We examined the hypothesis that respiratory sinus arrhythmia (RSA) is primarily a central phenomenon and thus that RSA is directly correlated with respiratory controller output. RSA was measured in nine anesthetized dogs, first during spontaneous breathing (SB) and then during constant flow ventilation (CFV), a technique whereby phasic chest wall movements and thoracic pressure swings are eliminated. Measurements of the heart rate and of the moving time averaged (MTA) phrenic neurogram during these two ventilatory modes were made during progressive hypercapnia and progressive hypoxia. RSA divided by the MTA phrenic amplitude (RSA.) showed a powerlaw relationship with both arterial carbon dioxide partial pressure (Paco2) and oxygen saturation (SaO2), but with different exponents for different conditions. However, the power-law relation between RSA. and respiratory frequency had an exponent indistinguishable from -2 whether hypoxia or hypercapnia was the stimulus for increased respiratory drive, and during both CFV and spontaneous breathing (-1.9±0.4, hypoxia, SB; -1.8±0.7, hypoxia, CFV; -2.1±0.8, hypercapnia, SB; -1.9±0.7, hypercapnia, CFV). We conclude that respiratory sinus arrhythmia is centrally mediated and directly related to respiratory drive, and that changes in blood gases and phasic afferent signals affect RSA primarily by influencing respiratory drive. (J. Clin.

We studied the effects of removing cyclic pulmonary afferent neural information on respiratory pattern generation in anesthetized dogs. Phrenic neural output during spontaneous breathing (SB) was compared with that occurring during constant-flow ventilation (CFV) at several levels of eucapnic hypoxemia. Hypoxia caused an increase in both the frequency and the amplitude of the moving time average (MTA) phrenic neurogram during both SB and CFV. The change in frequency as arterial saturation was reduced from 90 to 60% during SB was significantly higher than that during CFV [SB, 32.3 +/- 10.9 (SD) breaths/min; CFV, 10.3 +/- 5.8 breaths/min; P = 0.001]. By contrast, the increase in the amplitude of the MTA phrenic neurogram was smaller (SB, 0.62 +/- 0.68 units; CFV, 1.35 +/- 0.81 units; P = 0.01). The changes in frequency with hypoxia during both modes of ventilation resulted primarily from a shortening of expiratory time. Both inspiratory time and expiratory time were greater during CFV than during SB, but their change in response to hypoxia was not significantly different. We conclude that the amplitude response of the MTA phrenic neurogram to hypoxia is similar to that seen during hypercapnia; in the presence of phasic afferent feedback the MTA amplitude response is decreased and the frequency response is increased relative to the response observed in the absence of phasic afferents.

Background: An ideal neuromuscular blocking agent should have a high degree of potency, rapid onset of action and short duration of action. It should not cause haemodynamic changes through releasing histamine, ganglion block, anti-muscarinic effects on heart or sympathomimetic action. This study was designed to compare the effect of cisatracurium with or without magnesium pretreatment on neuromuscular blockade and hemodynamics in cardiac surgery patients on cardiopulmonary bypass with objective to assess and compare intubation conditions and timing of intubation in minutes and assess and compare clinical duration of blockade, haemodaynamics variable and side effect if any. Materials and Methods: Hospital based, prospective, randomized double blind observational study. Total 60 patients included in the study (30 in each group). Group A Pretreatment with mgso 4(50mg/kg BW in 100ml NS 10 mints before general anesthesia (GA) and muscle relaxant injection Cisatracurium 0.2mg/kg. Group B received 100ml NS 10 mints before general anesthesia (GA) and muscle relaxant injection Cisatracurium 0.2mg/kg. Results: The mean onset of action of injection cis-atracurium after intubating dose in group A was 4. 27+/1.68 minutes as compared to 5.48+/-1 minutes in group B(p-value<0.001) which showed that onset of action was significantly less in group A as compared to group B. Duration of action of intubating dose of cisatracurium was prolonged by pre-treatment of Mgso4. Haemodynamic parameters were comparable and there was no statistically significant difference among both the groups. No side effect reported in our study.