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Anesthesia, CRI and TIVA: What’s the difference?
by Marc Raffe, DVM, DACVAA, DACVECC

Anesthesia, by definition, has three elements; amnesia (sedation or sleep), analgesia and muscle relaxation.  How these elements are achieved is determined by the individual anesthetist.  Historically, anesthesia was induced by intravenous drugs and maintained by inhalation, or gas, anesthesia.  All inhalants, by definition, are complete anesthetics and satisfy the three elements.  However, the degree to which they satisfy each element is unequal.  Of the three elements, analgesia, or pain control, is the most difficult to achieve.  Most anesthetists understand this point having experienced patients that appeared to be anesthetized until pain stimulus provoked an immediate patient response.

In the past twenty years, numerous articles have been published describing anesthesia drug combinations.  Combinations have been studied, in part, to identify a way to shift pain control from inhalation anesthesia, a weak option, to drugs with known pain control properties.  For example, selecting pre-anesthetic drugs with pain control characteristics creates a “platform”, or foundation, of pain control during early phases of anesthesia.  It has been shown that better perioperative pain control is achieved because pain is treated prior to pain stimulus (preemptive analgesia).  In an effort to take this strategy an additional step, perioperative drug delivery via constant rate infusion (CRI) has become popular.  Combinations such as morphine, lidocaine and ketamine (MLK or “MiLK”) delivered as a CRI are noted to improve pain control during the perioperative period.  Because inhalant anesthesia drugs (isoflurane, sevoflurane, desflurane) have relatively low safety margins, an additional advantage of using the CRI strategy is to reduce vaporizer settings resulting in improved patient safety. 

In the final part of this strategy, drug protocols have been developed to administer general anesthesia by intravenous drug administration.  This approach, named total intravenous anesthesia (TIVA), was described approximately two decades ago in human medicine and has been adopted in veterinary medicine for specific case management.  Both CRI and TIVA require an initial, or loading, dose of each drug to establish an effective blood concentration followed by continuous IV infusion to maintain therapeutic drug level.  The advantage of TIVA is that is can be used to provide general anesthesia in patients where exposure to inhalation anesthesia may pose significant risk.  The main disadvantage of TIVA, and CRI, is that the anesthetist must be able to calculate drug doses for continuous infusion rates and have a way to provide constant drug delivery during the course of anesthesia. 

We will explore further details of CRI and TIVA in future postings.