The reporting of randomized controlled clinical trials and the Cochrane Collaboration.
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Humans↗
Biomedical subjects
J Ludbrook
Publications and source records attributed to J Ludbrook.
The decompensatory phase of acute hypovolaemia in rabbits involves a central delta 1-opioid receptor.
Graded caval occlusion in conscious rabbits caused a biphasic response. Phase I was characterized by a fall in conductance so that arterial pressure was maintained. When cardiac output had fallen to 69 +/- 2% of its baseline level, phase II supervened. During phase II, conductance rose abruptly and arterial pressure fell to a life-threatening level (< 40 mmHg). Centrally administered delta-opioid receptor antagonists prevented the occurrence of phase II. The relative order of potency was 7-benzylidene-naltrexone (BNTX, delta 1-selective) > N,N-diallyl-Tyr-Aib-Phe-Leu-OH (ICI 174,864) > naltrindole (delta 2-selective). It is concluded that a central delta 1-opioid receptor is involved in the onset of the second decompensatory phase of the haemodynamic response to haemorrhage.
Animals↗
Role of vagal afferents in the haemodynamic response to acute central hypovolaemia in unanaesthetized rabbits.
In unanaesthetized mammals, including rabbits, the response to acute central hypovolaemia is biphasic. An initial phase of baroreflex-mediated systemic vasoconstriction is succeeded by an abrupt failure of sympathetic vasoconstrictor drive and haemodynamic decompensation. We have tested whether a signal travelling in the cervical vagus nerves is responsible for the second phase. An inflatable vena caval cuff, an ascending aortic flow probe, and diaphragmatic electrodes were chronically implanted into 7 rabbits. Haemorrhage was simulated by gradual caval constriction so cardiac index (CI) fell linearly at 9% per minute. In Study 1, caval constriction was performed under control conditions, after muscarinic cholinoceptor blockade (MCB), and was repeated twice under MCB after a sham operation. In Study 2, the steps were identical but bilateral cervical vagotomy plus tracheostomy was substituted for sham operation. With or without MCB, caval constriction caused a progressive fall of systemic vascular conductance index (SVCI), and a small decline in mean arterial pressure (MAP) (Phase I). When CI had fallen by approximately 40%, there was an abrupt rise of SVCI and fall of MAP (Phase II). Sham operation had no effect on either phase. Vagotomy had no effect on Phase I, but the onset of Phase II was delayed until CI had fallen by approximately 53% in 6 rabbits. In 1 rabbit, Phase II did not occur, even though CI had fallen by 67%. We conclude that an afferent vagal signal does not contribute to the compensatory Phase I, and is not essential for the occurrence of the decompensatory Phase II, of acute central hypovolaemia in unanaesthetized rabbits.
Acute Disease↗
Repeated measurements and multiple comparisons in cardiovascular research.
In cardiovascular research, experiments are commonly performed in which repeated measurements are made in the same individual at predetermined intervals of time or at ascending levels of stimulus or dose of drug. The goal is usually to test the effects of treatments or disease state on the time course of the response, or on the stimulus-response relationship. Since the passage of time or the order of stimuli or doses is fixed, statistical analysis of the results of such experiments is associated with an excessive risk of false positive interferences (type I error) unless special precautions are taken. The nature of the statistical problems associated with repeated measures experimental designs, and several solutions to them, have been discussed. An approach much favoured by cardiovascular investigators is to make multiple pairwise contrasts between treatments at each time or dose, or between times or doses within each treatment. This greatly inflates the risk of type I error unless special precautions are taken, and the information provided by making multiple contrasts is of limited value. I believe that repeated measures analysis of variance, with a correction for multisample asphericity, usually provides the most informative and least biased test of the biological hypotheses proposed by cardiovascular investigators. Other analytical techniques, such as comparing areas under curves and regression analysis, have also been discussed. Summary recommendations are given in the table.
Cardiology↗
Interactions between the circulatory effects of central hypovolaemia and arterial hypoxia in conscious rabbits.
1. Eight conscious rabbits were repeatedly subjected to progressive reduction in central blood volume by gradually inflating a thoracic inferior vena caval-cuff so cardiac index (CI) fell at a constant 8.5% of baseline/min. 2. Caval-cuff inflations were performed after 10 min exposure to 100, 21, 12-14 and 8-10% O2, with and without the addition of 3-4% CO2, in randomized order. 3. The haemodynamic response to progressive reduction in central blood volume was biphasic. In Phase I, systemic vascular conductance index (SVCI) fell linearly, supporting mean arterial pressure (MAP). When CI had fallen to a critical level, Phase II occurred in which SVCI rose abruptly, MAP plummeted and respiratory drive progressively increased. 4. During Phase I, there were independent linear relationships between PaCO2 (but not PaO2) and the rates at which SVCI and MAP changed during the progressive fall of CI. The higher the level of PaCO2, the greater was the rate of fall of SVCI and the less the rate of fall of MAP. 5. There was an inverted U-shaped effect of PaO2 on the level of CI at which Phase II occurred: (a) during hyperoxia (100% O2), Phase II occurred later than during normoxia (21% O2); and (b) across the normoxic and hypoxic gas mixtures (21-8% O2, with and without added CO2), there was an independent linear relationship between PaO2 (but not PaCO2 or PaO2 x PaCO2) and the level of CI at which Phase II occurred. That is, the lower the level of PaO2, the later was the onset of Phase II. This interaction is best explained by an increased level of central sympathetic vasoconstrictor drive during hypoxia.
Analysis of Variance↗
Advantages of permutation (randomization) tests in clinical and experimental pharmacology and physiology.
1. The statistical procedures that are used most commonly in clinical and experimental pharmacology and physiology are designed to test for differences between two means. 2. The classical procedures for detecting such differences are those in which, under the population model of inference, the test statistic is referred to the t- or F-distributions. The validity of statistical inferences from these tests depends on a number of assumptions. Foremost among these is that the experimental groups have been constructed by taking random samples from defined populations. The statistical inferences then apply to the sampled populations. 3. In biomedical research this sampling process is seldom followed. Instead, samples are usually acquired by non-random selection, and are then divided by randomization into experimental groups. This being the case, it is theoretically invalid to use the classical t- or F-tests to analyse the experimental results. 4. The validity of inferences from the classical tests also depends on other assumptions, such as that the sampled populations are normal in form and of equal variance. It is difficult to be certain that these assumptions are fulfilled when group sizes are small, as they usually are in pharmacology and physiology. Breach of them, especially if the groups are unequal in size, can lead to serious statistical errors. 5. Exact permutation tests are designed to make statistical inferences under the randomization model. These conclusions apply only to the results of experiments actually performed. By permuting the statistic of interest, such as the difference between arithmetic means, geometric means, medians, mid-ranges or mean-ranks of randomized groups of observations, the probability is calculated that the observed difference or a more extreme one could have occurred by chance. This inferential process is consistent with the way most biomedical experiments are designed and conducted. 6. Exact permutation tests, or sampled permutation tests based on Monte Carlo random sampling of all possible permutations, can now be performed on personal computers. They are commended to biomedical investigators as being superior to the classical tests for analysing their experimental results when the central tendencies of two independent groups, or of two sets of measurements on the same group, are compared. 7. When there is doubt that the assumptions for t-tests are satisfied, investigators sometimes use non-parametric rank-order procedures such as the Wilcoxon-Mann-Whitney rank-sum test for independent groups or the Wilcoxon signed rank-sum test for paired observations.(ABSTRACT TRUNCATED AT 400 WORDS)
Clinical Trials as Topic↗
Issues in biomedical statistics: statistical inference.
The first step in making inferences under the frequentist system of statistical logic is to propose a null hypothesis. An experiment is then performed, or a set of observations made. The resulting data are subjected to statistical analysis to determine whether the null hypothesis should be rejected or not. If it is, then some alternative hypothesis must have been entertained. In biomedical work, the alternative hypothesis should usually be non-specific and it follows that the statistical test of the null hypothesis should be interpreted in a two-sided fashion. The decision to reject or accept statistical null hypotheses, whether on the basis of a P value or confidence intervals, is probabilistic in nature and always attended by the risk of error. It is argued that, in biomedical research, it is the risk of making false-positive statistical inferences (Type I error) that should be most closely controlled. The risks of Type I error cannot be considered in isolation from the model of inference under which the null hypothesis is tested. That which forms the basis for using the classical t, F and X2 tests is the population model, in which the inference is referred to a defined population that has been randomly sampled and which conforms to a specified frequency distribution. Under this model, serious errors in statistical inference can occur if the actual distributions of the populations do not conform to those specified by theory. More importantly, the population model is inappropriate to most biomedical research, in which treatment groups are created by randomization but not by random sampling.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Issues in biomedical statistics: analysing 2 x 2 tables of frequencies.
How best to analyse statistically experimental results that are set out as a 2 x 2 table of frequencies has been debated by statisticians for more than 50 years. The main issue is what framework of statistical inference should be adopted. The design of most biomedical experiments that result in 2 x 2 tables of independent observations is compatible with the randomization model of inference and with the Fisher exact test. It is rare that the Neyman-Pearson population model is applicable and that a case can be made for using the Pearson chi 2 test, or others that refer a test statistic to the chi-squared distribution. Even then, the adjustments for the mismatch between the test statistic and the chi-squared distribution so as to control the risk of Type I error are complex that the Fisher test is probably a safer option (or Yates' correction to the Pearson test if there is no access to a computer). When the 2 x 2 table results from two sets of measurements having been made on the same group, the population model of inference is inapplicable and the exact form of the McNemar test should be used. Confidence intervals for differences in proportions, the likelihood ratio, or the odds ratio, refer to randomly sampled populations and are not compatible with the randomization model of inference.
Animals↗
Problems with the design of observations and their statistical analysis.
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Research Design↗
Effects of 5-HT-receptor and alpha 2-adrenoceptor ligands on the haemodynamic response to acute central hypovolaemia in conscious rabbits.
1. We set out to elucidate the pharmacological mechanisms by which alpha 2-adrenoceptor and 5-HT-receptor ligands affect the haemodynamic response to acute central hypovolaemia in conscious rabbits. 2. Acute central hypovolaemia was produced by inflating an inferior vena caval cuff so that cardiac output fell at a constant rate of approximately 8.5% of its baseline level per min. 3. Drugs were administered into the fourth cerebral ventricle in either 154 mM NaCl (saline) or 20% w/v 2-hydroxypropyl-beta-cyclodextrin (beta-CDX). After vehicle treatments, the haemodynamic response to acute central hypovolaemia had the usual two phases. During Phase I, systemic vascular conductance fell in proportion to cardiac output so that mean arterial pressure fell by only 8 mmHg. Phase II commenced when cardiac output had fallen to approximately 60% of its baseline level, when vascular conductance rose abruptly and arterial pressure fell to < or = 40 mmHg. The haemodynamic response was not dependent on the vehicle used (saline or beta-CDX). 4. Methysergide delayed the occurrence of Phase II in a dose-dependent manner, and prevented it at a dose of 30- 600 nmol (geometric mean = 186 nmol). The effects and potency of methysergide were not dependent on the vehicle used, indicating that beta-CDX can be used as a vehicle for fourth ventricular administration of lipophilic drugs to conscious rabbits. Clonidine (10 nmol) reversed the effects of a critical dose of methysergide. 5. Phase II was also prevented by 8-hydroxy-2-(di-n-propylamino)tetralin (5-HT1A-selective agonist, geometric mean critical dose (range) = 13.1 (10-30) nmol), sumatriptan (5-HT1D-selective agonist, 72.1 (10-300) nmol), mesulergine (5-HT2/1C-selective antagonist, 173 (30-1000) nmol), idazoxan (alpha 2-adrenoceptor-selective antagonist, 548 (100-3000) nmol), and mianserin (5-HT2/1C-selective antagonist, 548 (100-3000) nmol). It was not affected by MDL 72222 (5-HT3-selective antagonist, 300 nmol) or ketanserin (5-HT2/1C-selective antagonist, 3000 nmol). 6. To characterize the nature of alpha 2-adrenoceptors in rabbit brainstem, we examined the binding of [3H]-rauwolscine to membrane homogenates of whole brainstem. [3H]-rauwolscine bound to a population of sites with the characteristics of alpha 2A-adrenoceptors. 7. From these results we suggest that activation of 5-HT1A receptors in the brainstem can prevent Phase II of the response to acute central hypovolaemia in conscious rabbits. Our results do not support the notion of an endogenous 5-hydroxytryptaminergic mechanism mediating Phase II. The mechanism by which the alpha 2-adrenoceptor antagonists yohimbine and idazoxan prevent Phase II remains to be elucidated. However, their potency relative to other 5-HT-receptor ligands indicates that an agonist action at 5-HT1A-receptors is more likely than an antagonist action at alpha 2-adrenoceptors.
2-Hydroxypropyl-beta-cyclodextrin↗
Factors confounding blockade of cardiac afferents by intrapericardial procaine in conscious rabbits.
Intrapericardial procaine has been used by several groups to block cardiac afferent nerves to study effects of cardiogenic reflexes. In eight conscious rabbits, procaine (17-113 mg ipc; median 32) blocked cardiac efferents. Procaine (17-113 mg ipc; median 39) abolished the reflex depressor effects of the cardiac C-fiber excitant 1-phenylbiguanide (PBG), and in four of eight rabbits prevented the hypotensive phase 2 of acute central hypovolemia, which has been attributed to a signal from the heart. However, in three of the rabbits respiratory incoordination and blood gas abnormalities developed. In another study of four rabbits, procaine (165-335 mg ipc; median 235) invariably caused phrenic nerve blockade and underventilation. In three rabbits, after intrapericardial (250 mg) or subcutaneous (50 mg) procaine, plasma procaine levels rose to 9.4 and 4.8 micrograms/ml, respectively. During intravenous infusion of procaine, the PBG chemoreflex was abolished at plasma levels > 3.1 micrograms/ml, and phase 2 of acute hypovolemia at levels > or = 4.3 micrograms/ml. There is a narrow margin between a dose of intrapericardial procaine that blocks cardiac nerves and one that can produce confounding effects from phrenic nerve blockade or absorption into the bloodstream.
Animals↗
Effects of alpha-adrenoceptor antagonists and clonidine on the haemodynamic response to acute hypovolaemia in conscious rabbits.
In conscious rabbits an inferior vena caval cuff was progressively inflated so cardiac output fell at a constant approximately 8% of its baseline value. There was a biphasic haemodynamic response, consisting of an initial compensatory phase during which there was progressive systemic vasoconstriction and tachycardia, followed by a decompensatory phase in which systemic vasoconstriction failed abruptly, blood pressure plummeted and heart rate declined. We tested the effects on the haemodynamic response of prior 4th ventricular, and in some cases intravenous, infusions of saline, yohimbine, clonidine, yohimbine plus clonidine, and bunazosin. From the results we conclude that a yohimbine-sensitive mechanism in the brainstem, possibly alpha 2-adrenoceptor-mediated, may be an essential element of the cardiac receptor-mediated decompensatory phase of acute central hypovolaemia, but does not contribute to the arterial baroreflex-mediated compensatory phase.
Adrenergic alpha-Antagonists↗
A CNS serotonergic mechanism in acute central hypovolemia in conscious rabbits?
We tested whether a brainstem serotonergic mechanism influences the hemodynamic response to acute central hypovolemia. An inferior vena caval cuff was gradually inflated so that mean cardiac index (MCI) fell at a constant rate (approximately 8%/min). Under control conditions, mean systemic vascular conductance index (MSVCI) fell progressively until MCI had fallen by approximately 50% (compensatory phase), at which point MSVCI rose abruptly and arterial pressure fell to approximately 40 mm Hg (decompensatory phase). Intravenous methysergide delayed the decompensatory phase and at a critical dose (300-3,000 nmol) abolished it. Methysergide had similar effects when injected into the fourth ventricle, pontomedullary cistern, or lateral ventricle in doses that were 7-10% of the critical i.v. dose, but had no effect when injected into the spinal subarachnoid space. LY53857 was equipotent to methysergide. None of these treatments attenuated the vasoconstriction of the first, compensatory, phase. Partial depletion of neuronal serotonin (after p-chlorophenylalanine or 5,7-dihydroxytryptamine treatment) had no effect on either phase. We conclude that a serotonergic mechanism, probably located in the brainstem, may be involved in the decompensatory phase of acute central hypovolemia.
5,7-Dihydroxytryptamine↗
Does the haemodynamic response to acute central hypovolaemia depend on the rate of fall of cardiac output?
1. In published studies of the effects of acute blood loss in conscious rabbits, the rates of haemorrhage have ranged for 3-9% of blood volume/min. This is potentially a confounding factor when it comes to comparing the results of different studies. We have therefore tested whether the haemodynamic response to acute central hypovolaemia depends on the rate of fall of cardiac output. 2. Cardiac output in six conscious rabbits was reduced by 4, 8 and 12% of baseline levels per min by gradual inflation of a cuff around the thoracic inferior vena cava. These rates correspond approximately to blood loss at rates of 3, 6 and 9% of blood volume/min. 3. The haemodynamic responses were biphasic. In Phase I (compensatory) there was progressive systemic vasoconstriction and tachycardia, and only a small fall in blood pressure. In Phase II (decompensatory), systemic vasoconstriction failed abruptly, arterial pressure plummeted and heart rate declined. 4. We could detect no effect of rate of fall of cardiac output on the pattern of the haemodynamic responses in either Phase I or Phase II. 5. We conclude that the rate of blood loss in different studies of haemorrhage in conscious rabbits, within the range 3 to 9 per cent of blood volume per minute, need not be regarded as a confounding factor when it comes to interpreting the results. It is likely that this conclusion can be generalized to studies of haemorrhage in other mammalian species.
Animals↗
New therapies for shock associated with gram-negative sepsis?
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Anti-Bacterial Agents↗
Influence of higher brain centres and vasopressin on the haemodynamic response to acute central hypovolaemia in rabbits.
We tested whether suprapontine brain centres contribute to the sudden failure of vasoconstriction that occurs in unanaesthetized rabbits during acute reduction in central blood volume. Haemorrhage was simulated by gradually inflating a cuff around the thoracic inferior vena cava so that cardiac output fell by about 8% per min. In intact rabbits, and in rabbits that had undergone craniectomy but not decerebration, the haemodynamic response to simulated haemorrhage was always biphasic. During the first, compensatory phase, systemic vascular conductance fell almost in proportion to the fall in cardiac output so that arterial pressure fell by only about 10 mmHg. When cardiac output had fallen by about 50%, a decompensatory phase supervened in which systemic vascular conductance rose abruptly, arterial pressure fell steeply to less than 40 mmHg, and the plasma arginine vasopressin (AVP) level rose. High mesencephalic decerebration did not affect the compensatory phase, but it abolished the decompensatory phase and there was no rise in the plasma AVP level. The decompensatory phase was not restored by intravenous administration of AVP. We came to two conclusions as a result of this study. Suprapontine brain centres do not influence the arterial baroreflex-mediated vasoconstriction that occurs during the first phase of acute central hypovolaemia. However, the sudden failure of vasoconstriction that occurs during the second phase of acute central hypovolaemia, attributable to a signal from the heart and mediated by a delta-opioid receptor mechanism in the brainstem, does depend on the integrity of suprapontine brain centres, though not on neurohypophysial AVP release.
Animals↗
Cardiac chemoreceptors: pharmacological curiosities or physiological tools?
1. We have tested in unanaesthetized rabbits two hypotheses regarding a physiological role for cardiogenic chemoreflexes in acute central hypovolaemia. 2. In rabbits, the sympathoinhibitory phase of acute central hypovolaemia depends on the activation of a brain-stem delta-opioid receptor mechanism by a signal from the heart. Blockade of this by fourth ventricular injection of the delta-receptor antagonist ICI 174864 had no effect on the reflex haemodynamic responses to left atrial phenylbiguanide or intrapericardial nicotine. 3. Intravenous administration of the 5-HT3 receptor antagonist MDL 72222, or intrapericardial administration of the nicotinic ganglionic cholinoceptor antagonist mecamylamine HCl, had no effect on the haemodynamic response to acute central hypovolaemia. 4. We conclude that phenylbiguanide-sensitive myocardial afferents and nicotine-sensitive epicardial afferents play no part in the response to acute hypovolaemia in rabbits, and that the reflex effects evoked by chemically exciting these afferents do not depend on a brain-stem delta-opioid mechanism.
Afferent Pathways↗