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 Table of Contents  
ORIGINAL ARTICLE
Year : 2015  |  Volume : 2  |  Issue : 3  |  Page : 62-67

Pulse co-oximetry perfusion index as a tool for acute postoperative pain assessment and its correlation to visual analogue pain score


1 Department of Anesthesia, Faculty of Medicine, Cairo University, Cairo, Egypt
2 Department of Anesthesia, Faculty of Medicine, Beni Suef University, Beni Suef, Egypt

Date of Submission14-May-2015
Date of Acceptance21-Sep-2015
Date of Web Publication30-Dec-2015

Correspondence Address:
Nashwa Nabil Mohamed
Assistant Professor in Cairo University, 3 Omarat El Shams, 8th District, Nasr City
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2356-9115.172783

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  Abstract 

Background
A painful stimulus can produce vasoconstriction and a decrease in perfusion index (PI). The visual analogue scale (VAS) is the most common pain assessment scale. However, it is affected by psychometric instability. This study was designed to evaluate the correlation between VAS as a subjective indicator of pain and PI as an objective indicator of pain.
Patients and methods
At postanesthesia care unit, a Masimo pulse co-oximetry perfusion index was attached to 70 adult patients of ASA I who underwent lumbar spine discectomy. At the time of the first request for analgesia (T1) VAS was recorded together with the PI, heart rate (HR), mean arterial blood pressure (MAP), peripheral oxygen saturation, and axillary temperature, following which analgesia was given. Thirty minutes thereafter (T2) second measurements for the mentioned parameters were taken.
Results
The PI was significantly higher at T2 than at T1 (mean increase% = 94.3 ΁ 82.7%). This increase was associated with a statistically significant decrease in VAS, HR, and MAP. The mean decrease% was 70.5 ΁ 19.88%, 11.1 ΁ 7.2%, and 3.96 ΁ 5.01% in VAS, HR, and MAP, respectively. This means that the PI increases with adequate relief from pain, as indicated by a decrease in VAS, HR, and MAP. A decrease in VAS was associated with an increase in PI, but the correlation was not statistically significant as the degree of the increase in PI in relation to the decrease in VAS was variable among patients.
Conclusion
PI can be added to other indicators of pain assessment in the postanesthesia care unit.

Keywords: Masimo, pain, perfusion index, postanesthesia care unit, visual analogue pain score


How to cite this article:
Mohamed SA, Mohamed NN, Rashwan D. Pulse co-oximetry perfusion index as a tool for acute postoperative pain assessment and its correlation to visual analogue pain score. Res Opin Anesth Intensive Care 2015;2:62-7

How to cite this URL:
Mohamed SA, Mohamed NN, Rashwan D. Pulse co-oximetry perfusion index as a tool for acute postoperative pain assessment and its correlation to visual analogue pain score. Res Opin Anesth Intensive Care [serial online] 2015 [cited 2017 Jun 27];2:62-7. Available from: http://www.roaic.eg.net/text.asp?2015/2/3/62/172783


  Introduction Top


The International Association for the Study of Pain (IASP) defines pain as 'An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage' [1].

Unrelieved postoperative pain can result in serious side effects that affect the respiratory system (atelectasis, retention of secretions, pneumonia), the cardiovascular system (hypertension, arrhythmias, coronary ischemia), the gastrointestinal system (decreased bowel movement, nausea, vomiting) and the endocrinal system (increased catecholamine secretion). It also promotes thromboembolism by delaying mobilization [2].

Effective pain management requires careful assessment and continuous review of pain. The objectives of pain assessment are to measure the severity of pain, select the appropriate analgesic, and estimate the response to treatment. Pain is a subjective symptom as the individual can describe his own feelings. Thus, emotional and psychological factors may interfere with the assessment of the physical component of pain. Self-report pain scales have been the most common pain assessment tools over the years. The visual analogue scale (VAS) is the most common pain assessment scale [3],[4],[5]. Both VAS and numeric rating scale have been proven to be superior to a four-point verbal categorical rating scale [6]. However, their validity cannot be established in every environment because of the difference in psychometric stability [7].

The Masimo set pulse oximetry system can measure the perfusion index (PI) at the monitored site by calculating the relation between pulsatile and static blood in peripheral tissues. The PI is an indirect, noninvasive, and continuous measure of peripheral perfusion. It ranges from 0.02% (very weak pulse strength) to 20% (very strong pulse strength). It can also measure PI in conjunction with oxygen saturation and pulse rate by simple application of the pulse oximeter probe to the finger. By knowing the highest recorded PI, the best monitoring site for pulse oximetry can be identified. The changes in sympathetic nervous tone affect smooth muscle tone and can alter the level of perfusion.

Temperature, volume, and anesthetics can affect the perfusion at the extremities by causing vasoconstriction and vasodilatation, which can cause a decrease in PI or an increase in PI, respectively. The measurement of PI is not affected by heart rate (HR) variability, SpO 2 , or oxygen consumption [8],[9].

Most anesthetics produce a vasodilator effect while pain induces vasoconstriction. A study [8] had investigated whether a painful stimulus can produce vasoconstriction and a decrease in PI in normothermic anesthetized patients. The researchers found that the PI decreased during painful stimuli in anesthetized volunteers at different concentrations of sevoflurane. They hypothesized that an increased PI after anesthetic administration can be an early indicator of successful anesthesia, whereas absence of this increase may be an early warning of anesthetic failure. Hence, it could be a valuable tool for pain assessment under anesthesia.

Previous studies [10],[11] had investigated the changes in PI after initiation of epidural anesthesia in adult and pediatric patients. They suggested that an increase in PI due to the peripheral vasodilatation can be an early indicator of successful epidural anesthesia.

Lumbar spine surgery is associated with severe acute postoperative pain unless it is well managed. As far as we know, no study has investigated the correlation between VAS as a subjective indicator of pain and PI as an objective indicator of pain. This correlation can be of great help in analgesic guidance in postanesthesia care unit (PACU) and unconscious patients in ICUs.


  Aim of work Top


The aim of the study was to correlate pulse co-oximetry PI with VAS and evaluate the possibility of its use as an objective tool for postoperative pain assessment.


  Patients and methods Top


A prospective, observational study was performed in Cairo and Beni Suef University Hospitals after obtaining the approval of the Ethical Committee and informed written consent from patients undergoing elective lumbar spine discectomy. The study was conducted from May 2014 to December 2014.

Inclusion criteria

Patients of ASA I, aged 18-50 years, who were conscious enough to cooperate and whose mental status was normal in the immediate postoperative period were enrolled in the study.

Exclusion criteria

Patients with pre-existing cardiovascular, pulmonary, or metabolic diseases, patients with a history of a neurological, psychiatric, or chronic pain disorder, who were taking psychotropic drugs, patients with allergy to any drug used in the study, those with unstable hemodynamic status, and unconscious patients were excluded.

The study was conducted on 82 adult patients undergoing lumbar spine discectomy. Preoperatively, patients were trained on how to express their pain level using VAS to increase their familiarity with the scale. VAS is a subjective tool that depends on the patient's self-expression. The scale consists of a 10 cm horizontal line. Patients can make a mark on the line according to their pain intensity that can range from 0 to 10.

The patients were premedicated with intravenous (i.v.) 2 mg midazolam, 50 mg ranitidine, 4 mg ondansetron, and 8 mg dexamethasone. In the operating room, standard monitors were applied: an ECG, pulse oximeter, and noninvasive arterial blood pressure monitor. Preoxygenation was carried out for 3 min by means of a face mask with 100% oxygen. Anesthesia was induced by i.v. fentanyl 2 μg/kg, propofol 2.5 mg/kg, and atracurium 0.5 mg/kg. After endotracheal intubation, capnography and a temperature nasopharyngeal probe were applied. The lungs were ventilated with a tidal volume of 6-8 ml/kg and the ventilatory rate was adjusted to maintain ETCO 2 between 35 and 40 mmHg. Maintenance was done with 1.5 MAC isoflurane and top-up doses of atracurium. Analgesia was maintained with i.v. fentanyl at 0.5 μg/kg/h. The operation was performed with the patient in the lateral or the prone position according to the preference of the surgeon. The intraoperative mean arterial blood pressure (MAP) was kept around 60 mmHg. Patients who required i.v. nitroglycerine or ephedrine were excluded from the study. Warm i.v. Ringer's acetate solution was infused to replace fluid deficit and basal fluid requirements. The patient was kept warm by maintaining the room temperature at 25°C and by providing a warming mattress and warm i.v. fluids. At the end of the operation the muscle relaxant was reversed and all patients were extubated and sent to the PACU.

At the postanesthesia care unit

The following monitors were attached to the patient: ECG, noninvasive arterial blood pressure monitor, and Masimo pulse co-oximeter (Radical 7 pulse co-oximetry; Masimo Corporation, Irvine, California, USA). The oximeter probe used to monitor the PI was attached to the middle fingertip of the hand contralateral to the site of blood pressure monitoring and was wrapped in a towel to decrease heat loss and interference by ambient light.

An oxygen mask was applied if SpO 2 was below 90%. The patients were kept warm with wool blankets, warm i.v. fluids, and a warm air-forced device. All patients were observed until they asked for analgesia.

At the time of the first request for analgesia (T1)

At the time of the first request for analgesia VAS for pain intensity was recorded, together with the PI. Simultaneously, HR, MAP, peripheral oxygen saturation, and axillary temperature (measured by axillary thermometer) were also noted.

For all patients analgesia was achieved with i.v. morphine at 0.07 mg/kg and i.v. 1 g perfalgan vial.

Thirty minutes from postoperative analgesia (T2)

Thirty minutes after postoperative analgesia, second measurements of the above-mentioned parameters were taken simultaneously: VAS for pain intensity, PI, HR, MAP, peripheral oxygen saturation, and axillary temperature.

Statistical analysis

Data were statistically described in terms of mean ± SD. Comparison of the time point values was done using the paired t-test. Correlation between various variables was determined using Pearson's moment correlation equation for linear relation in normally distributed variables. P values less than 0.05 were considered statistically significant. All statistical calculations were performed using the computer program SPSS (Statistical Package for the Social Science; SPSS Inc., Chicago, Illinois, USA) release 15 for Microsoft Windows (2006).

Power analysis

Power analysis was carried out by comparing all variables between the two study time points. The paired t-test was chosen to perform the analysis. α-Error level was fixed at 0.05 and the sample size at 70 participants. The statistical power of our comparisons is shown in the table below. Calculations were performed using PS Power and Sample Size Calculations Software, version 3.0.11, for MS Windows (William D. Dupont and Walton D. Vanderbilt, USA).


  Results Top


The study initially comprised 82 patients who underwent lumbar spine discectomy. All patients who met the inclusion criteria were enrolled in the study. Twelve patients were excluded as they required i.v. nitroglycerine or ephedrine intraoperatively. Finally, 70 patients completed the study.

The demographic characteristics of the patients were as follows: sex, 37 women and 33 men; age, 36.68 ± 10.33 years; and BMI, 24.76 ± 4.27 kg/m 2 .

(-) There was a statistically significant increase in PI at T2 than at T1. The mean increase% equalled 94.3± 82.7% [Table 1].
Table 1: The perfusion index, visual analogue scale, mean arterial pressure, and heart rate at T1 and T2, their difference
between T2 and T1


Click here to view


(-) There was a statistically significant decrease in VAS at T2 than at T1. The mean decrease% equalled 70.5 ± 19.88% [Table 1].

(-) There was a statistically significant decrease in MAP at T2 than at T1. The mean decrease% equalled 3.96 ± 5.01% [Table 1].

(-) There was a statistically significant decrease in HR at T2 than at T1. The mean decrease% equalled 11.1 ± 7.2% [Table 1].

(-) There was no statistically significant difference between SpO 2 % at T1 (97.4 ± 1.93%) and at T2 (97.2 ± 2.08%).

As regards the difference in axillary temperature there was no statistically significant difference between the measured axillary temperature at T1 (35.48 ± 0.34°C) and that at T2 (35.52 ± 0.28°C).

The correlation between PI difference and VAS difference was not statistically significant (P > 0.05) [Table 2].
Table 2: Correlation between the difference in perfusion index and the difference in visual analogue scale

Click here to view


There was a statistically significant negative correlation between HR and PI. A decrease in HR was associated with an increase in PI [Table 3].
Table 3: The correlations between MAP difference, HR difference, and PI difference

Click here to view


No correlation between MAP and PI. It was not statistically significant (P > 0.05) [Table 3].


  Discussion Top


Pain is a subjective and personal experience that makes objective measurements impossible [6]. However, the increase in sympathetic nervous tone caused by pain can affect the PI, which can be a guide for the given analgesics in PACU. This tool for pain assessment can eliminate the variations in personality, age, sex, and cultural background. It can also eliminate psychological factors such as fear, anxiety, depression, and anger.

This benefit can be more valid in patients suffering from cognitive impairment and dementia. In such patients, common pain behaviors and physiological manifestations of pain are the only available indicators. Physiological manifestations of pain include diaphoresis, pupil dilatation, increased HR, blood pressure, and breathing rate [12].

In this study the PI was significantly higher at T2 than at T1 (mean increase% = 94.3 ± 82.7%). This increase was associated with a statistically significant decrease in VAS, HR, and MAP. The mean decrease% was 70.5 ± 19.88%, 11.1 ± 7.2%, and 3.96 ± 5.01% in VAS, HR, and MAP, respectively. This means that the PI increases with adequate relief from pain as indicated by a decrease in VAS, HR, and MAP. A decrease in VAS was associated with an increase in PI but the correlation was not statistically significant as the degree of the increase in PI in relation to the decrease in VAS was variable among patients.

This study was similar to a study conducted by Hagar et al. [8] in which an electrical current was applied to the anterior thigh in two healthy volunteers anesthetized with propofol and maintained with sevoflurane at different concentrations (1, 1.5, 2, 2.5%). This painful stimulus produced a significant increase in HR and MAP with a significant decrease in PI. They concluded that the PI may be of clinical value in assessing pain in the anesthetized state.

New-generation pulse oximeters (Masimo) are the easiest of all peripheral perfusion assessment modalities. It enables physicians to obtain reliable measurements even under difficult clinical conditions: patient's movements, hypotension, hypothermia, or electromagnetic field of other devices because of the presence of reference signal calculations, the adaptive filter, and transformation of a single saturation signal [13].

Perfusion index is a numerical value reflects the strength of infrared signal returning from the monitoring site. In contrast to the conventional pulse oximeter which measures O 2 saturation from the ratio between transmitted red and infrared light, Masimo Signal Extraction Technology (SET) infrared signal depends upon the amount of blood at the monitoring site, not upon blood oxygenation. PI reflects the ratio of pulsatile to the non pulsatile amounts of blood at the monitoring site. Sympathetic nervous tone changes cause changes in smooth muscle tone and affect PI [13].

Pain can alter the endocrine system leading to increased catecholamine secretion causing vasoconstriction [2]. It was reflected as decreased PI with high VAS, but after receiving analgesia the PI increased significantly.

PI was used before for prediction of the onset of successful regional sympathetic blocks, by measuring PI before and after block, like the onset of the epidural anesthesia, which was associated with an increase in PI [10],[11]. Zifeng et al. [14] had used it in pediatric caudal block under basal ketamine anesthesia to assess the efficacy of the block. They found a decrease in PI after ketamine injection despite the increase in MAP and HR. This finding is explained by the sympathetic stimulation caused by ketamine and proves that the PI is more affected by pain and stressful stimuli rather than by changes in pulse pressure. After initiation of caudal block, the PI increased significantly, far beyond the preinduction PI.

In agreement with the current study; Nishimura T et al. [15] had studied the changes in perfusion index in response to noxious electrical stimulation in healthy subjects. They measured the PI and pulse rate in 70 healthy volunteers exposed to increasing electrical stimulation until they reached their pain tolerance threshold. They measured PI and pulse rate 10 seconds before and after electrical stimulation in 4 different group population (young men, young women, aged men and aged women). They observed a significantly decreased PI in response to electrical stimulation in spite of the stimulation was too small to increase the pulse rate. They concluded that the PI may be an independent parameter reflecting the perception of noxious stimuli and offers a noninvasive option for objectively evaluating pain perception.

Also, Korhonen I et al. [16] supported the use of photoplethysmography wave (PPG) in analgesia monitoring. They hypothesized that the use of PPG waveform is related to the volume changes in the measurement site and the peripheral blood circulation. So, it is linked to the vasomotor response and hence, nociception monitoring under general anesthesia.

Also, in agreement with the present study; Hamunen K et al. [17] had studied the use of photoplethysmographic pulse wave amplitude (PPGA) in detecting the changes in autonomic activity and assessing pain. They measured PPGA and its derived parameters {surgical pleth index (SPI), autonomic nervous system state (ANSS) and ANSS index (ANSSI)} and heart rate in 29 healthy volunteers during two heat stimuli (43° C and 48° C) and the cold pressor test. Their results showed that the three thermal stimuli produced a significant change in PPGA and its derived parameters even with mild noxious stimulus. They concluded that nociception induced sympathetic nervous system activation causing an increase in SPI and ANSSI, a decrease in PPGA and ANSS measures. So, they recommended their use to assess perioperative pain.


  Conclusion Top


Perfusion index can be added to other indicators of pain assessment in PACU. It is easy, non invasive, free of subjective interpretation and low time consuming.

For future studies

Post-operative recording of the perfusion index after different types of surgery and for longer duration. This would assist for optimum use of sedatives and analgesics and can be of great help in patients with cognitive impairment.


  Acknowledgements Top


Conflicts of interest

None declared.

 
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    Tables

  [Table 1], [Table 2], [Table 3]



 

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  In this article
Abstract
Introduction
Aim of work
Patients and methods
Results
Discussion
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Acknowledgements
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