Let's discuss a common misunderstanding: when to use intermittent positive pressure ventilation (IPPV)), how much to use, when and how to stop assisting a patient's ventilation.
Let's discuss a common misunderstanding: when to use intermittent positive pressure ventilation (IPPV)), how much to use, when and how to stop assisting a patient's ventilation. This can be administered by hand, manual IPPV, or by a machine, in which case we call it mechanical IPPV. I raise this issue in light of recently hearing the often cited, but completely misconstrued "don't breathe for the patient or the patient will not breath for themselves. Just let carbon dioxide accumulate." True, under non-anesthesia condition. False, under anesthesia. One can let carbon dioxide accumulate until the proverbial cows come home. If our anesthetic agents tell the brain not to take a breath as frequently as the patient would when awake, then we need to help them breath. Simple! The advantages are that breathing for them will make them equilibrate faster to the number on the isoflurane dial. When we are finished breathing for them on pure oxygen will help them to arouse and extubate. If we do not breath for them, they will likely have some degree of respiratory acidosis, experience mild to severe acidemia, and possible accumulate enough carbon dioxide in their blood to cause an injury. The downside is we have to focus on our patient to avoid iatrogenic injury.
The quickest summary is to give an example of how these misunderstanding hurt veterinary patients is that insufficient ventilation causes respiratory acidosis, this leads to patient acidemia. Our medications and inhalant agents SUPPRESS the normal drive to breath. The way to see if one needs to apply IPPV is to read a capnograph. When the patients’ need a manual breath or need a ventilator, it will require them until the medications have been discontinued and been metabolized. There is rational and straightforward way to stop breathing for the patient, same as manually or mechanically. Wait until the effects on the patient that dulled his ventilatory drive have left his system.
Let's run through a case example. Bear, is an aptly named healthy 6-year-old 33 kg chocolate Labrador retriever is premedicated with hydromorphone and dexmedetomidine, both IM. After Bear vomited and became sedate, a catheter is placed in his right cephalic vein and he is induced with midazolam and ketamine. He is intubated and placed on 2% isoflurane delivered in a circle system using 2 L/Min oxygen. He is clipped, prepped and received a morphine-saline epidural for his stifle surgery. He is moved and instrumented on the capnograph in the OR. There is no waveform, so his technician gives a breath, makes a nice "elephant holding the tail" waveform and the ETCO2 is 68 mm Hg. The seasoned staff reaches across and indicates that she should stop, because "he won't breath for himself if we keep breathing for him" so she stops. Because her employer is wonderful, and the patient is stable and large, she practices her art line skills on the tiny artery under the front paw and uses a point of care machine to test the acid base status. There she recognizes that the patient's pH is 7.2. He is spontaneously ventilating, but he has sharp little peaks that read 45 mmHg and few deep breaths. Bear’s technician gives Bear a breath, achieves that rectangular waveform and now his ETCO2 is 72 mmHg. What should she do? Ignore the seasoned tech and breath for the patient? If so, why? If she starts breathing for the patient, how will she stop, and will it delay his recovery? She has had enough education to know something is not correct, but not enough experience to figure out what to do or how to rationally explain why she FEELS breathing for the patient is her best choice, and no idea when to stop IPPV.
The answer is that she should breath for the patient. She is recognizing these peaks represent less than full exhalations or air escaping as the surgeon manipulates the stifle for the procedure. To start breathing for the patient of this size, she starts by checking the cuff on the ETT tube is full and closes the pop-off valve (adjustable pressure release valve) and inflate to a patient airway pressure of 15 or 20 cm H20, then opens the pop off valve again, watching his thorax rise and fall under the drapes. She starts googling "pop off occlusion valve" to make this process more streamlined. She begins with breathing every 10-15 seconds and assess if the ETCO2 can be brought down to approximately 45 mmHg. She continues this until there is a chance to discontinue the isoflurane. After that happens, she rouses the patient by continuing to breathe for the patient on pure oxygen. One can pause and see if Bear is ventilating on his, but then return to ventilating for Bear if he has not lightened his plane of anesthesia. The more frequent and full the breaths, the faster the isoflurane will be offloaded and exhaled into the scavenging side of the circle breathing system. Bear will wake up faster than if he had been left to breath room air on his own sluggish ventilation. In his medicated state, he could again fail to breath enough to keep his carbon dioxide levels at a healthy level. Bear could even have two reversal drugs, atipamezole and flumazenil if he failed to ventilate himself to a safe carbon dioxide level within a reasonable amount of time. But he does not need it.
Bear, still on oxygen, wakes up, opens his eyes and lifts his head. He is extubated and lays back down on his gurney. The surgeon comments that she doesn't smell isoflurane as strongly as usual. She's correct, Bear was exhaling into the scavenging system for a while. Everyone was happy. The pop-off occlusion valve and prep room capnography machine are ordered because the employer is so generous and helpful.
Next article, Bear goes on the ventilator for his other stifle surgery. How do we "wean" him off that apparatus, and when? What are the impacts on blood pressure and how do we balance them for short-term anesthesia ventilation?
Jennifer Hess is a board-certified anesthesiologist who has a life-long interest in helping high-risk patients survive and thrive after anesthesia.