A Novel Two-Step Dosing Induction Strategy (The Propofol Challenge Test) to Minimize Propofol Injection Pain

Background and Rationale Propofol is the most widely used intravenous hypnotic agent for the induction of general anesthesia owing to its rapid onset and short duration of action. Despite its favorable pharmacokinetics and widespread use, a notable drawback persists: the incidence of pain on injection. Statistics on the matter are widely variable, but some estimates indicate that the incidence of pain with propofol injection is around 70%1.

The mechanism behind propofol-induced injection pain is multifactorial. Propofol, an alkylphenol, directly irritates the venous endothelium upon contact. In addition, it activates the kallikrein-kinin system, leading to the release of bradykinin and other proinflammatory mediators, which increase vascular permeability and sensitize peripheral nociceptors. While various methods-such as pretreatment with lidocaine-have shown efficacy in reducing this discomfort, a substantial proportion of patients still report moderate to severe pain during administration.

To date, no studies have systematically evaluated whether dividing the induction dose of propofol-starting with a small sub-induction bolus (e.g., 20 mg) followed by the remainder-can reduce pain perception. The rationale for this approach draws on a parallel from thermos-sensory experience: gradual immersion in cold water is often less painful than immediate full-body exposure. Anecdotal and physiological evidence suggests that incremental exposure to a noxious stimulus allows for short-term adaptive responses, reducing the subjective intensity of the experience. On a mechanistic level, this effect may involve both peripheral and central nervous system plasticity. A small initial dose of propofol may serve as a moderate nociceptive stimulus that activates endogenous inhibitory pathways-such as descending modulation from the periaqueductal gray and rostroventral medulla-which dampen the transmission of subsequent pain signals. This phenomenon, known as homotopic or heterotopic noxious conditioning stimulation (HNCS) or preconditioning-induced hypoalgesia, relies on dynamic modulation of nociceptive input at spinal and supraspinal levels.

We hypothesize that a two-step administration of propofol-consisting of a small initial dose followed by the remaining dose-may serve as a nociceptive preconditioning stimulus and reduce postoperative patient-recalled injection pain compared to standard single-bolus administration.

Sample size:

The sample size was calculated to detect a clinically meaningful difference in the incidence of injection pain between the two groups in the proposed PROTECT trial. We plan to conduct an interim analysis at the midpoint of data collection (around 4 months) to assess conditional power based on the observed effect size. At that point, we will simulate 1,000 completed trials using the interim data to estimate conditional power. If conditional power is <10%, we would consider stopping the study early for futility. Conversely, if we observe strong early evidence of efficacy, we may stop early using a conservative O'Brien-Fleming boundary (interim p < 0.005, final p < 0.048). Assuming an incidence of pain of 10% in the control group and 5% in the intervention group (absolute difference of 5%) and using a two-sided chi-square test to compare independent proportions with alpha = 0.048 (adjusted for the interim look), we estimate that 880 participants (440 per group) would be required to detect this difference with 80% power. We propose enrolling 1,000 patients to allow for potential dropout.