When administered in large doses, propofol produces burst suppression in the EEG, an end point that has been used when propofol (and other intravenous anesthetics) after a single bolus dose or continuous infusion are best described by means of a three-compartment model. Such models have been used as the basis for developing systems of target-controlled infusions.
The context-sensitive half-time of a drug describes the elimination half-time after discontinuation of a continuous infusion as a function of the duration of the infusion. It is an important parameter in assessing the suitability of a drug for use as maintenance anesthetic. The context-sensitive half-time of propofol is brief, even after a prolonged infusion, and therefore, recovery occurs relatively promptly (Figure 25–8).
A. CNS Effects
Propofol acts as hypnotic but does not have analgesic properties. Although the drug leads to a general suppression of CNS activity, excitatory effects such as twitching or spontaneous
FIGURE 25–8 The context-sensitive half-time of common intravenous anesthetics. Even after a prolonged infusion, the half-time of propofol is relatively short, which makes propofol the preferred choice for intravenous anesthesia. Ketamine and etomidate have similar characteristics, but their use is limited by other effects.
administering intravenous anesthetics for neuroprotection during neurosurgical procedures. Evidence from animal studies suggests that propofol’s neuroprotective effects during focal ischemia are similar to those of thiopental and isoflurane.
B. Cardiovascular Effects
Compared with other induction drugs, propofol produces the most pronounced decrease in systemic blood pressure; this is a result of profound vasodilation in both arterial and venous circulations leading to reductions in preload and afterload. This effect on systemic blood pressure is more pronounced with increased age, in patients with reduced intravascular fluid volume, and with rapid injection. Because the hypotensive effects are further augmented by the inhibition of the normal baroreflex response, the vasodilation only leads to a small increase in heart rate. In fact, profound bradycardia and asystole after the administration of propofol have been described in healthy adults despite prophylactic anticholinergic drugs.
C. Respiratory Effects
Propofol is a potent respiratory depressant and generally produces apnea after an induction dose. A maintenance infusion reduces minute ventilation through reductions in tidal volume and respiratory rate, with the effect on tidal volume being more pronounced. In addition, the ventilatory response to hypoxia and hypercapnia is reduced. Propofol causes a greater reduction in upper airway reflexes than thiopental does, which makes it well suited for instrumentation of the airway, such as placement of a laryngeal mask airway.
D. Other Effects
Although propofol, unlike volatile anesthetics, does not augment neuromuscular block, studies have found good intubating conditions after propofol induction without the use of neuromuscular blocking agents. Unexpected tachycardia occurring during propofol anesthesia should prompt laboratory evaluation for possible metabolic acidosis (propofol infusion syndrome). An interesting and desirable side effect of propofol is its antiemetic activity. Pain on injection is a common complaint and can be reduced by premedication with an opioid or coadministration with lidocaine. Dilution of propofol and the use of larger veins for injection can also reduce the incidence and severity of injection pain.