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Just Ask the Expert: Light anesthesia anxiety
There are lessons for all veterinary practices in deciphering the cause of inadequate anesthetic depth in routine procedures. Here is a step-wise guide on what to consider as you perform your root-cause analysis.
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Are your surgical patients all sufficiently anesthetized? (Getty Images)Q.Recently, we have noticed that our anesthetized patients have heart rates that don't correspond with their respiration rates in terms of their plane of anesthesia. Their heart rates seem to reflect stage 3, plane 2 anesthesia, but their respiration rates reflect stage 3, plane 1 anesthesia (e.g. a 100-lb dog with a heart rate of 100 to 110 beats/minute but respiration is 60 breaths/minute or more). These patients are easily awakened by stimuli such as turning them over or stimulating a sore tooth when performing a dental cleaning. We find we are constantly having to turn up the isoflurane.
We originally thought we had a problem with our medical gas scavenging system. However, we recently performed a test to determine if the scavenging system has the correct amount of suction, and the results were normal. We are back to square one. I would greatly appreciate any thoughts you have on this.
A. This is a very interesting question with two major categories to consider-the circumstances regarding the patient and those of the scavenger system.
Based on the context of the question above it seems you have placed most of the concerns for the patient issues on the scavenging system. Granted, there are key points about the scavenging system to consider, but there are also important details concerning the patient that are of equal importance.
1. What are the typical anesthetic protocols being used for these patients?
Commonly, premedication protocols use the combination of a tranquilizer and an opioid (neuroleptanalgesia). Multiple publications state that analgesics used before and during surgery decrease the dose of both the induction drugs and the maintenance inhalants (mean alveolar concentration, or MAC). Local and regional anesthetic techniques are also effective in decreasing the MAC of inhalant anesthetics. Without preoperative and intraoperative analgesia, anesthetized patients will be subject to sudden light planes of anesthesia necessitating multiple top-up administration of induction drugs and increased doses of inhalant anesthesia.
In my opinion, in dogs and cats, if you are routinely running your isoflurane vaporizer greater than 1.75% (sevoflurane > 2.5%) during surgical procedures (e.g. ovariohysterectomy, orthopedics), your analgesia protocols should be reconsidered. Contrary to popular belief, butorphanol is NOT an effective primary analgesic (especially in dogs) and does not last very long. Morphine, hydromorphone, methadone, fentanyl, oxymorphone and other full mu agonist opioids are much more effective analgesics.
2. What is the time-frame between premedication, induction and maintenance anesthesia for surgery?
Is there enough time allotted for the premedications to work before induction and surgery? For example, buprenorphine, although an effective partial mu agonist, takes 20 to 30 minutes for full analgesic effects. Starting surgery before analgesic onset will result in elevated levels of nociception, contributing to patient arousal.
3. How are patients being monitored?
In response to an increase in sympathetic tone, heart rate will increase during light periods of anesthesia; however, judging anesthetic depth solely based on heart rate is inaccurate. Because heart rate can be affected by so many other variables, it is a poor indication of anesthetic depth. For example, a patient entering a deeper plane of anesthesia (level 3 to 4) will commonly become tachycardic because of increased sympathetic tone from decreased ventilation and elevated partial pressure of carbon dioxide (PCO2). The person monitoring the patient notices the tachycardia and mistakenly turns up the inhalant anesthetic, thinking the patients is arousing. Gradually the patient will reach deeper levels of anesthesia, which can become serious.
Besides heart rate, other monitoring parameters include arterial blood pressures, end-tidal PCO2, jaw tone, eye position, response to a toe pinch, respiratory rate and character, mucous membrane color and capillary refill time.
Other than tachypnea, no other parameters included in the question help judge the patient's level of anesthesia. However, a 100-lb dog with a heart of 100 to 110 beats/minute is inconsistent with a surgical plane of anesthesia (keeping in mind heart rate is a poor indication of anesthetic depth). Any external stimuli-nociception, position changes-will cause the patient to quickly arouse.
It is important to realize that anesthesia is not the same as analgesia. Many anesthetic drugs (e.g. isoflurane, sevoflurane, propofol) have little to no analgesic properties. In addition to monitoring a patient's cardiovascular status, respiratory status and depth of anesthesia, it is imperative to monitor the level of the patient's nociception. Movement during anesthesia could be an indication of inadequate anesthesia (light plane) or reflexes (nociception). When used alone, a high dose of inhalant anesthetic drug is required in order to eliminate all muscle movement reflexes due to surgical stimuli during general anesthesia (this is called “MAC no movement”). Administering analgesic drugs before and during surgery (intermittent or continuous rate infusions) decreases the necessity for high vaporizer outputs.
4. What breathing systems are being used? Are the breathing systems being checked for leaks? Are the patients intubated?
There are two major types of breathing systems used on conventional anesthetic machines for small animal patients: rebreathing and non-rebreathing. The rebreathing systems can be further divided into adult and pediatric circle systems. The choice of breathing system is based on patient size: patients weighing < 5 kg require a non-rebreathing system, those between 5 and 10 kg need a pediatric rebreathing system, and those > 10 kg require an adult rebreathing system.
Because oxygen flow rates and carbon dioxide removal differ between non-rebreathing and rebreathing systems, an important question needs to be clarified. Has the author noticed the difference only in large dogs (adult rebreathing) or small patients (non-rebreathing), or both? For example, a patient on a non-rebreathing system with an inadequate flow rate will rebreathe carbon dioxide (due to dead space ventilation within the expiratory tube), which, in turn, will cause the patient to initially become tachypneic and tachycardic.
Are the patient breathing systems (both rebreathing and non-rebreathing) checked for leaks? Leaks within the breathing system can cause the anesthetic gases (oxygen, carbon dioxide and anesthetic vapor) to leak out, resulting in an inadequate supply to the patient, or room air to enter in, resulting in dilution.
Are the patients intubated or are face masks used? Because face masks frequently do not provide a tight seal against the patient's snout, anesthetic gases and can leak out and room air can leak into the breathing system. Intubating the patient allows for controlled ventilation, which is helpful in controlling the depth of anesthesia. Combining the use of a mask for inhalant anesthesia and an active scavenger system would almost confirm air being pulled into the breathing system through the mask. This would result in a significant amount of anesthetic gas loss and/or dilution.
5. What kind of ventilation is being used?
Patients allowed to spontaneously ventilate will normally have cyclical anesthetic depth levels. A lightly anesthetized patient will have a higher minute ventilation, thus breathing more anesthetic gases into the lungs. Gradually the patient will breathe themselves into a deeper plane of anesthesia, resulting in a decreased minute ventilation and decreased uptake of anesthetic vapor. Eventually the patient's anesthetic depth will begin to lighten and minute ventilation will increase again, completing the cycle.
It's recommended to use the rebreathing bag to periodically apply a positive pressure breath (intermittent sighing) to the patient at least once every five minutes. This will help decrease positional pulmonary atelectasis and improve the cyclical levels of anesthesia in spontaneously ventilating patients. Controlled ventilation, employing a mechanical ventilator with intermittent positive pressure ventilation (controlled IPPV), provides the most consistent control level of anesthesia because the patient is not relying on its own minute ventilation to determine anesthetic vapor uptake.
Tachypneic anesthetized patients can develop ventilatory dead space, resulting in little to no anesthetic gas exchange in the alveoli. Positive pressure ventilation (intermittent or controlled) will help mitigate tachypnea and dead space ventilation. Some people use the oxygen flush valve to provide positive pressure ventilation to their anesthetized patients. Because the oxygen flush valve bypasses the flow meter and vaporizer, when pressed, 50 to 80 psi will be delivered to the patient's alveoli, causing barotrauma. In addition, using the oxygen flush valve as a mode of ventilation will cause the patient to gradually become light due to anesthetic vapor dilution.
6. Can you describe your active scavenging unit?
Is the active scavenging system made by a credible manufacturer? Active scavenging systems are manufactured to provide the correct draw against the patient breathing system and should include ample safety mechanisms. The interface between the scavenging system and breathing system contains its own reservoir bag, a pressure control needle valve, and a positive and negative pressure safety valve. If the negative pressure safety valve malfunctions, there will be an excessive draw against the patient breathing system, resulting in anesthetic gases being pulled away from the patient. This problem is typically manifested by a continually deflated breathing system reservoir bag and variable levels of patient anesthetic depths.
Ways to mitigate excess scavenging system draw against the patient breathing system are to turn up the flow rate to match the draw, adjust (close) the interface needle valve, or partially close the adjustable pressure limiting device (pop-off valve).
Scavenging systems problems are rare, but they can occur. I would recommend you continue to use the passive system until the problem is solved.
Dr. Claude is an assistant professor in the Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University.
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