The main focus of this prospective randomized clinical study was determination of the position dependent haemodynamic changes during the minor anorectal surgery under a low-dose spinal anaesthesia assessed by ICG. In our study, the reduction of CI by 33.3% (1.3 l/min/m2 (from the baseline 3.9 ± 0.8 to 2.6 ± 0.7)) and CO by 33.8% (2.5 l/min (from the baseline 7.4 ± 1.6 to 4.9 ± 1.2)) and the increase of SVR from the baseline immediately after the patient’s positioning was determined in the jack-knife position. CO, CI, SVR, SI remained stable or changed slightly in the lithotomy position. The MAP and HR remained stable in both positions.
Assuming that a low dose spinal anaesthesia has negligible haemodynamic effects, we initiated this study with the aim of evaluation of haemodynamic changes and effects of the patient’s positioning on haemodynamic parameters assessed by the transthoracic electric bioimpedance (TEB). The second goal to start this study was that we could find no currently available studies of haemodynamic effects assessed by the non-invasive method of CO monitoring during a minor anorectal surgery under the selective spinal anaesthesia in two different positions: prone or lithotomy position.
For anorectal surgery, the jack-knife position with the head down tilt of 15–20 degrees for easier surgical access is frequently used. The majority of surgeons in the Hospital of Lithuanian University of Health Sciences, Kaunas Clinics prefer to use the jack-knife position than the lithotomy position for the anorectal surgery.
The well – known haemodynamic effects of the conventional dose spinal anaesthesia are arterial hypotension and bradycardia [14,15]. They are caused by multiple mechanisms: 1) extensive neuraxial blockade including the sympathetic chain, 2) redistribution of intravascular volume due not only to vasodilation, but also to the patient’s positioning, and 3) paradoxical vasovagal reactions . The decrease in pre-load that may accompany spinal anaesthesia or the patient’s position, may initiate the following three reflexes that can eventually lead to a sudden onset of cardiovascular collapse and syncope. The first reflex involves direct stretching of the pacemaker cells in the sinoatrial node. The decrease in venous return produces less stretch and a lower heart rate. The second reflex involves baroreceptors located within the walls of the right atrium and the vena-cava–atrial junction. The stimulation of these receptors by an increase in venous return sends signals to the vasomotor centre. The decreases in venous return induce a decrease in the heart rate. The third (Bezold–Jarisch) reflex is mediated by cardiac baroreceptors located in the inferoposterior wall of the left ventricle. This reflex provokes a decrease of central blood volume with decreases of ventricular volume and an increase of ventricular contractility. This leads to a combination of the increase in vagal efferent activity from the vasomotor centre leading to varying degrees of bradycardia and a decrease in the efferent sympathetic output to the primary sympathetic neurons in the thoracolumbar spinal cord leading to marked vasodilation .
To provide a safe care, the anaesthesiologist must maintain tissue perfusion and haemodynamic stability by ensuring optimal preoperative fluid balance. This can be achieved by monitoring of cardiovascular function by using the invasive CO monitoring, such as pulmonary artery catheterization – thermodilution method, less invasive such as oesophageal Doppler monitor or non-invasive cardiac output monitoring achieved by TEB [16-18]. CO is a primary determinant of global oxygen transport from the heart to the body. So, monitoring of CO, CI and other haemodynamic parameters may have a role in monitoring circulation and forming management decisions. The patients in our study were awake, under selective spinal anaesthesia with low dose hyperbaric bupivacaine, admitted for elective minor anorectal surgery and relatively healthy (ASA class 1–2). Therefore, the non-invasive method of monitoring a cardiovascular function – TEB was chosen for this study due to a number of factors: safety, convenience, adaptability, cost, type of surgery and anaesthesia and the possibility of using it for these patients [16,17].
Sudden haemodynamic reactions occur despite that the selective low dose spinal anaesthesia affecting restricted dermatomes was used [5,19]. Gudaityte et al.  concluded that the selective spinal anaesthesia with a dose of 4 mg of hyperbaric bupivacaine produces an adequate level of sensory and motor block for anorectal surgery. Despite the fact that MAP was stable, acute bradycardia was recorded in 6 cases before a dural puncture in the sitting position and in 2 cases immediately after the injection of local anaesthetic. In our study, the patients were relatively healthy, and haemodynamic changes were not clinically significant despite the reduction of CO by 33.8%, and no further rescue treatment was needed. We failed to measure ICG during the cases of sudden bradycardia (three cases, all prior to dural puncture). The drawback of ICG is that it requires time to get a signal of sufficient quality. We could not proceed with prolonged investigation due to ethical issues, or use controlled measurements by thermodilution with a pulmonary catheter after the assessment of a potential risk/benefit ratio.
The prone or the jack-knife position itself is associated with physiological cardiovascular and respiratory changes, and the reduction of CI and CO is most frequent . The researchers suggest that the anaesthetic technique could affect haemodynamic variables in the prone position [5,7,20]. Our findings are comparable to other studies [6,9,12,21]. Several studies revealed the decrease of CO by 18.5% - 24.4% during a lumbar surgery in the prone position under general anaesthesia [6,10]. The authors concluded that the reduction was mainly due to a reduction in a stroke volume index. The transoesophageal dopller (TEE) was used to determine these findings . Others studies reported that changing the patient’s position to prone significant decreases CI by 17.2% and increases the total SVR, but there is little change in other haemodynamic variables [8,9]. The researchers of this study used the invasive method of cardiovascular function monitoring, i.e. thermodilution method. Ozkose et al. compared total intravenous anaesthesia (TIVA) with inhalation anaesthesia by measuring MAP and HR in patients undergoing spinal surgery. HR and MAP decreased significantly after induction of anaesthesia in the TIVA group compared to other groups when the patients were placed in the prone position . Sudheer et al. used a non-invasive cardiac output NICO™ system - partial rebreathing of the carbon dioxide technique to measure CI. They compared the impact of two different methods of anaesthesia (inhalation and TIVA) to CI and SVR during the lumbar spinal surgery and found the significant reduction of CI by 25.9% in TIVA group and by 12.9% in inhalation group and the increase in SVR on turning the patients prone .
Wadsworth and colleagues  used TEB to measure CO and HR. CI and the total vascular resistance index were derived from these data. No significant changes in HR or MAP occurred when the volunteers were positioned from the supine position to any of the four prone positions or when returned to the supine position again. CI decreased significantly on turning from the supine to the knee-chest position (20%) and onto the two props with a support (17%), but not onto the evacuatable mattress (11%) or two pillows (3%) . A reduction in CI is due to a decrease in venous circulation from venostasis in the lower extremities and an increase in thoracic pressure. However, the change in blood pressure is not common due to the result of the increase in total systemic vascular resistance caused by the activation of the sympathetic nervous system following a decrease in CO . Toytota and Amaki revealed controversial results . They demonstrated that the prone position did not cause a reduction in CI and changes in the stroke volume index, however, caused a reduction of the left ventricular volume, systolic pulmonary venous flow velocity and pulmonary venous velocity time integral via TEE .
The studies investigating haemodynamic effects of the lithotomy position are limited. Miyabe et al.  concluded that the lithotomy position after spinal anaesthesia reduces a decrease in blood pressure and has no effect on the analgesic level . They did not measure CO, CI or SVR. The results of our study revealed that MAP, HR, CO, CI, SVR, SI measured by ICG remained stable or changed slightly in the lithotomy position. The return of the pooled venous blood from the lower extremities (500–1000 ml) to the heart increased the afterload due to the elevation of the legs, assuming that baroreflex does not occur in the lithotomy position, explains the consistent result of lithotomy position .
There are many limitations to the validity of the calculation of cardiac output by ICG. These limitations can be divided into two main categories: 1) the difficulty of acquiring the signal because of spontaneous movements of the patient, interferences with electrocautery in OR, disorders of heart rhythm, or mechanical ventilation; the newest monitors, such as the Niccomo, include a quality indicator signal to eliminate real-time misinformation; and 2) physiologic and pathophysiologic situations in which the physical modelling of the system is no longer valid, in particular, during changes in the baseline thoracic impedance (pregnancy, obesity, gas or fluid pleural effusion, chronic congestive heart failure with pulmonary oedema), or when there is a severe aortic valve disease or modified mechanical properties of the arterial tree. These limitations must be kept in mind when using TEB at the bedside . The jack-knife or lithotomy position can also affect the lung compliance and thoracic volume which may have impact to the accuracy of TEB measurements . We avoided such conditions, as surgery in the chest, abdominal opening and closure, a vasodilation/vasoconstriction status, a lung fluid balance, an acute lung injury which can lead to the bias of ICG compared with thermodilution in the present study. The limitation of our study is that the single method of haemodynamic monitoring was used. We did not use a pulmonary artery catheter (PAC) as a controlled method for CO measurement. Monitoring with PAC is associated with complications and can hardly be justified if the risk for the patient and the cost of the device are taken into account. According to the risk/benefit ratio, the method of haemodynamic monitoring to this relatively healthy patient population should be non-invasive, accurate and should give reproducible results. The studies of ICG reported different results and are difficult to compare with each other because different ICG devices were used with different software and the subjects’ characteristics varied from healthy volunteers to the patients with heart failure, surgical procedures and medical treatment. In a meta - analysis, Peyton and Chong reported that the correlation between TEB and the pulmonary artery catheterization thermodilution method which is the “gold standard” for CO measurement is very good (r = 0.79). However, they demonstrated that the percentage error of ICG versus thermodilution is 42.9% with a significant precision error . The relative changes of CO, CI, and SI may be accurate, but the absolute values may be different versus thermodilution method. According to the fact that ICG is a non-invasive, continuous, operator – independent and a cost effective tool for CO monitoring, we regarded this method as most acceptable for our study.
The present study was focused on the assessment of ICG changes in a relatively healthy, ASA 1–2 population with a normal compensatory reserve who could well tolerate haemodynamic shifts caused by the jack-knife position. We hypothesize that the haemodynamic changes after the placement into the jack-knife or prone position could be even more dramatic in the cases of a limited cardiovascular reserve, i.e. for patients with cardiovascular compromise.