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Environmental Health and Safety

Anesthetic Gases

How anesthetic gases work:

  1. anesthetic gases travel to the lungs through a tube into the animal's windpipe (intubation).
  2. The anesthetic moves through the branching structure of the lungs until it can go no further, ending up in an alveolar sac.
  3. The anesthetic gas transfers form the lungs to the bloodstream by crossing from an individual alveolus into a neighboring blood vessel (a pulmonary capillary). Once in the bloodstream, the anesthetic travels to special receptors in the brain where it essentially places the animal unconscious.

To maintain a safe anesthetic state, the anesthetist monitors the depth of anesthesia; the rate and quality of the patient's pulse and heartbeat, the lungs, the body temperature (to prevent hypothermia), and the mucous membranes (for abnormal color). The monitors used are heart monitors, instruments to measure you pet's oxygen and carbon dioxide levels, respiratory rate, and blood pressure.

Anesthetic machine

The primary purpose of the anesthetic machine and system is to deliver an inhaled (gas) anesthetic to the animal to keep him unconscious through surgery. There are two basic types of anesthetic systems used for small animals: rebreathing and nonrebreathing. Rebreathing systems are for animals weighing over 10 pounds. Nonbreathing systems are for animals under 10 lbs.

Rebreathing advantages

  1. Less oxygen and anesthetic gases are used because of the lower flow rates.
  2. Less waste gases are produced.
  3. The animal's heat and moisture from respirations are conserved.

Nonbreathing advantages

  1. Depth of anesthesia can be changed more rapidly.
  2. Less resistance occurs during respirations (small animals may have difficulty inhaling with enough force to draw air through a rebreathing system)

Acute exposure to anesthetic waste gases has been associated with central nervous system effects such as headaches, irritability, nausea, and fatigue. Some test data indicates that impairments of perceptual, cognitive, and motor skills may result from exposure to concentrations of nitrous oxide as small as 50 PPM.The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values for the anesthetic agents are 50 PPM for nitrous oxide, 50 PPM for halothane and 75 PPM for enflurane. The National Institute of Occupational Safety and Health (NIOSH), however, felt that no safe level of waste anesthetic gases could be defined because information on health effects is not completely definitive. The levels set by NIOSH are those that are attainable with proper anesthetic techniques, well maintained equipment, scavenging systems and ventilation. The recommended exposure limit (REL) for nitrous oxide is 25 PPM Time Weighted Average (TWA) during the period of anesthetic administration. The REL for the halogenated agents is 2 PPM, but NIOSH believes that if nitrous oxide concentrations are maintained below 25 PPM then the levels of the halogenated agents can be maintained below 0.5 PPM. The NIOSH document states that the "environmental limits should be regarded as the upper boundary and every effort should be made to maintain exposures as low as is technically feasible." OSHA currently does not have a standard for waste anesthetic gas exposure.

Chronic exposure: Animal studies have shown adverse reproductive effects in female rats exposed to airborne concentrations of N2O [Corbett et al.1973; Vieira 1979; Vieira et al. 1980, 1983]. Data from these studies indicate that exposure to N2O during gestation can produce adverse health effects in the offspring. Several studies of workers have shown that occupational exposure to N2O causes adverse effects such as reduced fertility [Rowland et al. 1992], spontaneous abortions, and neurologic, renal, and liver disease [Cohen et al. 1980]. A recent study [Rowland et al. 1992] reported that female dental assistants exposed to unscavenged N2O for five or more hours per week had a significant risk of reduced fertility compared with unexposed female dental assistants. The exposed assistants had a 59 percent decrease in probability of conception for any given menstrual cycle compared with the unexposed assistants. For dental assistants who used scavenging systems during N2O administration, the probability of conception was not significantly different from that of the unexposed assistants. Since environmental exposures were not measured during these epidemiology studies, no dose-effect relationship could be established.