Heart Rate Variability For Evaluation The Effect Of Mechanical Ventilation Pressure On Autonomic Nervous System During Operation Under General Anesthesia

Objectives:The negative pressure of the pleural cavity changes with periodic breathing in a physiological manner followed by the changing of autonomic nervous system（ANS）activity through neural reflex mechanism,ending up in cardiac cycle variability named as respiratory sinus arrhythmia（RSA）.Mechanical ventilation can augment internal thoracic pressure（ITP）that is negative physiologically or even make it positive,which has a certain effect on hemodynamics and ANS activity.In this study,we monitored the hemodynamics indicators HR and MAP,bispectral index（BIS）,as well as continuous electrocardiosignal among patients those were about to undergo orthopedic surgery under intravenous-inhalation anesthesia.Heart rate variability（HRV）indictors including time-domain,frequency-domain,nonlinear indicator and deceleration capacity（DC）were calculated according to electrocardiograph（ECG）collected.Our aim was to explore the effect of augmented ITP caused by increasing airway pressure on the activity of ANS in patients with general anesthesia,thus providing a new view for adjusting mechanical ventilation during clinical anesthesia.Methods:Twenty-two patients ASAⅠ^Ⅱscheduled for orthopedic surgery under intravenous-inhalation anesthesia participated in this investigation,age range from 40 to 65 years old with an average of 58±4 years old.Patients with autonomic nervous system disease and taken cardiovascular medications recently were excluded.None of the subjects were premedicated.Upon arrival in the operating room,ECG,SpO₂,BIS and invasive blood pressure were initiated and continuously monitored,meanwhile,ECG were recorded continuously.General anesthesia was induced with intravenous midazolam,0.05 mg/kg,sufentanil,0.5μg/kg,propofol,2 mg/kg and rocuronium,0.6 mg/kg,endotracheal intubation was performed under visual laryngoscope for mechanical ventilation with tidal volume（Vt）settled to maintain end tidal carbon dioxide(P_ETCO₂)about 40mmHg and respiratory rate 14 times per minute.General anesthesia was maintained by propofol 6mg/kg·h and remifentanil0.1μg/kg·min which were adjusted according to the depth of anesthesia along with1.0 MAC of sevoflurane with BIS index monitor readings ranging from 40⁶0,rocuronium 0.15mg/kg was intravenously injected regularly to keep muscle relaxant.When the P_ETCO₂ and the depth of anesthesia as well as the patient’s vital signs were stable（at least 35 min after the beginning of the operation）,the airway pressure was settled to two modes successively without changing the respiratory rate.The mode of low airway pressure was sustained for 20 min which was adjusted to maintain the P_ETCO₂ 45mmHg or so and the Vt,peak inflation pressure（PIP）,mean airway pressure（Pm）were recorded.While the mode of high airway pressure for 20 minutes kept a low level of P_ETCO₂ about 35 mmHg and the Vt,PIP,Pm were recorded too.ECG signal was collected in 5 min at five points,before induction（T0）,endotracheal intubation（T1）,intraoperative smooth period（T2）,low Vt（T3）,high Vt（T4）,then HRV indicators were calculated by MatlabR2010a including time domain,frequency domain and nonlinear indicators as well as PRSA indicator deceleration capacity（DC）.HR,MAP,Vt,PIP,Pm,P_ETCO₂ and BIS for the five measurement intervals were recorded accordingly.Result:（1）Hemodynamic variables and BIS:compared with T0,HR significantly increased at T1 but decreased at T2,T3 and T4.same changed tendency to HR,MAP increased at T1 compared to T0 but make no sense,it significantly decreased at T2,T3 and T4 like HR.Compared to T3,the values of HR and MAP did not change significantly at T4.Compared with T0,the BIS values of T1,T2,T3 and T4 were significantly reduced.The BIS value of T3 and T4 showed no significant difference.（2）Mechanical ventilation related parameters:Vt,PIP,Pm decreased and P_ETCO₂ markedly increased at T3 in comparison to T2;an increase of Vt,PIP and a decrease of P_ETCO₂ was found at T4 in comparison with T2,no significant differences of Pm was found between T2 and T4.Vt,PIP,Pm significantly increased and P_ETCO₂ decreased at T4 in comparison to T3.（3）HRV analysis indicators:HRV fast varying indicators:compared to T0,a significant decrease for fast varying indicators including RMSSD,LgHF,SD1 and DC were found at T1,T2,T3 and T4.There was no significant difference for these indicators between T3 and T4.HRV slow varying indicators:compared to T0,SDNN,LgTP,SD2 increased markedly at T1,while LgLF significantly decreased;all the slow varying indicators at T2,T3 and T4 decreased significantly.HRV slow varying indicators also decreased in comparasion with T1.No significant difference was found for all slow varying indicators between T3 and T4.Conclusion:During the procedure of intravenous-inhalation general anesthesia,airway pressure of mechanical ventilation was augmented to increase the ITP with a certain depth of anesthesia and a normal range of P_ETCO₂.Results showed that there was no significant change in hemodynamics and HRV indicators,reveling that the change of ITP had no obvious effect on ANS activity under certain conditions above and do not influence the stabilization of ANS concering organ’s function during operation.