On the patterns of blood pressure, heart rate, and blood levels of noradrenaline and adrenaline during haemorrhage in the rat

Caudal midline medulla mediates behaviourally-coupled but not baroreceptor-mediated vasodepression

Within the caudal medulla there are two regions whose activation leads to vasodepression and bradycardia, the caudal ventrolateral medulla and a discrete region of the caudal midline medulla. This study investigated, in the halothane anaesthetized rat, the contribution of these two vasodepressor regions to "homeostatic" and "behaviourally-coupled" cardiovascular regulation. In an initial set of experiments the contribution of each of these two regions to the hypotension and bradycardia evoked by acute hypovolaemia (15% haemorrhage) was investigated. It was found that inactivation of the caudal midline medulla significantly attenuated (cobalt chloride) or completely blunted (lignocaine) the hypotension and bradycardia evoked by acute hypovolaemia. In contrast, inactivation of the caudal ventrolateral medulla using cobalt chloride, although attenuating the magnitude of the hypotension and completely blocking the bradycardia, did not delay the onset of the hypotension evoked by acute hypovolaemia. The caudal ventrolateral medulla is known to be critical in homeostatic cardiovascular control through the expression of the "baroreceptor reflex" and the hypotension and bradycardia evoked by activation of cardiopulmonary afferents. In a second series of experiments we found inactivation of the caudal midline medulla played no role in baroreflex-evoked bradycardia (i.v. phenylephrine) or the hypotension and bradycardia evoked by cardiopulmonary afferent activation (i.v. 5-hydroxytryptamine). These data suggest that the caudal midline medulla and caudal ventrolateral medulla play different roles in cardiovascular control. The caudal ventrolateral medulla is involved in mediating cardiovascular changes associated with a variety of stimuli including "homeostatic" and "behaviourally-coupled" cardiovascular changes, whereas the caudal midline medulla is critical for mediating "behaviourally-coupled" changes in arterial pressure and heart rate.

Reduced metabolic rate accompanies the hemorrhage-induced hypothermia in conscious rats

The mechanism for the hemorrhage-induced drop in body temperature is unknown. This study determined the alterations in cutaneous heat exchange and metabolic heat production caused by a moderate hemorrhage in conscious rats. Chronically instrumented rats were subjected to a 16 ml/kg hemorrhage, followed by a 4-h recovery period, while monitoring body core temperature and cutaneous temperature. Cutaneous heat transfer was disrupted by housing the animals at an elevated (28 degrees C) ambient temperature. A separate group of experiments measured the change in oxygen consumption in the post-hemorrhage period. Moderate hemorrhage caused a drop in body core temperature which stabilized at 0.7+/-0.3 degrees C below control in the second hour following hemorrhage. Disruption of cutaneous heat exchange by reducing the thermal gradient did not diminish the hemorrhage-induced hypothermia. Hemorrhage caused a significant decline of oxygen consumption (-0. 21+/-0.05 ml O(2)/g per h). This 16% drop in resting oxygen consumption was prevented by immediately retransfusing the aspirated blood back into the rat. These data indicate that a decrease in metabolic heat production mediates the drop in body core temperature caused by moderate hemorrhage in conscious rats.

Hypotension following acute hypovolaemia depends on the caudal midline medulla

Acute hypovolaemia evokes abrupt, life-threatening hypotension and bradycardia. Hypotension can be evoked also by excitation of the caudal midline medulla (CMM). This study investigated the possible contribution of the CMM depressor area to hypotension evoked by acute hypovolaemia. Inactivation of the CMM, with either lignocaine or cobalt chloride did not alter resting arterial pressure. However lignocaine injections blocked the fall in arterial pressure, and cobalt chloride injections delayed the onset and significantly attenuated the size of hypovolaemic-evoked hypotension. These findings suggest that the CMM is a key region triggering hypotension after blood loss, and that the brain areas mediating cardiovascular response to challenges such as acute hypovolaemia are not the same areas that regulate resting arterial pressure.

Glycyl-L-glutamine [beta-endorphin-(30-31)] attenuates hemorrhagic hypotension in conscious rats

The profound hypotension caused by acute hemorrhage is thought to involve opioid peptide neurons. In this study, we tested whether glycyl-L-glutamine [Gly-Gln; beta-endorphin-(30-31)], a nonopioid peptide derived from beta-endorphin processing, prevents the cardiovascular depression induced by hemorrhage in conscious and anesthetized rats. Previously, we found that Gly-Gln inhibits the hypotension and respiratory depression produced by beta-endorphin and morphine but does not affect opioid antinociception. Hemorrhage (2.5 ml/100 g body wt over 20 min) lowered arterial pressure in conscious rats (from 120.1 +/- 2.9 to 56.2 +/- 4.7 mmHg) but did not change heart rate significantly. Intracerebroventricular Gly-Gln (3, 10, or 30 nmol) pretreatment inhibited the fall in arterial pressure and increased heart rate significantly. The response was dose related and was sustained during the 35-min posthemorrhage interval. Pentobarbital sodium anesthesia potentiated the hemodynamic response to hemorrhage and attenuated the effect of Gly-Gln. Gly-Gln (10 or 100 nmol icv) did not influence arterial pressure or heart rate in normotensive rats. These data indicate that Gly-Gln is an effective antagonist of hemorrhagic hypotension.

Autonomic mediation of short-term cardiovascular oscillations after acute hemorrhage in conscious rats

The role of the autonomic and the renin-angiotensin (R-A) activities in short-term cardiovascular control during the bradycardic phase following severe hemorrhage was investigated in conscious rats. Spectral analysis of beat-to-beat fluctuations of the R-R interval (RRI), systolic (SBP) and diastolic (DBP) blood pressure in the 0.01-5-Hz range was carried out under control conditions and following a bleeding of 30% of total blood volume, with and without i.v. injection of atropine (2 mg/kg), prazosin (2 mg/kg), propranolol (5 mg/kg) or captopril (7 mg/kg). The bradycardic stage was characterized by: (i) an increase of the three oscillatory components exhibited by RRI variability which appears driven by vagal activity and buffered by beta-adrenergic activity, while the increase of the slower LF (0.01-0.2 Hz) oscillations seems driven by the slow alpha-adrenergic control; (ii) a decrease of SBP and DBP oscillations and absence of SBP-RRI correlation in the MF band (0.2-0.6 Hz) possibly related to a decrease in the sympathetic drive of SBP-MF and DBP-MF oscillations and in RRI baroreflex control; (iii) an increase in LF oscillations of SBP and even more of DBP that seems driven by the slow alpha-and beta-adrenergic control and buffered by the R-A control, responses possibly related to an increase of LF oscillations of peripheral resistance; (iv) the persistence of high SBP-RRI correlation in the LF band and in the respiratory band (1-3 Hz); the former seem to reflect the existence of a slow baroreflex control mediated by beta-adrenergic activity, the latter appears to be caused by feedforward mechanical effects of RRI changes on SBP.

The role of the adrenal medulla in cardiovascular responses to hypertonic saline in haemorrhaged conscious rats

The aim of this study was to determine if the adrenal medulla plays a role in mediating haemodynamic effects of hypertonic saline (HTS) resuscitation during haemorrhagic hypotension in conscious rats. Wistar-Kyoto rats were either adrenomedullectomized (ADMX, n = 11) or sham-operated (SHAM, n = 10) and implanted with intravascular catheters. Pre-haemorrhage resting mean arterial pressure (MAP) was lower in the ADMX than in the SHAM group. Haemorrhage was performed by withdrawal of blood through the venous catheter, and a MAP of 50 mmHg was maintained for 1 h by further withdrawal when necessary. Both groups responded to haemorrhage with marked bradycardia. Plasma adrenaline (A) rose 10-fold in response to hypotension in the SHAM group, while remaining at pre-haemorrhage levels in the ADMX group, indicating successful adrenal demedullation. Infusion of 2.0 ml kg-1 HTS (NaCl 8.0 mg ml-1 i.v.) produced an immediate increase in MAP and heart rate (HR) in both groups, to pre-haemorrhage values or higher. Plasma noradrenaline (NA) increased in both groups after HTS, while the high levels of A in the SHAM group slowly returned toward baseline. Plasma glucose and rate of haemodilution was higher in the SHAM group during and after hypotension. The maintenance of MAP above 60 mmHg was less effective in the SHAM group during the first 2 h after HTS, but after 24 h, prehaemorrhage MAP was established in both groups. In conclusion, the adrenal medulla does not play a major role in the response to HTS resuscitation.

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.

Cardiovascular and catecholamine responses to acute haemorrhage in anaesthetized potassium-adapted rats

The experiments were designed to determine whether potassium-loaded rats have a deficient recovery of blood pressure after a rapid arterial haemorrhage. Potassium loading was achieved by providing a 0.75% KCl solution as drinking fluid for 14 days, while control rats had either distilled water or tapwater. MAP, HR, Hct, and plasma electrolytes were determined before and after 1 and 2% body weight haemorrhage in anaesthetized Sprague-Dawley rats. Potassium-loaded rats had significantly reduced blood pressure recovery within 20 min after haemorrhage. HR was significantly reduced within 5 min only after 2% haemorrhage in potassium-adapted rats. Haemorrhage induced significant hyperkalaemia which was greater and significantly prolonged after 2% haemorrhage. The significant fall in Hct after haemorrhage was not affected by the magnitude of haemorrhage. In an additional group of rats, the pressor response to intravenous infusion of vasopressin was unaffected by potassium loading, whereas that to noradrenaline and angiotensin II was significantly reduced throughout the 20 min of infusion. The peak increase in blood pressure after phenylephrine injection was, however, unaffected by potassium loading. Basal plasma catecholamines concentration as well as concentrations after 1% haemorrhage were unaffected by potassium loading. It is concluded that the reduced vascular response to noradrenaline and angiotensin contributed to the reduced recovery of blood pressure after haemorrhage in potassium-loaded rats. Furthermore, the result with phenylephrine suggest a mechanism that is unrelated to direct vascular effects of noradrenaline and angiotensin II.

Bradycardia during reversible hypovolaemic shock: associated neural reflex mechanisms and clinical implications

1. Heart rate response to reversible central hypovolaemia can be divided into three stages. In the first stage (corresponding to a reduction of the blood volume by approximately 15%) a modest increase in heart rate (< 100 beats/min) and total peripheral resistance compensate for the blood loss, and a near normal arterial blood pressure prevails (preshock). During the second stage, a reduction of the central blood volume by approximately 30% results in a decrease in heart rate, total peripheral resistance and blood pressure due to activation of unmyelinated vagal afferents (C-fibres) from the left ventricle. In the third stage, blood pressure falls further as haemorrhage continues and tachycardia (> 120 beats/min) is manifest. This stage may proceed into irreversible shock with death from cardiac arrest probably related to the formation of free oxygen radicals. 2. Recognition of the vasodepressor-cardioinhibitory reaction to a reduced circulating blood volume is important and suggests the need for immediate treatment with volume expansion in critically ill patients.

Heart rate during haemorrhagic shock

In a prospective study, heart rate and mean arterial pressure were related to estimated blood loss in 34 consecutive patients aged 23-92 years during resuscitation from haemorrhagic shock. Eighteen patients with a blood loss of less than 31 (1.9 [0.9-3.0] l) (median and range), corresponding to 34 (16-46) % of estimated blood volume, had a heart rate of 83 (60-160) beats min-1 and a mean arterial pressure of 62 (35-73) mmHg. In 16 patients with a blood loss of more than 3 l (4.0 [3.3-5.0] l) corresponding to 89 (35-100) % of the estimated blood volume, heart rate was 120 (110-160) beats min-1 (P < 0.05) and mean arterial pressure 52 (0-70) mmHg (P < 0.05). Six patients died due to severe bleeding (3.1 [2.5-5.0] l) with a heart rate of 129 (110-160) beats min-1 and a mean arterial pressure of 40 (0-70) mmHg. It is concluded that reversible hypovolaemic shock is associated with a relatively low heart rate (approximately 80 beats min-1) and that tachycardia (approximately 120 beats min-1) is associated with profound bleeding. Hypovolaemic shock with tachycardia may represent a transition to an irreversible stage.

Reflex changes in post- and preganglionic sympathetic adrenal nerve activity and postganglionic sympathetic renal nerve activity upon arterial baroreceptor activation and during severe haemorrhage in the rat

The aim of the study was to compare pre- (pre-aSNA) and postganglionic adrenal sympathetic nerve activity (post-aSNA) and postganglionic renal sympathetic nerve activity (rSNA) in rats during arterial baroreceptor activation and haemorrhage. Adrenal multifibre nerve activity was recorded in chloralose-anaesthetized Wistar rats. To test for pre-aSNA or post-aSNA in adrenal nerves, a ganglionic blocker, trimethaphan (10 mg kg-1), was administered i.v. If the nerve activity in the adrenal nerve decreased or increased the nerve was considered to contain predominantly post- or preganglionic fibres, respectively. In contrast, the renal nerves exhibit an almost pure postganglionic activity. Baroreceptor activity was tested by activation of baroreceptors, with an alpha-receptor agonist, phenylephrine, which was slowly infused (0.5-2 micrograms kg-1 min-1), and to deactivate the baroreceptors the rats were bled down to 50 mmHg for 8 min. The experiments showed that all tested nerve types were baroreceptor dependent. There were no significant differences between the slopes relating nerve activity inhibition to increase in blood pressure (infusion of phenylephrine). During maximal inhibition there was a difference between the rSNA and pre-aSNA, 87 +/- 4%, n = 6, and 68 +/- 6%, n = 10 (P less than 0.01) of the control value, respectively. The maximal inhibition of post-aSNA was 80 +/- 3%, n = 7, of the control value. During haemorrhage there was a difference between the nerve populations. Pre-aSNA responded with a marked increase within 1.5 min (159 +/- 29% of control, n = 7) and was then maintained at that level until retransfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced responsiveness to noradrenaline in isolated rat atria exposed to hyperosmotic solutions

1. The effects of hyperosmotic NaCl and sucrose solutions on the responsiveness to noradrenaline (NA) were studied in isolated rat right and left atria. 2. Sucrose caused subsensitivity to NA and isoprenaline in right atria, which was abolished by atropine. 3. NaCl caused subsensitivity only to NA in both atria, which was reversed by imipramine. 4. Both solutions decreased the maximum tension of left atria after maximal NA and Ca2+ concentrations. 5. The results suggest that hyperosmolality reduces the atrial responsiveness to NA by directly depressing contractility and increasing acetylcholine release. However, if the solute is NaCl, the main mechanism seems to be an increase of the catecholamine neuronal uptake.

Cardiovascular and endocrine responses to haemorrhage in the pig

Heart rate (HR), mean arterial pressure (MAP), indices of sympathetic and parasympathetic activity (plasma concentrations of adrenaline, noradrenaline and pancreatic polypeptide, PP), vasopressin (VP) and aldosterone (ALDO) were measured in six pigs during continuous bleeding resulting in hypovolaemic shock, from which five survived. Three stages of haemorrhage could be defined. Stage I. Resting MAP was 85 +/- 6 mmHg and increased to 96 +/- 5 mmHg with a blood loss of 275 (range 250-300) (10 (9-12)% of the estimated blood volume) concomitant with an increase in HR from 105 +/- 5 to 113 +/- 6 beats min-1 (P less than 0.05). Stage II. After a blood loss of 375 (300-500) ml (15 (13-16)%) MAP fell to 62 +/- 9 mmHg and HR to 95 +/- 5 beats min-1 (P less than 0.05). Stage III. A blood loss of 1113 (825-1450) ml (44 (30-52)%) resulted in a MAP of 50 +/- 4 mmHg and an increase in HR to 206 +/- 3 beats min-1 (P less than 0.05). Adrenaline increased from 0.3 +/- 0.1 to 0.8 +/- 0.3 (stage II) and 3.6 +/- 1.1 nmol l-1 (stage III) (P less than 0.05); noradrenaline from 0.4 +/- 0.1 to 1.5 +/- 0.4 (stage II) and 5.9 +/- 1.7 nmol l-1 (stage III) (P less than 0.05); PP from 6.2 +/- 1.6 to 13.3 +/- 2.3 (stage II) and 20.9 +/- 7.8 pmol l-1 (stage III) (P less than 0.05). VP changed only marginally, but ALDO increased from 496 +/- 54 to 623 +/- 76 pmol l-1 (stage III) (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary supplementation of tyrosine prevents the rapid fall in blood pressure during haemorrhage

Previous studies have demonstrated the ability of tyrosine (TYR), the amino acid precursor of catecholamines, to increase blood pressure in rats made hypotensive by haemorrhage. Other studies have shown that supplementation of the diet with TYR can reverse certain neurochemical and behavioural consequences associated with acute stress. Such studies demonstrate that during conditions of enhanced neuronal firing catecholamine synthesis is accelerated when additional precursor TYR is made available. In these situations the rate-limiting enzyme of catecholamine synthesis, tyrosine hydroxylase, activated via phosphorylation, becomes responsive to additional TYR. Our experiments were designed to study the ability of dietary TYR (3.7%, or 4X the normal amount), to prevent the rapid fall in blood pressure observed during acute haemorrhage. Rats consuming the high TYR diet (5 days) maintained arterial blood pressure (systolic, diastolic and mean) at significantly greater values during the period of acute haemorrhagic insult than animals maintained on a control diet. Rats fed the high TYR diet had significantly greater levels of the amino acid in the heart, adrenal glands, liver, kidney, brainstem, spleen and semimembranosus pars caudalis muscle. We conclude that TYR can be stored and most likely utilized in the synthesis of catecholamines for the maintenance of arterial blood pressure during acute haemorrhage. These results are of particular importance in light of the fact that most total parenteral nutrition solutions contain very little if any TYR.

Relative cerebral ischemia in SHR due to hypotensive hemorrhage: cerebral function, blood flow and extracellular levels of lactate and purine catabolites

Cerebral blood flow (CBF, by laser Doppler flowmetry) and extracellular cortical concentrations (by microdialysis) of adenosine, inosine, xanthine, hypoxanthine, and lactate were measured together with somatosensory evoked potentials (SEP) in chloralose-anaesthetized spontaneously hypertensive rats (SHR) during relative cerebral ischemia induced by hypotensive hemorrhage. Reduction of mean arterial blood pressure (MABP) to 40-50 mm Hg, which decreased SEP to about 50% of prebleeding control level, decreased CBF only to about 75% of control due to cerebrovascular "autoregulation." A secondary, marked rise in cerebrovascular resistance (CVR) occurred after about 15 min in parallel with a striking increase in heart rate (after initial bradycardia). This late rise in heart rate is probably elicited by relative ischemia in medullary centers. The increase in CVR might indicate increased sympathetic nerve activity to the circle of Willis and large cerebral arteries. Cortical lactate increased initially but started to decline after about 30 min, and after 2 h it was not significantly higher than control. Cortical adenosine, inosine, hypoxanthine, and xanthine increased slowly and were significantly elevated after 50 min of hemorrhage. After 80 min, adenosine and inosine had returned to initial levels, while hypoxanthine and xanthine were further elevated. Despite the apparent partial recovery of metabolic disturbances during late hemorrhage, and with a blood flow maintained at 75% of resting control, SEP did not improve. It is suggested that the depression of SEP is not primarily caused by circulatory-metabolic derangements, but instead by activation of specific inhibitory systems.

Slowing of the heart during anaphylactic shock. A report of five cases

Five patients are reported with anaphylactic shock reactions due to i.v. administration of atracurium, methicillin, amidotrezoate (n = 2) and plasma. In these patients a fall in heart rate (HR) from 5-50 beats.min-1 was recorded during the hypotensive period. In three of the patients HR was 40 beats.min-1 or less, and in two of the patients a hypotensive bradycardiac episode spontaneously reappeared after normal HR and BP were established. Central venous pressure (CVP) fell in one patient from 0.8 to 0.6 kPa as HR and BP decreased while arterial oxygen tension remained normal. All patients were in a stable condition after treatment with volume expansion and sympathomimetic drugs. These observations indicate that anaphylactic shock is associated with a strong and sometimes persistent vagal tone to the heart in parallel with the early reaction to hypotensive hypovolaemic shock.

Acidosis, hypoxia and stress hormone release in response to one-minute inhalation of 80% CO2 in swine

The study pertains to a series of investigations on the effects of CO2 inhalation as used for pre-slaughter anaesthesia in swine. Acid/base parameters, blood oxygen tension, plasma Na, K, Ca and stress hormone concentrations were monitored in Yorkshire swine before, during, and for 10 min after the animals were descended for 1 min into 80% CO2 in air. Severe respiratory acidosis (PaCO2 approximately 50 kPa, arterial pH approximately 6.6) and hypoxia (PaO2 approximately 4kPa) had developed after 45 s of the CO2 inhalation. The corresponding changes in venous blood were less drastic (PvCO2 approximately 17 kPa, pH 7.1, PvO2 approximately 4 kPa). Readjustment to PaCO2 approximately II kPa, arterial pH 7.2, and PaO2 approximately 13 kPa had occurred at 1 min post CO2. Four minutes later the respiratory acidosis had become converted into metabolic acidosis subjected to partial respiratory compensation (arterial pH 7.3 in the presence of moderate hypocapnia and hyperoxaemia). The cause of this metabolic acidosis (present also at 10 min post CO2) was apparently hypoxia-induced anaerobic metabolism (= lactic acid production). Apparently due to hydrogen ion transport into the cells in exchange for other cations, hyperkalaemia (K approximately 6.6 mmol l-1), and a 7 mmol l-1 increase in plasma Na had developed at 1.5 min later. The CO2 inhalation did not change the total plasma Ca significantly. The transport of the swine from the stable to the immediate pre-experimental situation induced a 3-fold increase in plasma cortisol concentration (PC, to approximately 130 mmol l-1). No further increase in PC occurred in response to the CO2 inhalation. It indicates that no additional emotional strain was imposed upon the animals during the CO2 exposure.(ABSTRACT TRUNCATED AT 250 WORDS)