I from type II receptors, the receptors could not be divided into distinct subpopulations. Thus all receptors were treated as coming from a single population. For all receptors we observed that their proportional sensitivity (increases in firing produced by increases in lung expansion at a constant rate of inflation) declined as the lung was inflated. In contrast, the dynamic sensitivity (increases in firing produced by increased rates of inflation at constant transpulmonary pressure) increased or remained relatively constant with increasing lung expansion. Thus, as inflation volume increases, the pulmonary stretch receptor acts increasingly as a rate receptor. The rate of inflation may have a more important role in control of the inspiratory duration than previously realized.

Enalapril in congestive heart failure: acute and chronic invasive hemodynamic evaluation

Following hemodynamic evaluation using invasive and noninvasive methods, 73 patients were treated in an open, uncontrolled, multicenter study with single oral doses of enalapril maleate 1.25 to 40 mg until the optimal dose for each patient (based upon hemodynamic response) was achieved. Diuretics were withheld and reinstituted only if necessary. Hemodynamic measurements were made at 0 (predrug), 1, 2, 3, 4, 6, 8, 10, 12 and 24 hours postdrug. Patients were discharged on their optimal dose, treated 1 to 4 months and then rehospitalized for repeat hemodynamic measurements. The optimal enalapril single dose was associated with the following mean peak responses: increased cardiac index 42% (SE = 6) and decreased pulmonary capillary wedge pressure 40% (SE = 3), systemic vascular resistance 39% (SE = 2), and mean arterial pressure 23% (SE = 1.5). These changes persisted during chronic therapy. Chronic treatment with enalapril also improved exercise capacity 40% (P less than 0.01), ejection fraction 18% (P less than 0.05) and clinical status (N.Y.H.A. functional class, P less than 0.01). Ten and 20 mg/day, taken as once- or twice-daily regimens, were the most commonly effective doses.

Projection of pulmonary rapidly adapting receptors to the medulla of the cat: an antidromic mapping study

The activity of pulmonary rapidly adapting receptor (r.a.r.) neurones was recorded extracellularly in the nodose ganglion of the decerebrate cat. The receptors were identified by their rapid adaptation to 'ramp and hold' hyperinflations of the lung. The antidromic mapping technique was used to determine the sites of projection and branching patterns within the nucleus of the tractus solitarius (n.t.s.) of eleven r.a.r.s. The medulla was explored with a stimulating electrode to activate the r.a.r.s. antidromically. In each penetration, depth-threshold measurements were made for each antidromic response characterized by a distinct latency. Using the anatomical sites of the minimum threshold points, the locations of central branches of individual r.a.r.s. were determined. The main axons of all of them coursed within the tractus solitarius (t.s.) at levels from 2 mm rostral to 0.5 mm caudal to the obex. The axonal conduction velocities within the t.s. were 6.2-9.7 m/s, where the peripheral conduction velocities were 11.2-20.4 m/s (28 degrees C). Different latencies of response evoked in a single penetration were considered to indicate branching. The densest branching was found in the ipsilateral commissural subnucleus of the n.t.s. at levels 0.3-1.3 mm caudal to the obex and, to a lesser degree, in the contralateral commissural subnucleus. All r.a.r.s. sent a few branches to the medial n.t.s. rostral to the obex. Four r.a.r.s. ramified in the ventrolateral n.t.s. where inspiratory cells are located. Depth-threshold graphs were interpolated by best fitting parabolic equations: Ith = Ad2 + Bd + C; where Ith is the threshold current, d the corresponding depth of stimulation, and A, B and C are coefficients. Coefficient A is a measure of steepness of the parabola. The A coefficients were inversely related to the conduction velocity (v) of the stimulated branch. An analysis of the data from the present study (v = 5.0-9.7 m/s) combined with data from the literature (v = 2.2-85 m/s) led to a simple relationship between the A coefficient and the conduction velocity of the stimulated fibre: A = 6500/v, where A is expressed in microA/mm2 and v is expressed in m/s. Within the range 3-35 m/s, the formula is useful in predicting the effective current spread when the conduction velocity is known, or to estimate the conduction velocity from the shape of a depth-threshold curve. Two slowly adapting pulmonary stretch receptors (p.s.r.s) were studied.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of lung inflation on the excitability of dorsal respiratory group neurons

The effect of lung inflation on the excitability of inspiratory neurons of the dorsal respiratory group was studied in decerebrate, paralyzed, artificially ventilated cats. Variations in the antidromic latency (AL) were used as a measure of the changes in excitability. The antidromic responses of single cells were recorded extracellularly during electrical stimulation (20 Hz) of their spinal axons. Single-breath test inflations were delivered at the onset of inspiration (I) or expiration (E), and then maintained for the duration of that respiratory phase. In the absence of inflation during E, most of the inspiratory cells underwent progressive lengthening of the AL, indicating inhibition or disfacilitation. This effect was stronger in I beta than in I alpha cells but there was considerable overlap. In every cell (21 I alpha, 17 I beta), inflation during E caused a prompt AL shortening (excitation or disinhibition) that was evident in single tests. On average, I beta neurons were more strongly excited by the test inflation during E, but again there was considerable overlap. The excitation was maintained for the duration of the inflation, indicating that pulmonary stretch receptor afferents (PSR) were involved. The response to slow inflations (that preferentially excite PSR) was a progressive shortening of the AL that mirrored the increase in lung volume. The results emphasize the qualitative similarity in the responses of I alpha and I beta neurons to lung inflation and in their excitability changes during normal respiratory cycles.

[Worldwide experience with enalapril]

Up to now, the experiments carried out throughout the world with enalapril have been most encouraging. The drug gives good, even excellent responses in 54 to 66 % of patients with essential hypertension, and it is at least as effective as diuretics and beta-blockers. Compared with those of diuretics, the effects of enalapril confirm that the best responders are those patients who are most dependent on the renin-angiotensin system. When a diuretic is administered concomitantly with enalapril, almost all patients respond and the therapeutic effect is well maintained in long term. Blocadren or alpha-methyldopa can be added to hydrochlorothiazide, thus providing additional benefits to patients with severe hypertension. Enalapril reduces the undesirable metabolic effects of hydrochlorothiazide, particularly hypokalaemia. Altogether, enalapril and captopril have similar effectiveness, but enalapril is better tolerated and does not seem to produce the side-effects encountered with captopril, notably skin rashes and ageusia. As expected, enalapril and other angiotensin-converting enzyme inhibitors may be associated with azotaemia in patients with bilateral renovascular hypertension.

Nonsteroidal anti-inflammatory effect of sulindac sulfoxide and sulfide on gastric mucosa

Gastric injury resulting from nonsteroidal anti-inflammatory drugs is thought to require direct contact of the drug with the gastric mucosa. An inactive form of a drug (as a prodrug) should protect against mucosal damage. Because sulindac sulfoxide has little effect on prostaglandin synthesis until it is reduced to sulindac sulfide after absorption, we performed a double-blind, crossover endoscopic study in 15 normal subjects to compare the prodrug sulindac sulfoxide (200 mg b.i.d.), the active sulfide metabolite sulindac sulfide (100 mg b.i.d., which yields similar sulfide blood concentrations), a positive control (aspirin, 650 mg q.i.d.), and a negative control (placebo). Each drug was taken for 1 week and gastric mucosa were endoscopically assessed before and after 2, 5, and 7 days of dosing. Aspirin predictably damaged the gastric mucosa, whereas the effects of sulindac sulfoxide and sulindac sulfide could not be distinguished from those of the placebo. We conclude that sulindac sulfoxide as a prodrug is not directly responsible for the reduced severity of gastric mucosal lesions. Both sulindac sulfoxide and sulindac sulfide are poorly soluble in acid gastric contents and the reduced damage may relate to the inability of high concentrations of the drug to enter gastric mucosal cells.

Adaptation to reflex effects of prolonged lung inflation

Adaptation to the reflex effects of sustained changes in lung volume on inspiratory duration (TI), expiratory duration (TE), and the phrenic neurogram was examined. Test inflations in gallamine-paralyzed dogs anesthetized with pentobarbital sodium were made during a 6-min trial while the animal was not ventilated: 2 min at functional residual capacity (FRC), 2 min at elevated airway pressure, and 2 min back at FRC. The dogs were hyperoxygenated and arterial PCO2 was kept constant by an infusion of tris (hydroxymethyl) aminomethane. The maintained inflations produced minimal changes in TI. On return to FRC, TI was prolonged in proportion to the magnitude of the prior inflation. In contrast, inflation produced marked prolongation of TE, which then adapted back toward preinflation values. On return to FRC, TE shortened initially to values below control. This shortening increased with greater prior lung inflations. The times to reestablish steady-state values upon return to FRC differed for TI (14.8 +/- 4.6 s) and TE (33.8 +/- 12.7 s). The magnitude of the phrenic neurogram at a fixed time from onset of inspiration and its slope were unchanged with inflation. These results indicate that respiratory phase durations are influenced not only by pulmonary afferent input within each respiratory cycle but also by prior vagal afferent activity that engages central processes with long, although different, time constants. Afferent input to the slow central process controlling TI is not gated to only one phase of the respiratory cycle.

Enalapril in hypertension and congestive heart failure. Overall review of efficacy and safety

Multiclinic controlled studies have shown that enalapril alone 10 to 40 mg/day orally is effective in lowering blood pressure in patients with essential hypertension. Enalapril has been compared with thiazides and beta-blockers (propranolol, metoprolol and atenolol). The effect on systolic blood pressure has been greater with enalapril than with beta-blockers. The proportion of patients who respond to enalapril alone with a decrease in diastolic blood pressure (greater than or equal to 10mm Hg) is around 70%. When a thiazide is added to the treatment, the proportion is above 90%. Enalapril improves the signs and symptoms associated with congestive heart failure. Patients increased their exercise tolerance by an average of 148 sec and improved in their NYHA cardiac status and prognosis classification. The overall incidence of side effects is similar to that seen in the placebo control groups. Side effects such as agranulocytosis, taste loss, rash, proteinuria were not characteristic of enalapril. This supports the hypothesis that the improved safety profile of enalapril is the result of being a nonsulphydryl angiotensin-converting enzyme (ACE) inhibitor. The most common side effects reported were dizziness, headache and asthenia. Abnormalities in electrolytes, uric acid, glucose or in lipids have generally not been associated with enalapril.

Clinical pharmacology of enalapril

Enalapril is administered as the ethyl ester of the potent angiotensin converting enzyme (ACE) inhibitor enalaprilat. As such, it is 60% absorbed and 40% bioavailable as the active species. Absorption is not affected by food. Serum concentrations are proportional to dose, less a small drug residue apparently bound to the enzyme. The half-life for accumulation is approximately 11 h. The drug is eliminated by the kidney without further metabolism. Maximum inhibition of plasma ACE occurs 2-4 h after administration and persists for more than 24 h with clinical doses. ACE inhibition is associated with increases in plasma renin and angiotensin I levels, decreases in angiotensin II and aldosterone concentrations and reduction in blood pressure. All of these events are closely correlated when analysed statistically. The clinical and haemodynamic effects of these actions are addressed in other papers.

Enalapril worldwide experience

Overall, the worldwide experience on enalapril to date is very encouraging. The drug produces good to excellent responses in 54 to 66 percent of patients with essential hypertension and is at least as effective as either diuretics or beta blockers. The effects of enalapril compared with those of diuretics confirm that patients more dependent upon the renin-angiotensin system respond better. When hydrochlorothiazide is administered concomitantly with enalapril, almost all patients respond, with good long-term maintenance. In patients with severe hypertension, Blocadren or Aldomet may be added in addition to hydrochlorothiazide and will produce additional benefit. Enalapril attenuates the adverse metabolic effects of hydrochlorothiazide, particularly hypokalemia. Overall, although the efficacy of enalapril and that of captopril are similar, enalapril is better tolerated and does not appear to be associated with any significant occurrence of captopril-type side effects, particularly the skin rash and loss of taste. As expected, enalapril and other converting inhibitors may be associated with azotemia in patients with bilateral renovascular hypertension.

Overview: the role of angiotensin-converting enzyme inhibitors in cardiovascular therapy

Angiotensin-converting enzyme (ACE) inhibitors favorably modify control mechanisms that are disturbed in hypertension and congestive heart failure, principally, but perhaps not exclusively, through reduction in angiotensin II levels. Pharmacodynamic actions are vasodilation, increased sodium excretion, and lowering of blood pressure. Investigations with captopril and enalapril in the treatment of hypertension indicate efficacies comparable to each other and to current step 1 and 2 agents. Enalapril is more potent than captopril and has a longer duration of action. The hemodynamic mechanism of action is reduction in peripheral vascular resistance. Addition of a diuretic potentiates blood pressure lowering and proportion of patients responding. When used in congestive heart failure, ACE inhibitors exert a balanced vasodilator effect on arterial and venous beds and do not induce tachycardia or fluid retention. Cardiac output is increased whereas systemic vascular resistance, central pressures, and systemic blood pressure are reduced acutely and chronically. Although captopril is associated with certain side effects, possibly resulting from the sulfhydryl group in its structure, this profile has not been encountered thus far in clinical investigations with enalapril. The effects of ACE inhibitors on the natural histories of hypertension (independent of blood pressure lowering) and congestive heart failure are yet to be determined.

Tolerance and safety of enalapril

Enalapril is the result of a targeted research programme to develop a non-mercapto converting enzyme inhibitor with a long duration of action and an improved safety profile for use in the therapy of hypertension and congestive heart failure. Over 3500 patients world-wide have received enalapril or enalaprilat. Long-term experience at present includes over 2500 patients. While enalapril and captopril produce similar efficacy, enalapril is better tolerated and appears not to be associated with occurrence of captopril-type side-effects, particularly the skin rash, taste loss, leukopenia and proteinuria. Enalapril and other converting enzyme inhibitors may be associated with renal insufficiency when given to patients with bilateral renovascular hypertension.