Changes in cardiac output, stroke volume, and central venous pressure induced by atropine in man

Cardiovascular responses to hot skin at rest and during exercise

Integrative cardiovascular responses to heat stress during endurance exercise depend on various variables, such as thermal stress and exercise intensity. This review addresses how increases in skin temperature alter and challenge the integrative cardiovascular system during upright submaximal endurance exercise, especially when skin is hot (i.e. >38°C). Current evidence suggests that exercise intensity plays a significant role in cardiovascular responses to hot skin during exercise. At rest and during mild intensity exercise, hot skin increases skin blood flow and abolishes cutaneous venous tone, which causes blood pooling in the skin while having little impact on stroke volume and thus cardiac output is increased with an increase in heart rate. When the heart rate is at relatively low levels, small increases in heart rate, skin blood flow, and cutaneous venous volume do not compromise stroke volume, so cardiac output can increase to fulfill the demands for maintaining blood pressure, heat dissipation, and the exercising muscle. On the contrary, during more intense exercise, hot skin does not abolish exercise-induced cutaneous venoconstriction possibly due to high sympathetic nerve activities; thus, it does not cause blood pooling in the skin. However, hot skin reduces stroke volume, which is associated with a decrease in ventricular filling time caused by an increase in heart rate. When the heart rate is high during moderate or intense exercise, even a slight reduction in ventricular filling time lowers stroke volume. Cardiac output is therefore not elevated when skin is hot during moderate intensity exercise.

Cardiovascular responses to exercise when increasing skin temperature with narrowing of the core-to-skin temperature gradient

The decline in stroke volume (SV) during exercise in the heat has been attributed to either an increase in cutaneous blood flow (CBF) that reduces venous return or an increase in heart rate (HR) that reduces cardiac filling time. However, the evidence supporting each mechanism arises under experimental conditions with different skin temperatures (T_sk; e.g., ≥38°C vs. ≤36°C, respectively). We systematically studied cardiovascular responses to progressively increased T_sk (32°C-39°C) with narrowing of the core-to-skin gradient during moderate intensity exercise. Eight men cycled at 63 ± 1% peak oxygen consumption for 20-30 min. T_sk was manipulated by having subjects wear a water-perfused suit that covered most of the body and maintained T_sk that was significantly different between trials and averaged 32.4 ± 0.2, 35.5 ± 0.1, 37.5 ± 0.1, and 39.5 ± 0.1°C, respectively. The graded heating of T_sk ultimately produced a graded elevation of esophageal temperature (T_es) at the end of exercise. Incrementally increasing T_sk resulted in a graded increase in HR and a graded decrease in SV. CBF reached a similar average plateau value in all trials when T_es was above ~38°C, independent of T_sk. T_sk had no apparent effect on forearm venous volume (FVV). In conclusion, the CBF and FVV responses suggest no further pooling of blood in the skin when T_sk is increased from 32.4°C to 39.5°C. The decrease in SV during moderate intensity exercise when heating the skin to high levels appears related to an increase in HR and not an increase in CBF. NEW & NOTEWORTHY This study systematically investigated the effect of increasing skin temperature (T_sk) to high levels on cardiovascular responses during moderate intensity exercise. We conclude that the declines in stroke volume were related to the increases in heart rate but not the changes in cutaneous blood flow (CBF) and forearm venous volume (FVV) during moderate intensity exercise when T_sk increased from ~32°C to ~39°C. High T_sk (≥38°C) did not further elevate CBF and FVV compared with lower T_sk during moderate intensity exercise.

Does mean arterial blood pressure scale with body mass in mammals? Effects of measurement of blood pressure

For at least the last 30 years, it has been discussed whether mean arterial blood pressure (MAP) is independent of body mass or whether it increases in accordance with the vertical height between the heart and the brain. The debate has centred on the most appropriate mathematical models for analysing allometric scaling and phylogenetic relationships; there has been previously little focus on evaluating the validity of underlying physiological data. Currently, the 2 most comprehensive scaling analyses are based on data from 47 species of mammals, based on 114 references. We reviewed all available references to determine under which physiological conditions MAP had been recorded. In 44 (38.6%) of the cited references, MAP was measured in anaesthetized animals. Data from conscious animals were reported in 59 (51.8%) of references; of these, 3 (2.6%) were radiotelemetric studies. In 5 species, data were reported from both anaesthetized and conscious animals, and the mean difference in the MAP between these settings was 20 ± 29 mm Hg. From a literature search, we identified MAP measurements performed by radiotelemetry in 11 of the 47 species included in the meta-analyses. A Bland-Altman analysis showed a bias of 1 mm Hg with 95% confidence interval (from -35 to 36 mm Hg); that is, the limits of agreement between radiotelemetric studies and studies in restrained animals were double the supposed difference in the MAP between the mouse and elephant. In conclusion, the existing literature does not provide evidence for either a positive or neutral scaling of arterial pressure to body mass across taxa.

The principle of laplace and scaling of ventricular wall stress and blood pressure in mammals and birds

Maximum left ventricular wall stress is calculated at end-diastolic volume and systemic arterial diastolic blood pressure, according to a thick-walled model for the principle of Laplace. Stress is independent of body mass and averages 13.9 kPa (+/-2.3; 95% confidence interval) in 24 species of mammals weighing 0.025-4,000 kg and 15.5 kPa (+/-4.7) in 12 birds weighing 0.014-110 kg. Birds have higher arterial blood pressures and larger hearts than mammals. Systolic and diastolic arterial blood pressures increase with body mass according to M(0.05) in mammals, and heart mass increases according to M(1.06) in the same species, further supporting the principle. However, blood pressure in birds is independent of body mass, and heart mass scales isometrically. End-diastolic stress values, calculated according to Laplace, are about one-third of peak stresses recorded in isolated mammalian myocardial preparations.

Effects of atropine on left ventricular volumes and ejection and filling rates at rest and during exercise

1. The influence of the parasympathetic nervous system on left ventricular function including ejection and filling rates was studied by radionuclide cardiography in eight healthy young men at rest and during upright exercise. 2. After parasympathetic blockade induced by atropine, the mean heart rate (HR) at upright rest increased from 67 to 114 beats min-1, cardiac output (CO) from 4.05 to 5.17 1 min-1 (both P less than 0.001), and the diastolic blood pressure by 13 mm Hg (P less than 0.01). 3. Stroke volume (SV), left ventricular end diastolic and end systolic volume all decreased significantly after atropine. The relative ejection time increased from 0.33 to 0.51 of the cardiac cycle length (P less than 0.001), and the appropriate ejection and filling rates increased by 13% (P less than 0.05) and 147% (P less than 0.001), respectively. Haemodynamic changes in the supine position were virtually the same. 4. During exercise atropine increased HR from 115 to 146 beats min-1 (P less than 0.001) and CO by 12% (P less than 0.05), whereas SV decreased by 12% (P less than 0.05) and the systolic blood pressure by 16 mm Hg (P less than 0.001). Changes in ejection and filling rate of the left ventricle were of the same nature as those found at rest. 5. Thus apart from its HR limiting properties, secondary effects of parasympathetic nervous tone are dilatation of the left ventricle and enhancement of ejection, effects that are counteracted by atropine.

Effect of cholinergic blockade on heart rate, blood pressure and plasma catecholamine responses to mental stress in normal subjects

The effects of cholinergic blockade on haemodynamic reactivity to standardized mental stress has been studied in nine normotensive males during infusion of atropine (bolus dose 10 micrograms x kg-1 followed by a constant-rate infusion of 0.02 microgram x kg-1 x min-1) or placebo given in a randomized order on two different days. Partial cholinergic blockade increased resting heart rate by 25-30 beats per minute. The magnitude of the heart rate response to stress (reactivity) however was unaffected by the atropine infusion. Also, in four subjects who received a higher dose of atropine (approximately 1.8-1.9 mg), heart rate responses to stress were the same as during placebo infusion. Cholinergic blockade was associated with a small but prolonged increase in diastolic blood pressure. These findings suggest that parasympathetic withdrawal does not contribute to the tachycardia caused by mental arithmetic, and that the pattern of neurogenic activation may differ from that elicited during a classic defence-alarm reaction and by somatomotor activation.

Cardiovascular abnormalities accompanying acute spinal cord injury in humans: incidence, time course and severity

The frequency of cardiovascular abnormalities was evaluated in 71 consecutive patients with acute injury to the spinal cord. Persistent bradycardia was universal in all 31 patients with severe cervical cord injury and less common in milder cervical injury (6 of 17) or thoracolumbar injury (3 of 23) (p less than 0.00001). Marked sinus slowing (71 versus 12 versus 4%, respectively, p less than 0.00001), hypotension (68 versus 0 versus 0%, p less than 0.00001), supraventricular arrhythmias (19 versus 6 versus 0%, p = 0.05) and primary cardiac arrest (16 versus 0 versus 0%, p less than 0.05) were significantly more frequent in the severe cervical injury group. The frequency of bradyarrhythmias peaked on day 4 after injury and gradually declined thereafter. All observed abnormalities resolved spontaneously within 2 to 6 weeks. The primary mechanism underlying these observations appears to involve the acute autonomic imbalance created by the disruption of sympathetic pathways located in the cervical cord. Acute severe injury to the cervical spinal cord is regularly accompanied by arrhythmias and hemodynamic abnormalities not found with thoracolumbar cord trauma. These abnormalities are limited to the first 14 days after injury, a period in which life-threatening disturbances must be anticipated.

The hemodynamic effects of sympathetic stimulation combined with parasympathetic blockade in man

To define the effects of circulating norepinephrine and epinephrine on cardiac function and to determine whether left ventricular function is influenced by parasympathetic mechanisms during catecholamine stimulation, hemodynamic changes were investigated in healthy young human subjects who were supine and awake during infusion of intravenous norepinephrine alone (125 ng/kg/min) (n = 6), norepinephrine (125 ng/kg/min) plus epinephrine (50 ng/kg/min) (n = 6), and norepinephrine plus epinephrine plus parasympathetic blockade induced by atropine (2 mg intravenously) (n = 5). Ejection fraction and changes in cardiac volumes were measured by radionuclide ventriculography. During the infusion of norepinephrine plus epinephrine, plasma norepinephrine increased from 358 +/- 35 to 1782 +/- 123 pg/ml (mean +/- SE) and plasma epinephrine increased from 31 +/- 5 to 355 +/- 90 pg/ml (both p less than .01 vs baseline). These increases in plasma catecholamines were associated with increases in the heart rate (58 +/- 3 to 67 +/- 2 beats/min, p = NS), systolic blood pressure (113 +/- 3 to 140 +/- 6 mm Hg, p less than .01), ejection fraction (0.64 +/- 0.02 to 0.72 +/- 0.02 ejection fraction units, p less than .01), stroke volume (+41 +/- 5%, p less than .01), and cardiac output (+54 +/- 8%, p less than .01), and a decrease in systemic vascular resistance (-31 +/- 3%, p less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Integrated pharmacokinetics and pharmacodynamics of atropine in healthy humans. I: Pharmacokinetics

The pharmacokinetics of atropine in three healthy male volunteers after intravenous administration of 1.35 and 2.15 mg of the drug was determined. Pharmacodynamic effects of atropine were measured simultaneously. All the data were fitted to a novel integrated kinetic-dynamic model. Plasma concentrations of atropine and the amounts of atropine and its primary metabolite, tropine, excreted in the urine were measured by a sensitive gas chromatographic-mass spectrometric assay. The kinetics of elimination of atropine was first order. There was evidence that the kinetics of distribution of the drug was dose dependent. Two phases with apparent half-lives of 1 and 140 min were distinguishable in accordance with a linear two-compartment disposition model for atropine. The urinary excretion of unchanged drug was 57% of the dose. The steady-state volume of distribution was 210 L, implying extensive tissue binding and/or partitioning. Renal plasma clearance was 660 mL/min, suggesting significant tubular secretion. The renal clearance of atropine depended on urine flow. Urinary excretion of tropine amounted to 29% of the dose. The kinetics of the metabolite was first order.

Vagal activity is increased during intravenous isoprenaline infusion in man

Eight healthy normal subjects (19-23 years) received graded intravenous isoprenaline infusions PRE- and POST-atropine, 0.04 mg/kg i.v. The adequacy of atropinization was tested with bolus intravenous injections of phenylephrine. PRE-atropine, isoprenaline caused an increase in heart rate and systolic pressure, but a decrease in diastolic pressure with little change in mean pressure (+32.8 +/- 3.0 beats/min, +18.9 +/- 3.0 mm Hg, -13.5 +/- 1.9 mm Hg, -2.7 +/- 1.9 mg Hg respectively at isoprenaline 2 micrograms/min). POST-atropine, the increase in heart rate was enhanced, the rise in systolic pressure abolished and the falls in diastolic and mean pressures exaggerated (+47.0 +/- 2.8 beats/min, -8.9 +/- 2.9 mm Hg, -27.3 +/- 2.1 mm Hg, -21.1 +/- 1.9 mm Hg, respectively at isoprenaline 2 micrograms/min). During an isoprenaline infusion, when the heart rate and blood pressure changes are stable, there is an increase rather than a decrease in cardiac vagal tone.

Atropine-induced cardioacceleration in patients on chronic propranolol therapy: comparison with the positive chronotropic effect of isometric exercise

The hemodynamic effects of intravenous atropine administration were examined in 24 patients on chronic propranolol therapy. In the first 13 patients the safety of atropine administration was tested by giving the drug in small increments to a total dose of either 1.2 mg (five patients) or 1.7 mg (eight patients). The heart rate after atropine administration in these patients varied between 57 and 82 bpm and no adverse effects were noted. The other 11 patients received 1.2 mg atropine intravenously with hemodynamic measurements obtained prior to and 3 minutes after administration of the drug. Heart rate increased from 57.5 +/- 8.7 to 72.8 +/- 13.9 bpm, mean pulmonary arterial and left ventricular end-diastolic pressure declined, and cardiac index increased. Total systemic resistance decreased in most of the patients. Isometric exercise performed prior to atropine administration in the same 11 patients accelerated heart rate from 57.4 +/- 8.6 to 68.4 +/- 10.8 bpm. A close correlation, r = 0.909, was demonstrated between the postatropine heart rate and the rate during isometric exercise. It is concluded that atropine in a dose of 1.2 to 1.7 mg may be administered safely in patients on chronic propranolol therapy. Isometric exercise may be useful in unmasking vagal tone in beta-blocked patients.

Effect of atropine and methylatropine on human vaginal blood flow, sexual arousal and climax

No experimental data on the regulatory mechanism of the change in vaginal blood flow occurring at sexual arousal exist. Six women were in a controlled laboratory study given atropine 0.035 mg/kg intravenously. The basal vaginal blood flow was recorded by a heat probe kept at set temperature on the vaginal wall. During sexual stimulation the flow was increased as in women when no drugs are applied and orgasm was unaffected as well. The neurotransmitter has been supposed to be acetylcholine but the present experiments suggest that it is not an atropine sensitive traditional muscarinic transmission. Methylatropine was given in five subjects and neither in these cases any effect on the vaginal vascular response was observed.

Atropine and hyoscine

The drugs atropine and hyoscine are reviewed in the context of their use by anaesthetists. The results of recent studies are stressed and guidelines given for use of these drugs in modern anaesthetic practice.

Haemodynamic and electrocardiographic effects of intravenous disopyramide (rythmodan) following acute myocardial infarction

1 The haemodynamic and electrocardiographic effects of intravenous disopyramide were studied in fifteen patients with acute myocardial infarction. 2 Five minutes after drug injection a rise in heart rate, aortic mean and diastolic pressures and systemic vascular resistence was noted which persisted for at least 30 min. A small increase in pulmonary arterial diastolic pressure (mean = 1.5 mm Hg) occurred at 5 min only and no significant change of cardiac output was found throughout the period of the study (1 h). 3 Surface electrocardiograms revealed transient prolongation of the P-R interval and a sustained increase in the QTc interval. 4 The haemodynamic changes suggest an anticholinergic effect of the drug. There was no definite evidence of a negative inotropic effect in this study, however, these peripheral haemodynamic measurements might not have revealed a modest negative inotropic effect. 5 The electrocardiographic changes are similar to those previously reported in normals and in patients without acute myocardial infarction.

Effects of precurarization on the heart measured by systolic time intervals

The effects of precurarization, with or without prior atropine medication, on heart rate, systolic time intervals (preejection period (PEP) and left ventricular ejection time (LVET)) and derivatives from these were studied in 80 healthy patients before minor surgery. Atropine (0.5-0.7 mg i.v.) exerted a vagolytic action with no changes in systolic time intervals. D-tubocurarine (0.5 mg/kg b.w.) did not affect heart rate or systolic time intervals. Both gallamine (0.3 mg/kg b.w.) and pancuronium (0.015 mg/kg b.w.) increased heart rate due to the inherent vagolytic effect. Furthermore, a short-lasting decrease in PEP and an increase in LVET were demonstrated. Consequently, a fall in PEP/LVET and a rise in 1/PEP2 were induced, indicating an increase in cardiac pump performance. These changes were blocked by prior atropine medication, a result which is not easily explainable. Possible clinical implications are discussed.

Diastolic closure rate of normal mitral valve

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The influence of heart rate on the secretion of a substance closely resembling the 18-monoacetate of D-aldosterone by the hearts of cats under chloralose anaesthesia

1. A substance (HS), similar to the 18-monoacetyl derivative of D-aldosterone (18 MA) in its physico-chemical and biological properties, is shown to be secreted into the coronary circulation by cat hearts, in vivo.2. The concentrations found in coronary sinus blood from cats resting under chloralose anaesthesia ranged from 20 to 36 ng/ml. equivalent 18 MA activity.3. A positive correlation was found between heart rate and the concentration of HS in coronary sinus blood. This concentration was doubled by a rise in heart rate of 65 +/- 20.2 beats/min when the tachycardia was induced by electrical drive from the right atrial appendage. An increase of only 22.5 +/- 1.76 beats/min doubled the concentration of HS when the acceleration was induced by infusion of adrenaline, noradrenaline or isoprenaline or by atropinization.4. Two methods are described for bio-assay of HS. The reference compound used was 18 MA.