European Journal of Cardiovascular Medicine

General anaesthesia indiuction using general anaesthetic agents and opioid analgesics induces significant changes in peripheral vascular physiology and sympathetic or parasympathetic tone, thereby affecting peripheral perfusion, as observed by changes in the pulse oximetry plethysmographic waveform. PI (Perfusion Index) is a relatively new parameter estimating the pulsatility of blood in the extremities, calculated using the infrared spectrum as part of plethysmography waveform processing. It is the ratio of pulsatile to non-pulsatile blood flow in the extremities. It's an easy, non-invasive way to check peripheral perfusion. PI, then, is a measurement of peripheral perfusion that may be obtained constantly from a pulse oximeter without requiring any intrusive procedures. Opioid analgesics and general anesthetics like propofol, desflurane, and isoflurane raise the vasodilation threshold by suppressing the sympathetic nervous system, lowering the vasoconstriction threshold. Following the administration of opioid analgesics and general anesthetics, a reduction in peripheral sympathetic tone dilates peripheral arteries, boosts peripheral perfusion, and lowers arterial blood pressure. As a result, these medications may increase the patients' chances of experiencing hypotension brought on by general anesthesia. Pre-existing heart dysfunction or underlying volume depletion may exacerbate this hypotensive reaction. Clinicians may be better able to recognize and treat hypotension brought on by general anesthesia if they have a continuous, noninvasive way to gauge a patient's sympathetic or parasympathetic reaction to the induction of general anesthesia. Conventional pulse oximetry uses two light wavelengths-red and infrared-transmitted through the finger and detected by a photodetector. The LED's (Light Emitting Diode) nonabsorbed light is detected by the sensor's photodetector. An inverting operational amplifier reverses this signal. This signal, which is separated into DC (Direct Current) and AC (Alternating Current) components, indicates the light that has been absorbed by the finger. The tissue's absorption of light, venous blood, and non-pulsatile arterial blood is represented by the DC component, while the pulsatile arterial blood is represented by the AC component. Hence, PI is defined as AC/DC * 100% of the plethysmograph waveform. It reflects the peripheral vasomotor tone. A low PI suggests peripheral vasoconstriction, while a high PI suggests vasodilation. In addition, vasoactive medications, stroke volume, sympathetic activity (pain, anxiety), and finger temperature can all affect PI. A variety of circumstances influence the condition of the sympathetic nervous system-intense microcirculation, causing the microvasculature to dilate or constrict.^[1] It also serves as an indicator of SVR (Systemic Vascular Resistance). Though not linearly, the relationship between the vascular tone and the value of PI is inverse. Propofol-induced hypotension is caused by the autonomic nervous system, which affects both the sympathetic and parasympathetic branches.^[2] Additionally, there is proof that propofol has a direct impact via both endothelium-dependent and independent mechanisms. Both lead to vasodilatation at the end. The patient is more prone to experience hypotension if they have low peripheral PI, or substantial peripheral vasoconstriction. Similarly, there is less chance of hypotension if the patient already has low vascular tone, which is indicated by a greater PI and is vasodilated and relatively compensated blood volume. The baseline SVR is responsible for a considerable amount of the hypotension that occurs during the induction of anesthesia with propofol. Therefore, based on a person's pre-induction SVR, it might be possible to set a threshold value of PI that indicates hypotension. When life-threatening problems such as abrupt hypotension, rapid hypertension, arrhythmias, and cardiovascular collapse occur, especially in high-risk patients, the induction of anesthesia is an essential component of therapeutic management. Consequently, it is preferable to employ a safe induction agent with a negligible impact on hemodynamics. The most widely used induction drug is propofol (2, 6 diisopropylphenol), which has the advantages of a smooth and quick induction and recovery as well as a lower rate of nausea and vomiting. However, there is a strong correlation with hypotension. One common issue during propofol induction is hypotension. The goal of this study is to determine the pre-anaesthesia PI cut-off value, which may be helpful in predicting hypotension that occurs after propofol induction of anesthesia. If the anesthesiologist can predict which patients are most at risk of severe hypotension, they may be able to individualize treatment.^[3]

This was a hospital based prospective observational study conducted among forty-two adults aged between 18 and 65 years belonging to the ASA (American Society of Anesthesiologists)  physical status I and II undergoing elective surgery under general anaesthesia at the Department of Anaesthesiology, Government Medical College, Thrissur, Kerala, India, over a period of one year from January 2019-January 2020 after obtaining clearance from the institutional ethics committee and written informed consent from the study participants.

Inclusion Criteria

       Patients belonging to American Society of Anaesthesiologists physical status 1 and 2 aged between 18 and 65 years undergoing elective surgery under GA.

Exclusion Criteria

1. Patients with hypertension, on vasoactive medications.
2. Patients with difficult airways.
3. Pregnancy.

Statistical Methods

The data of the study was entered in Microsoft Excel sheet and statistical analysis was done using SPSS software version 16.

Sample Size

       Sample size is calculated using the formula

n≥ {[Z_1-α/2+ Z_1-β] /1/2 log e^{(1+r/1-r )}}^{2 +} 3

α=0.01

β=0.2

r=0.503

α=α error

β=β error

n=sample size

r=correlation coefficient

      Substituting the values in the equation, sample size, n=42.

Study Procedure

       With approval from the institutional ethical committee, a prospective observational study was carried out at the hospital affiliated with Thrissur Medical College. After obtaining written informed consent, 42 adults between the ages of 18 and 65 who were undergoing elective surgery under general anesthesia and fit into physical status I and II of the American Society of Anesthesiologists were enrolled.

       On reception in the Operation Theatre

1. With the Mindray Multipara iMEC 12 Monitor, baseline readings (heart rate [HR], PI, systolic [SBP], diastolic [DBP], and mean BPs [MAP]) were recorded. An electrocardiograph, a non-invasive blood pressure monitor, and a pulse oximeter were attached.
2. Ringer's lactate intravenous infusion was started at a rate of 100 ml/h.
3. The patient was given vecuronium intravenously (IV) at a dose of 0.1 mg/kg after a senior anesthesia care provider titrated the patient's intravenous propofol injection at a rate of 2-3 mg/kg until the patient no longer responded to vocal communication. Up to five minutes, the hemodynamic parameters were recorded every two minutes.
4. A senior anaesthesia care provider used a suitable sized endotracheal tube to intubate the patient after the patient's lungs were ventilated with 100% oxygen for three minutes.
5. The maintenance of anesthesia was established using 0.6% isoflurane and 50% N2O in oxygen. At one minute, three minutes (after induction), five minutes, ten minutes, and fifteen minutes (after intubation), hemodynamic data were measured. A decrease in SBP to <20% of baseline or an absolute mean arterial pressure of <60 mmHg was considered hypotension. Severe hypotension with a MAP <55 mmHg was promptly treated with IV boluses of injectable mephenteramine (6 mg) and fast intravenous fluid administration (10 ml/kg). IV boluses of Inj. Atropine (0.6 mg) were used to treat bradycardia, which was defined as a heart rate less than 50 bpm.

The incidence of hypotension was computed in two sets: the first 15 minutes following induction (impact of the induction process and endotracheal intubation) and the first 5 minutes following anesthesia induction (effect of induction drug). The result was a baseline PI cut-off value below which hypotension was predicted five minutes after introduction.

Continuous variables like age and BMI were summarized as mean and SD. Categorical variables like gender and ASA were summarized as proportions. Perfusion index, heart rate, blood pressure and SpO₂ at different time points were summarized as mean ± SD.

The incidence of hypotension was defined as mean arterial pressure < 60 mmHg or SBP less than 80% of the baseline value.

ROC analysis was done to identify the baseline perfusion index cut off to predict the incidence of hypotension within 5 minutes. Sensitivity, specificity, positive predictive value and negative predictive value were calculated based on the cut off identified in ROC analysis.

The incidence of hypotension at 15 minutes was summarized as a proportion

Hypotension Incidence

The incidence of hypotension is defined as mean arterial pressure < 60 mmHg or SBP less than 80% of the baseline value.

Out of the 42 participants, 26 (62%) developed hypotension within 5 minutes.

Hypotension and Baseline Perfusion Index

ROC analysis was conducted to identify a cut off for predicting hypotension.

The area under the ROC curve was 0.98, 95% CI [0.95 to 1.0]. The baseline perfusion index is an accurate predictor of hypotension within 5 minutes.

Baseline PI < 1.86 predicted hypotension with a sensitivity of 92.3% and a specificity of 93.75%. The positive predictive value was 96% and the negative predictive value was 88.2%.

SpO₂ was maintained at 100% for all patients post intubation.

The study evaluated 42 adults between 18 and 60 years old, with a with a BMI between 19 and 29 kg/m²and ASA physical status I and II, who underwent elective surgeries under general anaesthesia in Government Medical College, Thrissur.

Hypotension was defined as a reduction in systolic arterial pressure, to less than 100 mm of Hg or a reduction of 20% below baseline. For the purpose of this study, hypotension was defined as a reduction of systolic blood pressure to less than 20% of the baseline pressure or MAP less than 60 mmHg.

The age of the patients in the study ranged from 19 to 58 years. The mean age of the participants was 37.8 ± 10.5 years.

Out of the 42 participants, 25 (60%) were males and 17 (40%) were females.

The study's patients' BMI ranged from 19.5 to 28.2 kg/m². The mean BMI of the participants was 24.0 ± 2.2 kg/m².

Out of the 42 participants, 27 (64%) were in the ASA 1 category and the rest belonged to the ASA 2 category.

Out of the 42 participants, 26 (62%) developed hypotension within 5 minutes and 13 (31%) developed hypotension after 15 minutes.

In this study, severe hypotension necessitating rescue with mephenteramine and bradycardia necessitating atropine administration were not observed.

Our hypothesis was that a baseline prognostic index would be helpful in anticipating hypotension after propofol induction. Our results support this theory by demonstrating a relationship between PI and the prevalence of hypotension. The SBP criterion showed a link with baseline PI, but the absolute MAP criterion did not show a connection with hypotension. In this case, it makes more sense to define hypotension as a function of baseline SBP since PI is based on the vascular contractile state and absolute MAP only looks at the patient's pre-operative vasomotor tone.

ROC analysis was conducted to identify a cut off for predicting hypotension. The area under the ROC curve was 0.98, 95% CI [0.95 to 1.0].

Hypotension was predicted with a baseline PI < 1.86, showing 92.3% sensitivity and 93.75% specificity. There was a 96% positive predictive value and an 88.2% negative predictive value.

Therefore, we could infer from the study that the baseline perfusion index is an accurate predictor of hypotension within 5 minutes.

Loss of pulsatility is a simple explanation for low PI and low SBP. Hypovolemia and the use of vasopressors are associated with it. On the other hand, concurrent increases in the non-pulsatile component may be the cause of high SBP and low PI.

Low PI levels may be the consequence of compensatory vasoconstriction brought on by relative hypovolemia, and propofol, through vasodilatation, may induce hypotension in these patients. The intubation stress likely caused blood pressure to return to normal, which decreased the incidence of hypotension at 15 minutes, as seen by the difference in the size of the correlation between the hypotension that occurred at the first 5 minutes and that at 15 minutes.

Propofol administration causes mild reductions in systolic, mean, and diastolic arterial blood pressure without a corresponding increase in heart rate.

Selective premedicants substantially modify this typical response. Billard V et al. concluded that premedication with fentanyl reduced hypertension after intubation but increased the incidence of hypotension after propofol induction.^[4]

Propofol induction can cause severe hypotension in patients who are hypovolemic or have a low cardiac reserve. We excluded patients with a severely contracted state or cardiovascular compromise from the study, and restricted it to adequately optimized patients scheduled for elective procedures.

Studies have shown that the perfusion index and pleth variability index, obtained from non-invasive pulse oximetry can be useful parameters in predicting the hypotensive response to anaesthetic agents like propofol.

The study was conducted to determine whether the perfusion index could predict hypotension after anesthetic induction with propofol. The perfusion index was found to be a significant predictor of hypotension following propofol induction and a cut off value (baseline PI<1.86) was obtained to predict the same in this study.

• van Genderen ME, Bartels SA, Lima A, Bezemer R, Ince C, Bakker J, et al. Peripheral perfusion index as an early predictor for central hypovolemia in awake healthy volunteers. Anesth Analg 2013;116:351-6.
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