INTRODUCTION
Anaesthesia is a state of analgesia, unconsciousness, and muscle paralysis.[1 2
Perfusion index (PI) is a non-invasive and continuous measure of peripheral perfusion.[3 4
Limited literature is available on the correlation between PI and intraoperative nociception under general anaesthesia. In this study, we aimed at investigating the changes in PI corresponding to painful stimuli under general anaesthesia.
METHODS
This prospective, non-randomised, single-blind study was conducted from April to June 2021 in a general hospital after institutional ethics committee approval and Clinical Trials Registry-India registration [CTRI/2021/04/033195]. Twenty patients of either sex, aged 20–45 years, belonging to the American Society of Anesthesiologists (ASA) class I, and scheduled for laparoscopic cholecystectomy were included. Patient refusal, those with neurological or psychiatric illness, chronic pain disorder, peripheral vascular disease, hypertension, diabetes mellitus, ischaemic heart disease, autonomic dysfunction or on drugs which affect autonomic dysfunction, allergy to any drug used in the study, and unstable haemodynamic status were excluded.
The primary outcome of the study was to compare the change in PI to pain stimulus (laparoscopic port insertion) and its variability to subsequent pain stimulus after intravenous (IV) administration of 0.5 mg/kg injection of fentanyl. The secondary outcome was to compare PI with haemodynamic parameters such as HR and mean arterial pressure (MAP).
The sample size was calculated using the standard deviation of PI as 0.9, which was obtained from results by Mohamed et al .[5
On the day of surgery, all the monitors were attached: an electrocardiogram, non-invasive blood pressure (NIBP) monitor, and SedLine pulse oximetry (Root, Masimo Corporation®, Irvine, CA, USA) was placed on the index finger contralateral to the side of the blood pressure cuff. Pre-induction values of PI, Pleth-Variability Index (PVi), HR, NIBP, and MAP were noted. Observations were single-blinded to eliminate bias at the observer level.
Premedication was done with 0.05 mg/kg of injection midazolam, 0.004 mg/kg of injection glycopyrrolate and 2 μg/kg of injection fentanyl intravenously, and induction with injection propofol at 2 mg/kg, along with muscle relaxation using injection vecuronium 0.1 mg/kg given intravenously. PI, PVi, HR, NIBP, and MAP were noted soon after induction. Endotracheal intubation was confirmed with capnography and a nasopharyngeal temperature probe was applied. The patient was ventilated with a tidal volume of 6–8 ml/kg, and the ventilatory rate was adjusted to maintain end-tidal carbon dioxide (EtCO2 ) between 35 and 40 mmHg. Injection paracetamol 15 mg/kg IV was given post-intubation over 15 minutes. Anaesthesia was maintained with isoflurane and top-up doses of injection vecuronium (0.05 mg/kg). The intraoperative MAP was maintained between 60–65 mmHg, and patient state index (PSI) was maintained between 35–50. The thermoneutrality was maintained by keeping the room temperature at 25°C and by providing a warming mattress and warmed IV fluids.
PI, PVi, HR, NIBP, and MAP were noted before intubation, after intubation, and at the time of pneumoperitoneum (P0). Later, the values were recorded at the time of insertion of the first laparoscopic port (P1), following which a 0.5 μg/kg injection of fentanyl IV was administered, and the values were recorded at the time of insertion of second (P2) and third (P3) laparoscopic port. After the insertion of the third laparoscopic port, the PI, PVi, HR values were recorded every minute up to 10 minutes and later every five minutes until 30 minutes of surgery. However, the NIBP and MAP values were recorded every five minutes until 30 minutes of surgery. The residual neuromuscular blockade was reversed at the end of surgery by using an injection of neostigmine 0.05–0.07 mg/kg and an injection of glycopyrrolate 0.05 mg/kg.
All values observed were on the continuous scale. Since each value was repeated several times for the same patient before administration of anaesthesia, pairs of values were taken and tested for statistical significance in their difference. The null hypothesis was that the difference between these pairs of values was 0, and a 95% confidence interval (CI) was used to determine deviation from the null hypothesis. Paired t test calculations were conducted using data analysis tool in Microsoft Excel (version 15.12.3 ©2015 Microsoft). Pearson’s correlation coefficients were also computed when comparing PI to MAP and HR using the correlation function in Microsoft Excel.
RESULTS
A total of 20 patients were enroled for the study out of which twelve were females and eight were males with a mean age of 36.5 ± 8.3 years, and all participants who enroled completed the study. All tests were conducted with a degree of freedom 19 and a confidence level of 95%. PI values after administering fentanyl between P1 and P2 increased from 5.33 ± 2.67 to 5.99 ± 2.8 with a P value less than 0.001 [Table 1 ]. Similarly, these values saw a significant increase between P1 and P3, with P3 at 6.3 ± 2.88 (P < 0.001) [Table 1 ]. The heart rate (HR) decreased from 101.42 ± 12.53 at P1 to 87.93 ± 10.98 at P2 (P < 0.001), and to 83 ± 10.8 at P3 (P < 0.001) [Table 1 ], whereas the changes in systolic blood pressure (SBP), diastolic blood pressure (DBP), and MAP were not significant [Tables 2 and 3 ]. A Pearson’s correlation (r) test was also run on the difference in PI, HR, and MAP values between P2 and P1, and P3 and P1 [Table 4 ]. The increase in PI values between both these port insertions correlated to the decrease in HR and MAP and was found to be statistically significant (r = −0.734, P < 0.001) [Figures 1 and 2 ].
Table 1: Haemodynamic changes (Mean±SD)
Table 2: Haemodynamic changes (Mean±SD)
Table 3: P values from paired t test
Table 4: Correlation between change in PI with change in heart rate and mean arterial pressure before and after administration of intravenous injection Fentanyl 0.5 μg/kg
Figure 1: Changes in PI in comparison to heart rate at various time points. PI = Perfusion Index; HR = Heart Rate measured per minute
Figure 2: Changes in PI in comparison to MAP at various time points. PI = Perfusion Index; MAP = Mean Arterial Pressure measured as mmHg
DISCUSSION
PI is the ratio of pulsatile blood flow to non-pulsatile blood flow in a person’s peripheral tissue such as finger tip or ear lobes. The finger photoplethysmographic waveform relies on red and infrared light absorption which includes two components. The first component is the constant amount of light, which is absorbed by the skin, bone, tissue, pigment, and non-pulsatile blood.[6 7 8
Pain induces vasoconstriction due to sympathetic stimulation which also results in a decrease in PI.[9 10
Lee et al .[11 et al .[12 et al .[11 et al. ,[12
The findings of our study could be further confirmed by the findings of another study in which it was concluded that the PI may be of clinical value in assessing pain in the anaesthetised state by applying electrical current to the anterior thigh in two healthy individuals anaesthetised with propofol and maintained with sevoflurane at different concentrations (1%, 1.5%, 2%, 2.5%). The painful stimulus produced an increase in HR and MAP with a decrease in PI.[4 13
In a prospective observational study on 87 sedated, non-intubated patients in a surgical intensive care unit, it was concluded that the application of a painful stimulus was associated with a decreased PI. Additionally, arterial blood pressure, HR, Richmond Agitation-Sedation Scale (RASS), and Behavioural Pain Scale for Non-Intubated (BPS-NI) values before and after the application of a standard painful stimulus (changing the patient position) were recorded in this study. The authors reported that changing the patient’s position resulted in a significant increase in SBP, DBP, HR, and BPS-NI values and good correlation was found between the change in the PI and the change in BPS-NI values after the application of a painful stimulus.[14
The literature available on the effects of painful stimulus on the PI under general anaesthesia is limited. But there are studies that have proven that there is an increase in PI after administration of analgesia in the post-operative care unit. One such study was conducted on 70 ASA class I patients who underwent lumbar discectomy, in a post-operative room with Masimo pulse co-oximetry connected.[5 15
Further studies done with regional anaesthesia showed that an increase in PI from baseline proved the onset of epidural action. One such study investigated the relationship between labour pain level and PI in 30 women undergoing spontaneous vaginal delivery under epidural analgesia.[16 17
In the above mentioned studies, there was a significant correlation between increased PI and decrease in HR, MAP, VAS, SBP, and DBP post administration of analgesia and vice-versa on painful stimulus, and in our study, the increase in PI and decrease in HR post-administration of analgesia was significant (P < 0.001).[16 17 P > 0.05) post administration of analgesia, unlike other studies.
In a study conducted on 65 patients undergoing upper limb surgery under ultrasound-guided supraclavicular block, it was found that the increase in PI from baseline at subsequent time intervals was high in a successful block compared to a minimal change in PI in a failed block, indicating the ability of PI to assess the painful stimulus.[18
One limitation of this study is that with the use of several exclusion criteria, there is a limited utility in using ASA grade I or II cases with surgeries that do not involve fluid shifts since it becomes a confounding factor. Another limitation is the small sample size of the study. A larger sample size may provide more evidence to support the preliminary results found here.
CONCLUSION
From the conducted study, it was found that the change in PI could be a surrogate monitoring tool to determine the nociception intraoperatively after excluding the confounding factors in relatively fit (ASA physical status class I and II) surgical patients.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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