Effects of fentanyl, diazepam, and the combination of both on arterial baroreflex and sympathetic nerve activity in intact and baro-denervated dogs

"The peripheral perfusion index discriminates haemodynamic responses to induction of general anaesthesia"

Induction of general anaesthesia is often accompanied by hypotension. Standard haemodynamic monitoring during anaesthesia relies on intermittent blood pressure and heart rate. Continuous monitoring systemic blood pressure requires invasive or advanced modalities creating a barrier for obtaining important information of the circulation. The Peripheral Perfusion Index (PPI) is obtained non-invasively and continuously by standard photoplethysmography. We hypothesized that different patterns of changes in systemic haemodynamics during induction of general anaesthesia would be reflected in the PPI. Continuous values of PPI, stroke volume (SV), cardiac output (CO), and mean arterial pressure (MAP) were evaluated in 107 patients by either minimally invasive or non-invasive means in a mixed population of surgical patients. 2 min after induction of general anaesthesia relative changes of SV, CO, and MAP was compared to the relative changes of PPI. After induction total cohort mean(± st.dev.) MAP, SV, and CO decreased to 65(± 16)%, 74(± 18)%, and 63(± 16)% of baseline values. In the 38 patients where PPI decreased MAP was 57(± 14)%, SV was 63(± 18)%, and CO was 55(± 18)% of baseline values 2 min after induction. In the 69 patients where PPI increased the corresponding values were MAP 70(± 15)%, SV 80(± 16)%, and CO 68(± 17)% (all differences: p < 0,001). During induction of general anaesthesia changes in PPI discriminated between the degrees of reduction in blood pressure and algorithm derived cardiac stroke volume and -output. As such, the PPI has potential to be a simple and non-invasive indicator of the degree of post-induction haemodynamic changes.

Potentiation of GABAergic synaptic transmission by diazepam acutely increases resting beat-to-beat blood pressure variability in young adults

Resting beat-to-beat blood pressure variability is a powerful predictor of cardiovascular events and end-organ damage. However, its underlying mechanisms remain unknown. Herein, we tested the hypothesis that a potentiation of GABAergic synaptic transmission by diazepam would acutely increase resting beat-to-beat blood pressure variability. In 40 (17 females) young, normotensive subjects, resting beat-to-beat blood pressure (finger photoplethysmography) was continuously measured for 5 to 10 min, 60 min after the oral administration of either diazepam (10 mg) or placebo. The experiments were conducted in a randomized, double-blinded, and placebo-controlled design. Stroke volume was estimated from the blood pressure waveform (ModelFlow) permitting the calculation of cardiac output and total peripheral resistance. Direct recordings of muscle sympathetic nerve activity (MSNA, microneurography) were obtained in a subset of subjects (N=13) and spontaneous cardiac and sympathetic baroreflex sensitivity calculated. Compared to placebo, diazepam significantly increased the standard deviation of systolic (4.7±1.4 vs. 5.7±1.5 mmHg, P=0.001), diastolic (3.8±1.2 vs. 4.5±1.2 mmHg, P=0.007) and mean blood pressure (3.8±1.1 vs. 4.5±1.1 mmHg, P=0.002), as well as cardiac output (469±149 vs. 626±259 ml/min, P<0.001) and total peripheral resistance (1.0±0.3 vs. 1.4±0.6 mmHg/l/min, P<0.001). Similar results were found using different indices of variability. Furthermore, diazepam reduced MSNA burst frequency (placebo: 22±6 vs. diazepam: 18±8 bursts/min, P=0.025) without affecting the arterial baroreflex control of heart rate (placebo: 18.6±6.7 vs. diazepam: 18.8±7.0 ms/mmHg, P=0.87) and MSNA (placebo: -3.6±1.2 vs. diazepam: -3.4±1.5 bursts/100Hb/mmHg, P=0.55). These findings suggest that GABA_A receptors modulate resting beat-to-beat blood pressure variability in young adults.

Injury characteristics and hemodynamics associated with guideline-compliant CPR in a pediatric porcine cardiac arrest model

BACKGROUND

Guidelines for depth of chest compressions in pediatric cardiopulmonary resuscitation (CPR) are based on sparse evidence.

OBJECTIVE

We sought to evaluate the performance of the two most widely recommended chest compression depth levels for pediatric CPR (1.5 in. and 1/3 the anterior-posterior diameter- APd) in a controlled swine model of asphyxial cardiac arrest.

METHODS

We executed a 2-group, randomized laboratory study with an adaptive design allowing early termination for overwhelming injury or benefit. Forty mixed-breed domestic swine (mean weight = 26 kg) were sedated, anesthetized and paralyzed along with endotracheal intubation and mechanical ventilation. Asphyxial cardiac arrest was induced with fentanyl overdose. Animals were untreated for 9 min followed by mechanical CPR with a target depth of 1.5 in. or 1/3 the APd. Advanced life support drugs were administered IV after 4 min of basic resuscitation followed by defibrillation at 14 min. The primary outcomes were return of spontaneous circulation (ROSC), hemodynamics and CPR-related injury severity.

RESULTS

Enrollment in the 1/3 APd group was stopped early due to overwhelming differences in injury. Twenty-three animals were assigned to the 1.5 in. group and 15 assigned to the 1/3 APd group, per an adaptive group design. The 1/3 APd group had increased frequency of rib fracture (6.7 vs 1.7, p < 0.001) and higher proportions of several anatomic injury markers than the 1.5 in. group, including sternal fracture, hemothorax and blood in the endotracheal tube (p < 0.001). ROSC and hemodynamic measures were similar between groups.

CONCLUSION

In this pediatric model of cardiac arrest, chest compressions to 1/3APd were more harmful without a concurrent benefit for resuscitation outcomes compared to the 1.5 in. compression group.

Low-dose fentanyl reduces pain perception, muscle sympathetic nerve activity responses, and blood pressure responses during the cold pressor test

Our knowledge about how low-dose (analgesic) fentanyl affects autonomic cardiovascular regulation is primarily limited to animal experiments. Notably, it is unknown if low-dose fentanyl influences human autonomic cardiovascular responses during painful stimuli in humans. Therefore, we tested the hypothesis that low-dose fentanyl reduces perceived pain and subsequent sympathetic and cardiovascular responses in humans during an experimental noxious stimulus. Twenty-three adults (10 females/13 males; 27 ± 7 yr; 26 ± 3 kg·m^-2, means ± SD) completed this randomized, crossover, placebo-controlled trial during two laboratory visits. During each visit, participants completed a cold pressor test (CPT; hand in ∼0.4°C ice bath for 2 min) before and 5 min after drug/placebo administration (75 μg fentanyl or saline). We compared pain perception (100-mm visual analog scale), muscle sympathetic nerve activity (MSNA; microneurography, 11 paired recordings), and beat-to-beat blood pressure (BP; photoplethysmography) between trials (at both pre- and postdrug/placebo timepoints) using paired, two-tailed t tests. Before drug/placebo administration, perceived pain (P = 0.8287), ΔMSNA burst frequency (P = 0.7587), and Δmean BP (P = 0.8649) during the CPT were not different between trials. After the drug/placebo administration, fentanyl attenuated perceived pain (36 vs. 66 mm, P < 0.0001), ΔMSNA burst frequency (9 vs. 17 bursts/min, P = 0.0054), and Δmean BP (7 vs. 13 mmHg, P = 0.0174) during the CPT compared with placebo. Fentanyl-induced reductions in pain perception and Δmean BP were moderately related (r = 0.40, P = 0.0641). These data provide valuable information regarding how low-dose fentanyl reduces autonomic cardiovascular responses during an experimental painful stimulus.

Effects of anesthetic and sedative agents on sympathetic nerve activity

BACKGROUND

The effects of sedative and anesthetic agents on sympathetic nerve activity (SNA) are poorly understood.

OBJECTIVE

The purpose of this study was to determine the effects of commonly used sedative and anesthetic agents on SNA in ambulatory dogs and humans.

METHODS

We implanted radiotransmitters in 6 dogs to record stellate ganglion nerve activity (SGNA), subcutaneous nerve activity (ScNA), and blood pressure (BP). After recovery, we injected dexmedetomidine (3 μg/kg), morphine (0.1 mg/kg), hydromorphone (0.05 mg/kg), and midazolam (0.1 mg/kg) on different days. We also studied 12 human patients (10 male; age 68.0 ± 9.1 years old) undergoing cardioversion for atrial fibrillation with propofol (0.77 ± 0.18 mg/kg) or methohexital (0.65 mg/kg) anesthesia. Skin sympathetic nerve activity (SKNA) and electrocardiogram were recorded during the study.

RESULTS

SGNA and ScNA were significantly suppressed immediately after administration of dexmedetomidine (P = .000 and P = .000, respectively), morphine (P = .011 and P = .014, respectively), and hydromorphone (P = .000 and P = .012, respectively), along with decreased BP and heart rate (HR) (P <.001 for each). Midazolam had no significant effect on SGNA and ScNA (P = .248 and P = .149, respectively) but increased HR (P = .015) and decreased BP (P = .004) in ambulatory dogs. In patients undergoing cardioversion, bolus propofol administration significantly suppressed SKNA (from 1.11 ± 0.25 μV to 0.77 ± 0.15 μV; P = .001), and the effects lasted for at least 10 minutes after the final cardioversion shock. Methohexital decreased chest SKNA from 1.59 ± 0.45 μV to 1.22 ± 0.58 μV (P = .000) and arm SKNA from 0.76 ± 0.43 μV to 0.55 ± 0.07 μV (P = .001). The effects lasted for at least 10 minutes after the cardioversion shock.

CONCLUSION

Propofol, methohexital, dexmedetomidine, morphine, and hydromorphone suppressed, but midazolam had no significant effects on, SNA.

Effect of Diazepam on 24-Hour Blood Pressure and Heart Rate in Healthy Young Volunteers

Aim: To assess the effects of evening chronic administration of diazepam on 24-h blood pressure (BP) and heart rate (HR) in healthy young adults. Methods: This randomized double blind, cross-over study evaluated the effects of diazepam 5 mg or placebo, both ingested in the evening, on 24-h ambulatory BP and HR in healthy subjects aged 21–30. Results: A total of 30 subjects were included in the analysis. At the end of 4-week diazepam intake, an increase in 24-h HR mean values was found (+5.2 beats/min, p < 0.05). Analysis of subperiods showed that diazepam produced a 10.1% increase in night-time HR (+6.1 beats/min, p < 0.01) without affecting BP. A significant HR rise (+4.9 beats/min, p < 0.05) and SBP reduction (–3.8 mm Hg, p < 0.05) were observed in the morning hours. The HR increase persisted in day-time hours (+4.6 beats/min, p < 0.05), while BP values resulted unaffected. Conclusions: In healthy subjects, diazepam taken as a hypnotic agent induces a significant HR increase, possibly mediated by a decrease in vagal tone. This effect might be of clinical relevance due to the role that HR plays as an independent cardiovascular risk factor.

Tonic GABAergic inhibition of sympathetic preganglionic neurons: a novel substrate for sympathetic control

The sympathetic tone is primarily defined by the level of activity of the sympathetic preganglionic neurons. We report a novel inhibitory influence on sympathetic activity, that of tonic GABAergic inhibition which could have a profound global effect on sympathetic outflow. Recording from identified SPNs in the intermediolateral cell column (IML) of rat spinal cord slices, application of the GABA receptor antagonist bicuculline, but not gabazine, elicited a change in voltage that lasted for the duration of application. This response was mediated by a direct effect on SPNs since it persisted in tetrodotoxin and low Ca(2+)/high Mg(2+) and the amplitude of responses were related to Cl(-) concentration in patch solutions. Such tonic inhibitory responses were not observed in interneurons, the other neuronal type in the IML, although ongoing IPSPs were antagonized in these neurons. The effects of bicuculline were enhanced by diazepam but not zolpidem or the GABA modulators THIP and THDOC suggesting a role for alpha5 subunits. PCR using primers for the alpha5 and delta subunits indicated the presence of alpha5, but not delta subunits in the IML. Firing rates of SPNs were enhanced by bicuculline and decreased by diazepam indicating that this tonic inhibition has a profound effect on the excitability of SPNs. These data indicate a novel influence for controlling the activity of SPNs regardless of their function.

Pilot safety study of low-dose vasopressin in non-septic critically ill children

OBJECTIVE

To assess the safety of low-dose vasopressin infusion in critically ill children requiring prolonged mechanical ventilation (MV) at risk of developing sedation/analgesia-related hypotension.

METHOD

Randomized pilot safety study in children expected to require MV for at least 3 days. Children received either vasopressin (0.0005 U/kg/min) or sodium chloride (0.9%) infusion for a period of 48 h. Haemodynamic variables, urine output and serum electrolytes were closely monitored and analyzed.

RESULTS

Twelve children in each group had similar baseline characteristics. Vasopressin infusion was associated with an 8 mmol/L fall in serum sodium concentration (p < 0.01) and with higher incidence of hyponatraemia (8 vs. 66%, p < 0.01). In normotensive children, low-dose vasopressin also induced a reversible decrease in urine output, and acutely increased blood pressure (p < 0.01). After stopping the vasopressin there was rebound hypotension (p < 0.01).

CONCLUSION

Low-dose vasopressin infusion in haemodynamically stable, but critically ill, children is associated with reduction in urine output and decreased serum sodium level, yielding a high incidence of hyponatraemia. We conclude that these effects limit further study of prophylactic vasopressin for sedation-related hypotension in a randomized controlled trial.

Effect of fentanyl on heart rate variability during spontaneous and paced breathing in healthy volunteers

BACKGROUND

Many studies have been conducted to assess the effects of fentanyl on the autonomic nervous system (ANS) by heart rate variability (HRV) analysis, but disagreement on the results is still present due to confounding influences introduced by concomitant administration of other drugs or opioid-related transitory respiratory depression. We performed a single-drug controlled-breathing study on healthy volunteers to assess the impact of fentanyl on ANS.

METHODS

HRV as a measure of sympathovagal balance was prospectively analyzed with an autoregressive model in 11 subjects during spontaneous and paced breathing (PB) at 20 breaths/min both before and after fentanyl 1 mcg/kg administration. HRV total power, sympathovagal balance (low-frequency/high-frequency ratio) and normalized spectral powers were considered (LFnu, HFnu).

RESULTS

Fentanyl led to a reduction of LFnu (from 55.2+/-23.3 to 43.2+/-24.1, P<0.05) without HFnu increase during PB. A decrease in R-R interval variance (from 3345.6+/-3333.4 to 1806.9+/-1328.6 ms(2), P<0.05) was shown after fentanyl administration during spontaneous but not PB. PB alone decreased the HRV total power and R-R interval variance.

CONCLUSIONS

Low-dose fentanyl administration in healthy volunteers leads to sympathetic and overall ANS modulation decrease, with a trend toward vagal activation.

Biphasic cardiac output changes during onset of spinal anaesthesia in elderly patients

BACKGROUND AND OBJECTIVE

In most studies of cardiac output changes after spinal anaesthesia, the time-resolution is limited. The aim of this study was to demonstrate cardiac output changes with high time-resolution during onset of spinal anaesthesia in elderly patients.

METHODS

We investigated 32 patients aged 60 yr scheduled for elective lower limb surgery. Fourteen received concurrent cardiovascular medication. Cardiac output was measured every 10 s using a pulse wave algorithm derived from the radial artery pressure curve, after calibration with lithium chloride (LiDCOplus). Data collection ended when the patients were ready for surgery, or if ephedrine was given to raise the mean arterial pressure.

RESULTS

Cardiac output increased initially reaching a maximum after a mean of 7 min. The average increase was 1.1 L min(-1) (P<0.0001). This occurred when mean arterial pressure was reduced 14 mmHg on average. At the end of data collection, cardiac output decreased 0.5 L min(-1) from baseline (P=0.02). Mean arterial pressure decreased progressively in all patients, and only minimal changes in heart rate were found.

CONCLUSIONS

Using this high time-resolution method, we detected biphasic changes in cardiac output during onset of spinal anaesthesia. Initially, cardiac output increased. Subsequently, it was significantly reduced from baseline, although this decrease was of minor clinical importance.

Redistribution of cardiac output during hemorrhagic shock in sheep

OBJECTIVE

Our goal was to evaluate the robustness of one of the assumptions used by esophageal Doppler monitors to compute systemic stroke volume and cardiac output; i.e., a constant flow proportion between supra-aortic vessels and descending aorta. For this purpose, we measured ascending and descending aortic blood flows during acute hemorrhage in anesthetized ewes.

DESIGN

Prospective, experimental study.

SETTING

Animal research facility.

SUBJECTS

Adult ewes.

INTERVENTIONS

Anesthetized animals were implemented with an aortic pressure transducer and two ultrasound transit time flowmeters placed around ascending and descending aorta, respectively. After baseline measurements, three incremental blood withdrawals were followed by progressive blood restitution in three similar steps.

MEASUREMENTS AND MAIN RESULTS

Ascending and descending aortic blood flows were reduced in a proportional manner after hemorrhage (-48% and -46%, respectively; p < .05 vs. baseline). Following blood restitution, flows were not fully restored, but ascending aortic flow was reduced by 27% with respect to initial control values while descending aortic flow was only 15% below. The agreement between ascending aortic flow and cardiac output calculated as descending aortic flow divided by 0.7 was characterized by a bias of 0.07 L/min and limits of agreement of +1.24 L/min and -1.10 L/min.

CONCLUSIONS

Minor blood flow redistribution between supra-aortic and descending aortic territories was seen only following blood restitution but not during hemorrhage in these anesthetized ewes. This observation supports the robustness of the assumption of constant flow proportion used by the esophageal Doppler monitor to calculate systemic stroke volume from descending aortic flow measurements.

Short-term electroacupuncture at Zusanli resets the arterial baroreflex neural arc toward lower sympathetic nerve activity

Although electroacupuncture reduces sympathetic nerve activity (SNA) and arterial pressure (AP), the effects of electroacupuncture on the arterial baroreflex remain to be systematically analyzed. We investigated the effects of electroacupuncture of Zusanli on the arterial baroreflex using an equilibrium diagram comprised of neural and peripheral arcs. In anesthetized, vagotomized, and aortic-denervated rabbits, we isolated carotid sinuses and changed intra-carotid sinus pressure (CSP) from 40 to 160 mmHg in increments of 20 mmHg/min while recording cardiac SNA and AP. Electroacupuncture of Zusanli was applied with a pulse duration of 5 ms and a frequency of 1 Hz. An electric current 10 times the minimal threshold current required for visible muscle twitches was used and was determined to be 4.8 +/- 0.3 mA. Electroacupuncture for 8 min decreased SNA and AP (n = 6). It shifted the neural arc (i.e., CSP-SNA relationship) to lower SNA but did not affect the peripheral arc (i.e., SNA-AP relationship) (n = 8). SNA and AP at the closed-loop operating point, determined by the intersection of the neural and peripheral arcs, decreased from 100 +/- 4 to 80 +/- 9 arbitrary units and from 108 +/- 9 to 99 +/- 8 mmHg (each P < 0.005), respectively. Peroneal denervation eliminated the shift of neural arc by electroacupuncture (n = 6). Decreasing the pulse duration to <2.5 ms eliminated the effects of SNA and AP reduction. In conclusion, short-term electroacupuncture resets the neural arc to lower SNA, which moves the operating point toward lower AP and SNA under baroreflex closed-loop conditions.

Evaluation of induction by use of a combination of oxymorphone and diazepam or hydromorphone and diazepam and maintenance of anesthesia by use of isoflurane in dogs with experimentally induced hypovolemia

OBJECTIVE

To compare induction with hydromorphone and diazepam (HydroD) or oxymorphone and diazepam (OxyD) followed by maintenance with isoflurane in dogs with induced hypovolemia.

ANIMALS

6 healthy mixed-breed dogs.

PROCEDURE

The study used a crossover design. Measurements were obtained in normovolemic dogs during isoflurane. Hypovolemia was induced (blood loss of 30 mL/kg) and measurements repeated following recovery from anesthesia, after HydroD (hydromorphone, 0.1 mg/kg; diazepam, 0.2 mg/kg; i.v.) or OxyD (oxymorphone, 0.05 mg/kg; diazepam, 0.2 mg/kg; i.v.), after another dose of the same opioid, during administration of isoflurane (end-tidal concentration, 0.9%), and after glycopyrrolate (0.01 mg/kg, i.v.). Significant changes were identified.

RESULTS

Induction effect was evident within 1 minute. All dogs were intubated after the second dose of opioid. No significant differences were found between inductions. The HydroD decreased heart rate (mean +/- SEM, -41 +/- 9.8 beats/min), whereas both inductions increased stroke index (0.4 +/- 0.09 mL/kg/beat) and caused moderate respiratory depression. Cardiac index was decreased (-30.2 +/- 6.04 mL/kg/min) and there was minor metabolic acidosis during isoflurane following HydroD, compared with values for anesthetized normovolemic dogs. Glycopyrrolate increased heart rate (50 +/- 8.6 beats/min) and decreased systolic blood pressure (-23.2 +/- 4.87 mm Hg) in dogs induced with HydroD and decreased stroke index (-0.3 +/- 0.08 mL/kg/beat) for both inductions.

CONCLUSIONS AND CLINICAL RELEVANCE

Similar effects were detected after administration of HydroD or OxyD in hypovolemic dogs. Either combination should be safe for use in hypovolemic dogs. Administration of glycopyrrolate was not beneficial.

Diazepam reduces both arterial blood pressure and muscle sympathetic nerve activity in human

It is known that benzodiazepines have a hypotensive effect, but the mechanism has not been well elucidated yet. To clarify whether this effect is due to central or peripheral mechanism, we administered 5 mg of diazepam or saline intravenously to healthy volunteers and assessed the change in blood pressure, heart rate, muscle sympathetic nerve activity and heart rate variability. After diazepam administration, systolic and mean blood pressure decreased significantly. Muscle sympathetic nerve activity was also significantly reduced but heart rate did not change, whereas the variables of spectral analysis of heart rate variability did not show significant change. We concluded that the hypotensive effect of diazepam in human is mainly due to the central mechanism.

The recovery profile of baroreflex control of heart rate after isoflurane or sevoflurane anesthesia in humans

Volatile anesthetics attenuate baroreflex function in a concentration-dependent manner. This study was designed to determine how long full recovery of baroreflex control of heart rate takes after isoflurane or sevoflurane anesthesia in healthy volunteers. We assessed baroreflex sensitivity in 20 subjects randomized to receive either isoflurane or sevoflurane (n = 10 each). After an 8- to 10-h fast and no premedication, mea- surements of R-R intervals obtained from the electrocardiogram (lead II) and systolic blood pressure (SBP) measured through a radial artery catheter were made at conscious baseline and 20, 60, and 120 min after the induction during end-tidal isoflurane 1.3% or sevoflurane 2.0% in air and oxygen, and 20, 60, 120, and 180 min after the emergence from general anesthesia. Baroreflex responses were triggered by bolus IV injection of phenylephrine and nitroprusside to increase and decrease SBP by 15-30 mm Hg, respectively. The linear portions of the baroreflex curves relating R-R intervals and SBP were determined to obtain baroreflex sensitivity. During anesthesia, baroreflex sensitivities of both the pressor and depressor tests were decreased by 50%-60% compared with conscious baseline values in both groups (P <0.05). Pressor test sensitivities returned to the baseline values at 120 min, whereas depressor test sensitivities returned to the baseline values at 60 min, after general anesthesia in both groups. There were no significant differences in baroreflex sensitivities between groups at any interval. Our results indicate that the recovery characteristics of baroreflex sensitivity are similar after isoflurane and sevoflurane anesthesia and that the depressor test sensitivity is restored more rapidly than the pressor test sensitivity after both anesthetic techniques.

IMPLICATIONS

Arterial baroreflex function is an important neural control system for maintaining cardiovascular stability. The authors found that 2 h was required for full recovery of baroreflex function and that recovery characteristics were similar after isoflurane and sevoflurane anesthesia in healthy volunteers not undergoing surgery.

Suppressive effects of remifentanil on hemodynamics in baro-denervated rabbits

PURPOSE

To elucidate mechanisms by which remifentanil, an ultra-short-acting mu-opioid receptor agonist, causes hypotension and bradycardia.

METHODS

Mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were measured and recorded after bolus injections of 1, 2 or 5 microg x kg(-1) of remifentanil in neuraxis intact (n=6 for each dose) and baro-denervated rabbits (n=6 for each dose). Arterial baroreflex sensitivity was assessed by depressor tests. An additional six baro-denervated animals received remifentanil, 5 microg x kg(-1) after pretreatment with naloxone, 40 microg x kg(-1).

RESULTS

All values were expressed in % change from baseline. In the neuraxis intact animals, MAP and HR were decreased briefly immediately after remifentanil injection. RSNA was increased dose-dependently: 137 +/- 8% (mean +/- SE), 170 +/- 14% (P < 0.05) and 225 +/- 29% (P < 0.05) after 1, 2 and 5 microg x kg(-1) remifentanil, respectively. RSNA was increased even after MAP and HR had returned to baseline values. The depressor tests revealed that remifentanil did not attenuate arterial baroreflex sensitivity. In the baro-denervated animals, MAP and HR decreased gradually to 77 +/- 3% (P < 0.05) and 94 +/- 1% (P < 0.05), respectively 300 sec after 5 microg x kg(-1) remifentanil. At that time, increased RSNA (159 +/- 9%, P < 0.05) had returned to baseline. Pretreatment with naloxone in the baro-denervated animals abolished these changes.

CONCLUSION

Remifentanil decreases HR and MAP by its central vagotonic effect and by stimulating peripheral mu-opioid receptors. These effects appear to be counteracted and masked by its central sympathotonic effect and by maintaining arterial baroreflex integrity.

Surgical stress attenuates reflex heart rate response to hypotension

PURPOSE

The baroreflex-mediated increase in heart rate (HR) in response to acute reduction of systolic blood pressure (SBP) was studied in order to assess whether the changes in arterial baroreflex sensitivity depend on the intensity of surgical stress, and location of visceral and somatic stimulation during surgery.

METHODS

Patients were divided into visceral stimulation groups [upper (n = 30) and lower (n = 30) abdominal surgery] and somatic stimulation groups [upper (n = 25) and lower (n = 25) limbs, and chest wall (n = 25) surgery]. Acute hypotension as a baroreflex depressor test was induced by prostaglandin E1 (PGE1) 10 min before surgical incision (control) and during surgical manipulation under isoflurane-N2O anaesthesia or isoflurane-N2O-fentanyl anaesthesia. Plasma level of ACTH was measured in an additional 40 patients who underwent upper abdominal surgery.

RESULTS

During upper abdominal surgery, the heart rate baroreflex sensitivity (delta HR/delta SBP) was depressed from -0.47 +/- 0.05 (control) to -0.01 +/- 0.04 (P < 0.05). The reflex heart rate baroreflex sensitivity remained unchanged and was similar among the remaining groups of patients. The concentration of ACTH increased from 12.5 +/- 1.0 (control) to 343 +/- 78.6 pg.ml-1 (P < 0.05) with isoflurane-N2O anaesthesia but did not change with isoflurane-N2O-fentanyl anaesthesia during upper abdominal surgery.

CONCLUSION

Upper abdominal surgery is associated with the most stressful stimulation to attenuate heart rate baroreflex sensitivity. Integrity of the baroreflex can be preserved by adding opioids to supplement inhalation anaesthesia.

Diazepam-sensitive GABA(A) receptors in the NTS participate in cardiovascular control

The present study employed neuropharmacological and receptor binding protocols to determine if diazepam-sensitive (DS) gamma-aminobutyric acid-A (GABA(A)) receptors in the nucleus tractus solitarius (NTS) participate in autonomic regulation of cardiovascular function. The first set of protocols was designed to determine if GABA(A) receptors in the NTS were functionally modulated by the benzodiazepine agonist, diazepam. Mean arterial pressure and heart rate responses to microinjection of GABAergic substances into the NTS were examined in urethane-anesthetized rats. Microinjection of the GABA(A) agonist isoguvacine into the NTS increased mean arterial pressure and heart rate, and these effects were blocked by the GABA(A) receptor antagonist, bicuculline. Preadministration of diazepam into the NTS potentiated the pressor actions of isoguvacine and had variable effects on heart rate changes. Flumazenil, a benzodiazepine antagonist, blocked the diazepam-induced potentiation of the pressor response to isoguvacine. The second protocol employed receptor autoradiography to examine the presence of DS and diazepam-insensitive (DI) GABA(A) receptors in the NTS. Autoradiography confirmed that DS GABA(A) receptors were present in the NTS; however, no measurable levels of DI GABA(A) receptors were detected. We conclude that GABA(A)-mediated integration of central autonomic control in the NTS is mediated solely by DS GABA(A) receptors.

Effect of high-dose fentanyl on renal function in dogs

Our objective was to determine the effects of high-dose fentanyl on canine renal function (RF). We anesthetized with sodium pentobarbital (SP) 16 dogs, randomly divided into 2 groups: in G1, SP was given alone, and in G2, combined with 0.05 mg.kg-1 fentanyl. All animals were ventilated artificially and had catheterized left and right femoral veins and left femoral artery for fluid infusion, drug administration, blood collection, and hemodynamic measurement. Urine was collected throughout the experiment. Attributes of RF were studied. SP did not alter RF, which was significantly altered by fentanyl. In G2, slower heart rates, mean arterial pressure, creatinine clearance, urinary output, osmolar clearance and fractional excretion of sodium and potassium were observed. G1 had a behavior attributed to extracellular volume expansion and no RF alterations. In G2, we observed significant decreases in RF due to opioid-induced hemodynamic changes, not discarding the possible action of aldosterone.

Pressure-heart rate relationship in intact and after stepwise elimination of three major baroreflex loops in dogs

Systemic arterial blood pressure (BP)-heart rate (HR) relationship (the pressor test) is often used as an index of baroreflex. We evaluated this index by simultaneously comparing BP-HR, right carotid sinus pressure (CSP)-nerve action potentials (NAP), and NAP-HR relationship in dogs anesthetized with pentobarbital. BP was increased or decreased stepwise by intravenous (IV) infusions of phenylephrine or sodium nitroprusside, respectively. In nine dogs BP-HR and CSP-NAP relationships were measured and NAP-HR relationship was constructed before and after sequential and stepwise sectioning of the left aortic depressor nerve (LADN), the right aortic depressor nerve (RADN), and blockade of the left carotid sinus nerve (BLK) with 1% lidocaine. We found that HR was a negative sigmoidal function of BP in intact dogs. Linear slope of this relationship was significantly reduced after sectioning of LADN and RADN, but returned toward baseline after BLK. NAP was a positive sigmoidal function of CSP in intact dogs. Linear slope of this relationship was significantly depressed after sectioning of LADN and RADN. However, after BLK, the slope surpassed control, suggesting the existence of a central communication between the two carotid sinuses. HR was a negative function of NAP in intact dogs. However, as the other baroreflex feedback loops were eliminated, the slope of the NAP-HR relationship approached zero indicating that a closed integrated parallel feedback system is required for reflex regulation of HR. Our findings suggest that under normal conditions the pressor test is a valid index for baroreceptor function, but its use may not be warranted in chronic pathological states, such as atherosclerosis and hypertension. However, in contrast to the present acute experimental model, chronic pathological processes may not develop in sequence, and baroreceptor function on the affected site may not be completely eliminated from the baroreceptor loop such as performed in this study.

The effects of ketamine on renal sympathetic nerve activity and phrenic nerve activity in rabbits (with vagotomy) with and without afferent inputs from peripheral receptors

One reason for the reported conflicting results of the effect of ketamine on hemodynamics and respiration may be variations in afferent inputs from peripheral receptors to the central nervous system. In order to evaluate unmasked direct effects of ketamine on sympathetic nerve and phrenic nerve outflow, totally deafferented (involving vagus, sinus nerve, aortic depressor nerve) rabbits (n = 18), rabbits with vagotomy (n = 21), and neuraxis-intact rabbits (n = 6) were used in this study. The animals were anesthetized with urethane and mechanically ventilated. Ketamine 0.5, 1, or 2 mg/kg was injected intravenously and mean arterial pressure (MAP), heart rate (HR), and integrated renal sympathetic nerve and phrenic nerve activity (IRSNA, IPNA) were recorded before, and 1, 2, 3, 5, and 10 min after injection. MAP and IRSNA were significantly decreased, even by the smallest dose of ketamine, in the totally deafferented group. IPNA was decreased by the largest dose of ketamine only in the totally deafferented group. On the other hand, spontaneous respiratory frequency was decreased in the totally deafferented and vagotomy groups, but more so in the totally deafferented group. In the neuraxis-intact group, the only significant change with the largest dose of ketamine, 2 mg/kg was a slight increase in HR. We conclude that ketamine can suppress vasomotor and respiratory centers directly, and that the suppression is counterbalanced by afferent inputs from peripheral receptors.

Choice of anaesthetic regimen influences haemodynamic response to cemented arthroplasty

Haemodynamic changes during bilateral cemented arthroplasty (BCA) were compared in dogs anaesthetized with isoflurane/N2O (ISOF) or diazepam/fentanyl (100 microg x kg(-1))N2O(FENT). Eight animals were anaesthetized with each regimen. After establishing monitoring and recording baseline values, BCA was performed. Haemodynamic measurements included aortic blood pressure (ABP), pulmonary artery pressure (PAP), right and left atrial pressures, and cardiac output. These were recorded at 30, 60, 180 and 300 sec after BCA. Lungs were removed and examined postmortem using quantitative morphometry. Groups demonstrated similar increases in PAP (ISOF 15 +/- 2 to 32 +/- 7, FENT 19 +/- 4 to 38 +/- 13; P> 0.05 between groups, P< 0.05 vs baseline). The proportion of lung vasculature occluded by fat was not different between groups (ISOF 9.63 +/- 3.38%, FENT 8.85 +/- 2.20%). Stroke volume decreased similarly in both groups (P> 0,05 between groups, P< 0.05 vs baseline). However, ABP decreased within one minute of BCA in ISOF (111 +/- 17 to 55 +/- mmHg, P< 0.05 and two of eight dogs died. All FENT dogs survived and hypotension (118 +/- 20 to 102 +/- 24 mmHg) was transient and less severe (P< 0.05 vs ISOF). Increased heart rate (HR) was noted in FENT following BCA (73 +/- 8 to 108 +/- 25 beats x min(-1); P< 0.05). Baseline HR was higher in ISOF (P< 0.05) and no increase in HR was noted. Systemic vascular resistance decreased in ISOF (P< 0.05), but not FENT (P> 0.05 vs baseline, P< 0.05 vs ISOF). To assess the role of slower baseline HR in FENT (73 +/-8) versus ISOF (131 +/- 5), six FENT dogs were paced (130 beats x min(-1)) with epicardial leads and an AV sequential pulse generator to simulate the ISOF group's baseline HR. Haemodynamic stability was maintained in this group in spite of a more rapid baseline HR. The choice of anaesthetic regimen strongly influenced acute haemodynamic changes in response to BCA.