Abdominal vascular responses to changes in carbon dioxide tension in the cephalic circulation of anaesthetized dogs

Influence of hypercapnia on cardiovascular responses to tracheal intubation

BACKGROUND

Laryngoscopy and tracheal intubation are often associated with tachycardia, hypertension, and arrhythmias. There is a risk of hypercapnia in the case of difficult mask ventilation. The circulatory response to hypercapnia is increases in arterial pressure and heart rate. We evaluated the difference of cardiovascular responses to tracheal intubation between normocapnia and hypercapnia during mask ventilation before tracheal intubation.

METHODS

We studied 40 ASA physical status I to II patients under general anesthesia. Induction of anesthesia was achieved with midazolam 0.05 mg/kg, propofol 1.5 mg/kg, alfentanil 10 microg/kg, and rocuronium 0.6 mg/kg IV. The lungs were mechanically ventilated with a tidal volume of 10 mL/kg and 6 to 10 bpm in the hypercapnia group (n = 20) or 12 to 15 bpm in the normocapnia group (n = 20) during the induction period. Intubation was performed 3 minutes after the induction, and anesthesia was maintained using 1.5% sevoflurane (inspired) and 75% N(2)O in oxygen. Heart rate, systolic arterial pressure (SAP), and diastolic arterial pressure were recorded every minute throughout the study.

RESULTS

The proportion of the patients whose increase of SAP between just before intubation and 1 minute after intubation was more than 30 mm Hg in the hypercapnia group (40%) was greater than that in the normocapnia group (9.5%) (P = .0325). There were no differences in heart rate and diastolic arterial pressure between hypercapnia and normocapnia groups. For the SAP of the patients, the trend of changes was increased (P = .024).

CONCLUSIONS

Hypercapnia during mask ventilation before tracheal intubation may exaggerate the increase of SAP during intubation compared to normocapnia. Ventilation was important in minimizing hemodynamic responses during induction regardless of using drugs.

Protective effects of recombinant human interleukin-4 administration on the hypoxia-reoxygenation-induced gastric and intestinal injury in rat pups

A total of 40 rat pups were randomly divided into 4 groups. Group 1 (sham) rats served as nonhypoxic controls. Group 2 (rhIL-4 treated/control) rats were administrated rIL-4 alone. Group 3 (hypoxia-reoxygenation [H-O]/untreated) rats were subjected to H-O and were then returned to their mothers. Group 4 (H-O/rhIL-4 treated) rats were subjected to H/O and were treated with rhIL-4 for the next 3 days. All animals were killed on day 4 and gastric and intestinal specimens were obtained to determine the tissue level of malondialdehyde (MDA) and histological changes. In group 3 MDA levels were significantly increased compared with groups 1,2, and 4. In group 4, MDA levels were not significantly different compared with group 3. The gastric and intestinal injury score were increased significantly in group 3 and 4 rats compared with group 1 and 2. However, this increase was lower in group 4 rats compared with group 3. In group 3, after hypoxia following reoxygenation, exfoliation and necrosis of superficial cells, blood cell infiltration, and structural alterations on the two-third parts of the glandular pits, and bleeding erosions developed in all stomachs. Treatment with rhIL-4 produced a reduction of exfoliation of superficial cells, hemorrhage, and blood cell infiltration. In group 3 animals, destruction of villi and crypts of ileum and in some cases extension to the muscularis were noticed. In contrast, in the rats treated with rhIL-4, lesions were limited essentially to the very tips of the villi. This study found beneficial effects of rhIL-4 in an experimental model of hypoxia-induced gastric and intestinal injury.

Interaction of chemoreceptor and baroreceptor reflexes by hypoxia and hypercapnia - a mechanism for promoting hypertension in obstructive sleep apnoea

Asphyxia, which occurs during obstructive sleep apnoeic events, alters the baroreceptor reflex and this may lead to hypertension. We have recently reported that breathing an asphyxic gas resets the baroreceptor-vascular resistance reflex towards higher pressures. The present study was designed to determine whether this effect was caused by the reduced oxygen tension, which affects mainly peripheral chemoreceptors, or by the increased carbon dioxide, which acts mainly on central chemoreceptors. We studied 11 healthy volunteer subjects aged between 20 and 55 years old (6 male). The stimulus to the carotid baroreceptors was changed using graded pressures of -40 to +60 mmHg applied to a neck chamber. Responses of vascular resistance were assessed in the forearm from changes in blood pressure (Finapres) divided by brachial blood flow velocity (Doppler) and cardiac responses from the changes in RR interval and heart rate. Stimulus-response curves were defined during (i) air breathing, (ii) hypoxia (12% O(2) in N(2)), and (iii) hypercapnia (5% CO(2) in 95% O(2)). Responses during air breathing were assessed both prior to and after either hypoxia or hypercapnia. We applied a sigmoid function or third order polynomial to the curves and determined the maximal differential (equivalent to peak sensitivity) and the corresponding carotid sinus pressure (equivalent to 'set point'). Hypoxia resulted in an increase in heart rate but no significant change in mean blood pressure or vascular resistance. However, there was an increase in vascular resistance in the post-stimulus period. Hypoxia had no significant effect on baroreflex sensitivity or 'set point' for the control of RR interval, heart rate or mean arterial pressure. Peak sensitivity of the vascular resistance response to baroreceptor stimulation was significantly reduced from -2.5 +/- 0.4 units to -1.4 +/- 0.1 units (P < 0.05) and this was restored in the post-stimulus period to -2.6 +/- 0.5 units. There was no effect on 'set point'. Hypercapnia, on the other hand, resulted in a decrease in heart rate, which remained reduced in the post-stimulus period and significantly increased mean blood pressure. Baseline vascular resistance was significantly increased and then further increased in the post-control period. Like hypoxia, hypercapnia had no effect on baroreflex control of RR interval, heart rate or mean arterial pressure. There was, also no significant change in the sensitivity of the vascular resistance responses, however, 'set point' was significantly increased from 74.7 +/- 4 to 87.0 +/- 2 mmHg (P < 0.02). This was not completely restored to pre-stimulus control levels in the post-stimulus control period (82.2 +/- 3 mmHg). These results suggest that the hypoxic component of asphyxia reduces baroreceptor-vascular resistance reflex sensitivity, whilst the hypercapnic component is responsible for increasing blood pressure and reflex 'set point'. Hypercapnia appears to have a lasting effect after the removal of the stimulus. Thus the effect of both peripheral and central chemoreceptors on baroreflex function may contribute to promoting hypertension in patients with obstructive sleep apnoea.

PET(CO(2)) inversely affects MSNA response to orthostatic stress

Arterial hypocapnia has been associated with orthostatic intolerance. Therefore, we tested the hypothesis that hypocapnia may be detrimental to increases in muscle sympathetic nerve activity (MSNA) and total peripheral resistance (TPR) during head-up tilt (HUT). Ventilation was increased approximately 1.5 times above baseline for each of three conditions, whereas end-tidal PCO(2) (PET(CO(2))) was clamped at normocapnic (Normo), hypercapnic (Hyper; +5 mmHg relative to Normo), and hypocapnic (Hypo; -5 mmHg relative to Normo) conditions. MSNA (microneurography), heart rate, blood pressure (BP, Finapres), and cardiac output (Q, Doppler) were measured continuously during supine rest and 45 degrees HUT. The increase in heart rate when changing from supine to HUT (P < 0.001) was not different across PET(CO(2)) conditions. MSNA burst frequency increased similarly with HUT in all conditions (P < 0.05). However, total MSNA and the increase in total amplitude relative to baseline (%DeltaMSNA) increased more when changing to HUT during Hypo compared with Hyper (P < 0.05). Both BP and Q were higher during Hyper than both Normo and Hypo (main effect; P < 0.05). Therefore, the MSNA response to HUT varied inversely with levels of PET(CO(2)). The combined data suggest that augmented cardiac output with hypercapnia sustained blood pressure during HUT leading to a diminished sympathetic response.

Contribution of carotid chemoreceptors to mesenteric venoconstriction during acute hypercapnia in rabbits

The contribution of carotid chemoreceptors to hypercapnia-induced mesenteric venoconstriction was examined in 12 alpha-chloralose-anesthetized rabbits (1.0-1.6 kg). Surgical preparation consisted of a tracheotomy, femoral arterial and venous cannulation, and a midline laparotomy through which a 13-cm loop of ileum was exteriorized and superfused with physiological salt solution. Mesenteric vein diameter and intravenous pressure (using a servo-null measurement system) were measured in 500- to 1,000-micron mesenteric veins during 40-s periods of 15%, 20%, and 25% CO2 inhalation. Measurements were then repeated following bilateral ablation of the carotid chemoreceptors. Before denervation, mesenteric vein diameter constricted 6.5 +/- 1.1%, 11.9 +/- 1.1%, and 17.9 +/- 2.2% during the 15%, 20%, and 25% CO2 inhalation, respectively. After denervation, these values were reduced to 5.0 +/- 0.9%, 6.9 +/- 1.2%, and 8.4 +/- 1.3%, respectively. We conclude that activation of the carotid chemoreceptors by hypercapnia induces active mesenteric venoconstriction. After denervation of the carotid baroreceptors and chemoreceptors, there was also a small decrease in venule diameter proportional to the level of inspired CO2. We further conclude that noncarotid body chemoreceptor activation contributes to mesenteric venular constriction.

Two populations of sympathetic neurons project selectively to mesenteric artery or vein

The objective of this study was to determine whether sympathetic neurons of the inferior mesenteric ganglion (IMG) projecting to mesenteric arteries could be distinguished by their localization, neurochemical phenotype, and electrophysiological properties from neurons projecting to mesenteric veins. In an in vitro intact vasculature-IMG preparation, neurons were labeled following intraluminal injection of Fluoro-Gold or rhodamine beads into the inferior mesenteric artery (IMA) or vein (IMV). The somata of neurons projecting to IMA were localized in the central part of the IMG, whereas those projecting to IMV were localized more peripherally. None of the labeled neurons was doubly labeled. Neuropeptide Y immunoreactivity was found in 18.9% of neurons innervating the IMA, but not in neurons innervating the IMV. Identified neurons were dissociated and characterized using whole cell patch-clamp recording. After direct soma depolarization, all of the labeled arterial and venous neurons were classified as tonic firing, compared with only 40% of unlabeled neurons; the remaining 60% of unlabeled neurons were phasic firing. The results indicate that IMG neurons projecting to mesenteric arteries are distinct from neurons projecting to mesenteric veins.

Effect of halothane and isoflurane on in situ diameter responses of small mesenteric veins to acute graded hypercapnia

The purpose of the present study was to quantify the inhibitory effect of inhaled halothane and isoflurane on acute hypercapnia-induced responses of capacitance-regulating veins and related cardiovascular variables in response to sequential 40-s periods of 5%, 10%, 15%, and 20% inspired CO2 (FICO2). Measurements were made in normoxic alpha-chloralose-anesthetized rabbits before, during, and after either 0.75 minimum alveolar anesthetic concentration inhaled halothane or isoflurane. The graded hypercapnia caused graded venoconstriction and bradycardia but minimal pressor responses. Hypercapnia-induced venoconstriction was blocked by prior local superfusion of the exposed veins with 3 x 10(-6) M tetrodotoxin. Both the hypercapnia-induced venoconstriction and bradycardia responses were significantly attenuated by halothane or isoflurane and did not fully recover after removal of the anesthetics from the circulation. Both anesthetics produced a significant baseline (i.e., prehypercapnia) hypotension and a tendency toward a resultant tachycardia. The baseline hypotension did not recover completely after elimination of the anesthetic. Neither anesthetic altered baseline vein diameter. These results agree with previous studies demonstrating that hypercapnic acidosis produces mesenteric venoconstriction by elevating excitatory sympathetic efferent neural input via activation of peripheral and central chemoreceptors and that bradycardia results from activation of compensatory baroreflexes. The neural components of these reflexes are possible primary sites for attenuation of these cardiovascular responses by halothane and isoflurane.

Comparison of the frequency dependence of venous and arterial responses to sympathetic nerve stimulation in guinea-pigs

1. Intracellular potentials and measurements of contractions were recorded in adjacent veins and arteries in the colonic mesentery of the guinea-pig in vitro during stimulation of post-ganglionic nerve trunks. 2. Repetitive stimulation (0.5-5 Hz) of lumbar colonic nerve trunks produced frequency-dependent slow depolarizations in all venous and in 92% of arterial smooth muscle cells. Excitatory junction potentials were observed for each nerve shock in arteries, but not in veins. 3. Low-frequency stimulations produced slow depolarizations of greater amplitude and longer duration in veins than in arteries. The frequencies at which half-maximal depolarizations and contractions occurred were always lower for veins than for arteries. 4. The alpha 1-adrenergic antagonist prazosin (5 X 10(-7) M) reduced the mean arterial slow depolarizations by 82% and reduced mean venous slow depolarizations by 58% for 5 Hz stimulations. Arterial contractions were completely inhibited by prazosin but venous contractions were incompletely reduced in a frequency-dependent manner. 5. These findings suggest that functional differences in activation between mesenteric veins and arteries during sympathetic stimulation are a result of differences in neuromuscular transmission.

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.

The effect of pregnancy on the changes in hind-limb vascular resistance following haemorrhage in the rabbit

The change in vascular resistance of the skinned hind limb perfused at constant flow has been measured following rapid removal of 10% of the blood volume in anaesthetized pregnant rabbits (27-29 days gestation) and compared with that of non-pregnant rabbits. 5 s after haemorrhage was completed vascular resistance had risen in all rabbits, but in the pregnant ones this increase was only one-fifth of that in the non-pregnant ones. After section of either the carotid sinus and aortic nerves or the sympathetic innervation to the hind limb the increases of vascular resistance at this time were abolished. It is concluded that the previously described diminution during pregnancy of the protective function of the baroreflexes during haemorrhage results, to some extent, from their diminished role in a vascular bed not associated with the reproductive process. The blood withdrawn was not re-infused for 10 min, throughout which, in all rabbits with sinus, aortic and sympathetic nerves intact, the increase in vascular resistance was sustained. After section of the carotid sinus and aortic nerves and the sympathetic innervation there was, during this 10 min, a slow increase in vascular resistance comparable in final size to that which occurred immediately in the respective innervated preparations, i.e. it was smaller in the pregnant than in the non-pregnant rabbits. A possible difference in action during pregnancy of a vasoconstrictor substance released during haemorrhage is discussed briefly.