After obtaining the approval of the ethical committee of Faculty of medicine, Ain-Shams University (FMASU R35/2021), informed consent was taken from 60 patients ≥ 18 years old, with ASA physical status II–III, BMI ˃ 35 kg/m2 and scheduled to undergo laparoscopic Roux-en-y gastric bypass at Ain-Shams University Hospitals. Simple randomization was done using computer generated random number tables with sealed opaque envelopes.

Exclusion criteria

Patients’ refusal, hypersensitivity to the study medications, patients with known history of hepatic disease, severe renal impairment (estimated glomerular filtration rate: eGFR < 30 ml/min/1.73 m2), seizure disorder, heart failure; left ventricular ejection fraction < 35%, any cardiac dysrhythmias; Adam-Stokes syndrome; Wolff-Parkinson-White syndrome, atrio-ventricular block with heart rate < 50 beat per minute (bpm), hypertensive patients on beta blocking drugs, chronic pain, or substance abuse (Fig. 1).

Fig. 1

Flow chart for patients’ enrollment and allocation

Pre-operative evaluation included history taking, physical examination, and investigations; complete blood count, the coagulation profile, liver and kidney functions tests, electrocardiography (ECG), and echocardiography (Echo). Preoperative fasting instruction was nothing per orally 8 h for solids and 2 h for clear fluids. Also, during the pre-anesthetic visit, the NPRS for pain assessment was explained to the patients.

Preparation of the study drugs

The loading doses of 2% Lidocaine hydrochloride (Sigma Tec Industries, Co-packed by Al-Debeiky pharmaceutical Industries, Obour City Ind Zone, A.R.E.) or morphine sulphate (10 mg/ml; Misr Co For pharmaceuticals, Alexandria, Egypt); calculated according to the patient's body weight and diluted to a 10-ml syringes, were prepared and labeled as loading-1 and loading-2 respectively.

Also, 2 syringes of 50 ml volume; containing 2% Lidocaine hydrochloride or 0.9% sodium chloride (normal saline NS) infusion labeled as infusion-1 and infusion-2 respectively were also prepared.

The anesthetic technique

In the induction room, patients had an 18 G intravenous cannula inserted in the dorsum of the hand. Patients were preloaded with 10 ml/kg of Ringer acetate solution, until the start of the abdominal insufflation. One milligram granisetron (Granitryl 1 mg/ml; Alex Co, for Egy-pharma, Egypt) and 40 mg pantoprazole sodium (Controloc 40 mg IV vial; Takeda GmbH, D-78224 Singen, Germany) were given. On arriving to the operating theater five-leads ECG, non-invasive blood pressure, and pulse oximetry monitoring were started, using General electric-Dash 5000 (GE Medical Systems Information Technologies Inc., Milwaukee, WI, USA). Capnography was connected with induction of anesthesia. Patients were sedated with 0.05 mg/kg midazolam hydrochloride (Dormicum 5 mg/ml; Roche, Basel, Switzerland).

Patients then were divided into 2 equal groups of 30 patients each

Group L

The lidocaine group. At induction of anesthesia, patients received a loading dose of IV 1.5 mg/kg lidocaine hydrochloride 2% slowly over 3 min followed by IV infusion of 2 mg/kg/h lidocaine hydrochloride 2% via infusion pump, through a second IV line secured after induction of anesthesia. The infusion continued till the end of surgery (Eipe et al. 2016; Dunn and Durieux, 2017).

Group M

The morphine group. At induction of anesthesia, patients received a loading dose of IV 0.1 mg/kg morphine sulfate slowly over 3 min followed by IV infusion of NS via infusion pump, through a second IV line secured after induction of anesthesia. The infusion rate was calculated as for the study group so that both groups received equal volumes of infusion. The infusion continued till the end of surgery (Al-Tamimi et al. 2009).

After proper airway assessment and difficult airway anticipation, pre-oxygenation with 8 L/min of 100% O2 for 3 min was started; anesthesia with endo-tracheal intubation was induced with, IV 2 mg/kg propofol (Propofol 1%; Fresenius Kabi, Deutschland, GmbH Grazia) and 1 mg/kg rocuronium bromide-hameln (50 mg/5 ml; Sunny Pharmaceutical).

Maintenance of anesthesia was done with oxygen (4 L/min of 60% O2 in air) and 2% sevoflurane, further neuromuscular blockade was maintained with intermittent boluses of 0.1 mg/kg rocuronium bromide every 30 min. One gram paracetamol (Perfalgan vial; 100 ml of 10 mg/ml) was IV infused over 10–20 min. Maintenance rate of Ringer acetate solution (4 ml/kg/h) was IV infused (Idit and Andrei, 2021). Intra-operative additional IV titrated doses of 2.5–5 mg morphine sulphate with 10 min intervals was given (Choi et al. 2000; Charghi et al. 2003), when needed to maintain the MBP within 20% of the baseline value. Patients were mechanically ventilated by pressure controlled volume guarantee mode (PCVG). Ventilation was adjusted to maintain an end-tidal carbon dioxide (EtCO2) value between 30 and 35 mmHg, positive end expiratory pressure of 5 cm H2o was added. Pneumoperitoneum was achieved with carbon dioxide and intra-abdominal pressure maintained to 14 mmHg throughout the procedure.

After completion of surgery, neuromuscular blockade was antagonized with IV injection of 2 mg/kg sugammadex (200 mg/2 ml solution; Sunny Pharmaceutical). The trachea was extubated when the patient regained consciousness and patients were transferred to the PACU. 1 g of paracetamol was IV infused over 10–20 min every 8 h. A patient with a NPRS ˃ 3 was given IV bolus of 2 mg morphine sulphate, which was repeated if the NPRS still ˃ 3, with maximum total morphine dose given not exceeding 0.25 mg/kg (Al-Tamimi et al. 2009). The patients were observed for 90 min in the PACU, and then transferred to the surgical unit when the Modified Aldrete score was ≥ 9.

Primary outcome

Post-operative pain score at rest

Intensity of pain was monitored at regular intervals; on arrival to the PACU (0 min), at 30, 60, and 90 min after arrival to the PACU by the NPRS (Closs et al. 2004). The NPRS is a segmented numeric version of the visual analog scale (VAS) in which the patient selects a whole number (0–10 integers) that best reflects the intensity of pain felt (from 0 = no pain, to 10 = worst imaginable pain). The common format is a horizontal bar or line (Fig. 2).

Fig. 2

The Numeric Pain Rating Scale (NPRS)

Secondary outcomes

Hemodynamic parameters (MBP and HR) were recorded before induction of anesthesia 5 min after endotracheal intubation, before pneumoperitoneum (PP), 15 min after PP, 30 min after PP, 45 min after PP, 60 min after PP, after release of pneumoperitoneum, and after extubation and in the PACU.

Morphine requirements: number of patients who required intra-operative and post-operative additional morphine doses and the total dose of morphine given.

SpO2 recorded before induction of anesthesia and in the PACU.

Respiratory rate (RR) recorded before induction of anesthesia and in the PACU.

Post-operative sedation score in the PACU (Chiruvella et al. 2014)

0 = Awake and agitated.

1 = Awake and comfortable.

2 = Asleep and arousable.

3 = Asleep with sluggish response to verbal commands or touch.

4 = No response to verbal command or touch.

The Modified Aldrete Score before discharge from the PACU.

Modified Aldrete score (Aldrete, 1995).

Activity

2 = moves all extremities voluntarily or on command.

1 = moves two extremities voluntarily or on command.

0 = unable to move extremities.

Respiration

2 = breathes deeply and coughs freely.

1 = dyspneic, shallow, or limited breathing.

0 = apneic.

Circulation

2 = BP ± 20 mmHg of pre-anesthetic level.

1 = BP ± 20–50 mm of pre-anesthetic level.

0 = BP ± 50 mm of pre-anesthetic level.

Consciousness

2 = fully awake.

1 = arousable on calling.

0 = not responding.

Oxygen saturation

2 = SpO2 ˃ 92% on room air.

1 = Supplemental O2 required to maintain SpO2 ˃ 90%

0 = SpO2 < 90% with O2 supplementation.

Sample size calculation

Sample size calculation was done by G* program, setting alpha error at 5% and power at 80%, reviewing results from the study by Weibel and his colleagues in 2018; which showed the reduced pain score in the lidocaine group compared to the control group was (SMD = 0.5). Based on these results, a sample size of at least 25 patients per group was needed. Five patients were added to each group for possible dropouts so that each group included 30 patients.

Statistical methods

Data were analyzed using Statistical Package for Social Science (SPSS) version 21.0. Chicago, IL, USA. Mean ± standard deviation expressed quantitative data. Count expressed qualitative data. Comparison between means in the two groups was done by the independent-samples t test. Median and interquartile range expressed the non-normally distributed data, and independent samples median test applied for analysis. Comparison of proportions between two qualitative parameters was done by the chi-square test. P < 0.05 was considered significant.