In this post, Nancy Brock DVM DACVAA discusses some of the ways that anesthetic equipment can complicate the conduct of general anesthesia.
Much of my consulting time is spent responding to inquiries about adverse anesthesia events. Colleagues reach out because they want to understand what has gone wrong in the hopes of preventing a recurrence. Debriefing after an adverse anesthetic event helps all of us enhance our understanding of safety in anesthesia.
Sometimes, what appears to be a medical problem is actually an equipment problem. You might be surprised by how often this happens!
A nicely premedicated 4 kg lean, young adult healthy female cat was uneventfully induced with IV propofol while receiving oxygen supplementation by face mask, intubated (using a laryngoscope) and placed on 1 L/min oxygen flow rate through a Bain circuit for maintenance inhalant anesthesia with isoflurane in preparation for spay surgery.
Surgical prep and local anesthetic placement took about 15 minutes (we were teaching 😊) and was conducted outside the surgical suite. During this time, anesthesia was uneventful and all vital signs were stable. The patient was moved to the operating room. That’s when the fun started!
About 2-3 minutes after relocating to surgery, the patient developed a brisk palpebral reflex, tachypnea, and tachycardia indicating that anesthetic depth was insufficient. We attributed the change in depth to stimulation during the move to the OR. Additional IV propofol was administered and the delivered concentration of isoflurane increased.
Everything settled down, until a few minutes later when the patient began to move on the table. Additional IV propofol was given to regain an appropriate anesthetic depth. At this point, the anesthetist noticed that the patient’s mucous membranes were no longer as pink as they were at induction and the pulse oximeter was reading 92%.
What are your differentials for a light plane of anesthesia combined with hypoxemia? What steps will you take to resolve this issue?
Due to the unstable anesthetic plane and developing hypoxemia, the astute anesthetist quickly checked the anesthesia machine and noticed that the common gas outlet from the vaporizer was connected to a pediatric circle system but the patient was connected to the nonrebreathing circuit. The patient was receiving at best room air with no inhalant and at worst a hypoxic gas mixture that could have resulted in death.
The anesthetist quickly switched the circuits and connected the patient to the pediatric circle. Mucous membrane color returned to normal (PHEW!). Surgery and recovery then proceed uneventfully.
In our group discussion after the patient was extubated and awake, we reviewed all the potential rule-outs that needed to be considered in a situation where a patient won’t stay asleep:
1. Had the patient been agitated before the beginning of induction?
2. Is the equipment functioning appropriately?
Is the anesthesia machine set up properly?
Is the endotracheal tube ETT positioned correctly?
Was the vaporizer empty?
Was the flowmeter turned on and was there oxygen in the cylinder?
Let’s explore each of these possibilities:
1. Had the patient been agitated before the beginning of induction?
Nope, she’d had a smooth loss of consciousness and transition from injectable to inhalant anesthesia. Sometimes, induction can be stormy. The patient becomes agitated and combative during handling because of insufficient chemical restraint. I find that when the patient has this kind of induction experience, the surge of adrenaline (think fight or flight response) acts as an “upper” and interferes with the patient achieving a proper and stable plane of anesthesia. It can take me up to 30 minutes to calm things down. As long as I have verified that nothing is wrong with the equipment and that my patient’s vital signs don’t suggest cardiovascular depression (usually, it's the opposite), I forge ahead and remind myself yet again why it is so important to use effective premedication.
2. Is the equipment functioning properly?
In this instance, the anesthesia machine was the source of the problem. We had performed a proper anesthesia machine check in the surgical prep area but no one had thought to check the machine in surgery and to verify which circuit this cat was supposed to be connected to - she would do well on either a Bain or a pediatric circle so it was just failure to communicate and to use a routine machine checklist that got us into trouble.
At this point you might wonder why it took so long for hypoxemia to develop since the patient was not receiving any supplemental oxygen in the operating room. Two reasons: First, she had been receiving 100% oxygen in the prep area which meant that she had a reserve of oxygen in her lungs to rely on after disconnection from the prep room machine. Second, there was enough oxygen in room air of the Bain circuit hose to provide oxygen delivery for a little while.
An endotracheal tube issue is a distinct possibility given that hypoxemia did eventually develop: If the endotracheal tube is accidentally pushed further down the trachea because of jostling during transport, it may enter a bronchus whereby the patient receives oxygen only to one lung rather than both. This is an important rule out when your healthy patient is successfully intubated and is receiving oxygen through the anesthetic machine but cyanosis develops. Two other observations by the anesthetist can increase their suspicion that this has happened: a) the partially full reservoir bag is barely moving with each breath and b) when the reservoir bag is squeezed to provide ventilation support, it feels “tight” as if it is hard to deliver the breath. When the endotracheal tube is gently retracted, there is a rapid return to normal.
Proper placement of the endotracheal tube requires that the cuffed end of the tube be outside the thoracic inlet. You can use the point of the shoulder as an external landmark when assessing tube length in advance. It is easy to advance the endotracheal in too far especially in cats and cat-sized dogs unless you shorten the ETT length beforehand. Practice tip: after securing the ETT, take a moment to observe the alignment of one of the ETT’s external markings with one of the teeth such as an upper canine. Over the course of the procedure, you can then easily and quickly verify that your tube has not moved - especially useful during long dental procedures when the patient’s head may be repeatedly repositioned.
Esophageal intubation is another possibility here. It can happen surprisingly easily especially if a laryngoscope is not being used (or used properly - you should be able to clearly see the arytenoid cartilages). This seriously interferes with oxygen delivery for obvious mechanical reasons. However, if the patient is able to get room air into its lungs by inhaling around the tube, then hypoxemia may take a surprising amount of time to develop (10+ minutes if oxygen is delivered by mask before and during induction) or may not develop at all. The patient will just keep waking up because no isoflurane is reaching the brain.
Whenever I need to verify proper ETT placement (immediately after intubation and any time it looks like to tube has shifted, I simply palpate the throat: if I can identify only one rigid structure, then the ETT is in the trachea. If I identify two rigid structures then the ETT is in the esophagus. The gold standard for confirming appropriate tracheal intubation is the use of a capnograph however.
Other equipment issues that can lead to a patient that wont stay anesthetized include inadequate inhalant supply in the vaporizer. Part of routine machine check should be that the vaporizer is adequately filled and any machine that will be used during the anesthetic event should be checked prior to use.
We don’t need any extra drama during anesthesia and surgery. There is enough excitement even when things go well! So, be sure to verify that all your equipment is in proper working order and do this before every case, not just at the beginning of the day. Since there are so many aspects of our work and our patients’ conditions, we owe it to ourselves to control what we can. This approach to anesthesia delivery will help us troubleshoot effectively when the unavoidable curve balls get thrown at us.
Originally from Montreal, Dr. Nancy Brock graduated from the Ontario Veterinary College at the University of Guelph in 1982. She completed a residency in anesthesia and critical care at the
University of California, Davis in 1988. In 1995, she completed the specialty certification
process and was awarded the status of Diplomate of the American College of Veterinary
Anesthesia and Analgesia (DACVAA).
The focus of Dr. Brock’s practice is anesthesia safety. She helps veterinarians and their nursing
staff deliver safe, effective anesthesia and pain management for their surgical patients through the use of telephone consultations, telemedicine video support and hands-on training. Dr. Brock is also the editor of a quick reference anesthesia manual entitled Veterinary Anesthesia Update for Small Animal Practitioners now in its third edition. She is an enthusiastic contributor to the Anesthesia/Analgesia folder on the Veterinary Information Network (VIN).