Injection pain may predict postop pain outcomes

During induction of anaesthesia in gynaecological laparoscopic surgery, the incidence of propofol injection pain (PIP) and rocuronium‑induced withdrawal movement (RIWM) may predict the severity of postoperative pain and total opioid consumption, a prospective observational study has shown.

Compared with patients without PIP, those with PIP had significantly higher mean Visual Analogue Scale (VAS) pain scores at 1 hour (45.7 vs 40.4; p=0.017) and 24 hours postop (36.6 vs 31.8; p=0.024), and higher mean cumulative opioid consumption at 24–48 hours (50.7 vs 49.7; p=0.047) and for 48 hours postop (103.9 vs 99.9; p=0.017).

Similarly, postoperative pain scores (VAS pain scores at 1 hour and 24 hours) and opioid consumption (0–24, 24–48, and 48 hours postop) were significantly higher in patients with vs without RIWM (p<0.000 for all).

There was a significant but weakly positive correlation between PIP and VAS pain scores at 1 hour (r=0.234; p=0.013) and 24 hours (r=0.227; p=0.016), and between PIP and total opioid consumption in the 48‑hour postop period (r=0.234; p=0.00).

RIWM correlated more positively with VAS at 1 hour, VAS at 24 hours, and total opioid consumption for 48 hours postop (r=0.408, r=0.398, and r=0.457, respectively; p=0.00 for all), and emerged as the stronger predictor than PIP given its positive correlation with VAS pain scores at 48 hours as well (r=0.330; p=0.04).

“[These findings suggest] that preoperative pain perception may be used to predict pain outcomes after surgery,” said the researchers.

Of the 100 women enrolled, 95 (mean age 38.8 years, mean basal bispectral index [BIS] score 95.1) were included in the analysis. Anaesthesia was induced using 2 mg/kg of 1% propofol (long‑chain triglyceride [LCT] emulsion) over 15 seconds. Upon reaching a BIS score of 60, 0.6 mg/kg of 1% rocuronium was injected over 10 seconds. Participants were not given any analgesic (including opioids) prior to anaesthetic administration. Mean PIP incidence was 51. Forty women had grade 3 RIWM (arm only) while 14 had grade 4 (generalized movement).

Sevoflurane and an oxygen/air mixture (fraction of oxygen: 50 percent) were used to maintain anaesthesia. Pyridostigmine and glycopyrrolate were administered after completion of surgery to reverse neuromuscular blockade. [Singapore Med J 2024;65:326-331]

For the 48-hour postop period, the patient‑controlled analgesia pump comprised fentanyl 800 mcg, ketorolac 150 mg, and ramosetron 0.6 mg in 150 mL of saline. Pain severity was measured using a 100‑mm linear VAS and analgesics were given on demand. Pain corresponding to ≥50 mm on VAS warranted treatment with IV fentanyl 100 mcg, while for those with a pain score of <40 mm, IV ketorolac 15–30 mg was given.

In the study, gynaecological surgery* was performed in non-menopausal women to minimize selection bias. “However, we should have considered the hormonal state according to the stages of their menstrual cycles … [T]his may have affected the results of this study,” the researchers pointed out.

The female study population also limits extrapolation of the findings to males. Further research in gender‑specific surgery is thus warranted to ascertain preoperative pain sensitivity and its association with postoperative pain and analgesic use and to assess the influence of sex differences on these associations, they said.

Another factor that may have limited the findings is the different propofol formulations, they added, as results may differ depending upon the propofol formulation used (eg, medium‑chain triglyceride/LCT propofol or triglyceride‑free microemulsion propofol).

“[Nonetheless, the findings show that] the perception of PIP and RIWM at anaesthesia induction has significant and positive correlations with postoperative pain outcomes, with RIWM emerging as the stronger predictor for postoperative pain severity and total opioid consumption,” said the researchers.

“Both PIP and RIWM may be used to identify patients who will require more intensive pain management postoperatively,” they concluded.