Clinical implications of sedation in animals

Sedation follows a drug-induced depression of CNS causing a downturn of the level of consciousness and a reduction of motor activity, arousal and surrounding awareness. Various levels of consciousness ranging from alertness to sleep naturally fluctuate throughout a standard circadian cycle in humans and animals. Sedation is thus hard to demarcate, but is generally graded as minimal, moderate or deep (1), according to patient cognitive function and airway control. Sedation is intended to reduce anxiety fear and distress, to avoid episode recall and to facilitate the successful performance of diagnostic or therapeutic procedures, providing patient comfort and cooperation. Persistence of respiratory reflexes and of ventilation mechanics warrant respiratory homeostasis only during light and moderate sedation. Patient cooperation is of paramount importance in humans, to differentiate normal responses to verbal commands (minimal sedation) from purposeful responses to verbal commands following light tactile stimulation (moderate sedation), from purposeful responses following repeated or painful stimulation (deep sedation). Reflex withdrawal from a painful stimulus during deep sedation is not considered a purposeful response (1). Unfortunately animals, as well infants or non-cooperative human patients, lack collaboration such as purposeful responses to verbal commands. They may open their eyes following some kind of stimulation but won’t tell their name, weekday or else, demanding species specific sedation scales along with vital parameters’ monitoring to assess sedation depth. Terms like conscious sedation or twilight anesthesia can’t therefore be used in non-human patients. Still veterinarians are able to induce standing sedation (2) where anxiety and fear are prevented, allowing a certain degree of immobility and the performance of diagnostic or therapeutic procedures. Painful procedures are also carried during standing sedation, mainly on large animals, with the combined use of local or regional analgesia. The term procedural sedation is appropriate for most clinical procedures in animals since none can be pleasant for such patients. By combining opioids with sedatives, analgosedation favors the synergistic effects of single molecules reducing their dosages and side effects. Distress, fear, agitation and aggressive behavior can be expected even in peaceful domesticated animals. Sympathetic nervous system activation, hyperarousal and the fight-or-flight response are not unusual in animals under procedural circumstances, enforcing sedation even for routine actions such as clinical examination of untamed patients. Other than behavioral and security needs, sedation allows for the highly desirable low cardiac output needed for a prevalent drug distribution to CNS. Regrettably, such hemodynamic requisite can’t be exploited when chemical restraint is needed in animals either escaping or fighting human contact. Under those circumstances, very high doses leading to deep sedation or general anesthesia are needed. Disinhibition, refractory agitation and delirium are quite frequent with animal sedation as a consequence of anxiolytic, sedative or anesthetic molecules, as well of insufficient dosage or poor pharmacokinetics. Under those circumstances anesthesia induction or deep sedation by alternative molecules is generally the best choice if followed by a very slow, guarded recovery procedure. Like pediatric anesthesia, animal sedation involves a wide range of age and body weight, demanding for expert allometric calculations within the same species and among different species and animal classes. Other than such huge variables, the impact of underlying critical illness, potential organ dysfunction and drug interaction, as well availability of facilities and equipment impose on the veterinary choice of the sedation protocol (3).