By Dr. Truchetti Geoffrey, DMV, MSc, DES, DACVAA
- What is the recommended fresh gas flow (Bain circuit)?
- What is the recommended fresh gas flow (rebreathing circuit)?
- When should I check the anesthesia machine?
- How can I verify the anesthesia machine?
- When should I use a mechanical ventilator?
- What temperature is acceptable under anesthesia?
- Should a complete monitoring be used for every patient?
- When should soda lime be changed?
- What is the risk of using an endotracheal tube that is too long?
- What is dead space?
- How can exposure to waste anesthetic gases be minimized?
- How to choose between a Bain circuit and a rebreathing circuit?
- What are the differences between a Bain circuit and a rebreathing circuit?
- What should be the size of the breathing bag?
- What are the risks if the breathing bag is deflated? What if it is over-inflated?
What is the recommended fresh gas flow (Bain circuit)?
In a Bain circuit, high fresh gas flow is the source of oxygen and inhalant agent, but more importantly, is the only mean to prevent CO2 rebreathing.
The need to eliminate CO2 forces the use of high fresh gas flows. If the fresh gas flow is too low, rebreathing will occur. A fresh gas flow of 100-660 ml/kg/min is recommended (Seymour and Duke-Novarkovski 2007, Tranquilli, Thurmon et al. 2007) and needs to be adjusted to the minute volume of the patient (i.e. respiratory rate x tidal volume). A fresh gas flow of 150-300 ml/kg/min is often enough, but keep in mind that it needs to be adjusted based on clinical signs. A capnograph is highly recommended to adjust fresh gas flow to the need of the patient. It is even possible to use a fresh gas flow below 100 ml/kg/min, but it is not recommended without the use of a capnograph.
What is the recommended fresh gas flow (rebreathing circuit)?
In a rebreathing circuit, fresh gas flow is the oxygen and inhalant source. CO2 is eliminated from the circuit by the soda lime.
At the beginning of anesthesia (or when changing anesthesia machine), a high fresh gas flow should be used to load the breathing circuit in inhalants and removed nitrogen (denitrogenation) from the circuit and the lungs of the patient. A fresh gas flow of 100 ml/kg/min is recommended during the first 15-20 minutes. Subsequently, a fresh gas flow of 20-40 ml/kg/min is recommended (Seymour and Duke-Novarkovski 2007, Tranquilli, Thurmon et al. 2007). A lower fresh gas flow (5-10 ml/kg/min) can be used, but that requires increased monitoring, such as making sure the breathing bag is filled and checking the level of anesthesia.
A minimum fresh gas flow of 200-300 ml/min should be used, especially if no gas analyzer is used, to make sure the vaporizer output is stable and can be relied upon.
When should I check the anesthesia machine?
Every machine should be checked before anesthesia. Minimally, this should include a leak test, a visual integrity check, a verification that nothing is missing, that the appropriate breathing circuit, with the proper breathing bag, is attached to the machine and finally, that there are no cracks or holes in any tubing (including internal tubes if coaxial). This should take less than a minute.
A complete check should be done every day or after the disassembly and reassembly of a machine. The next question and this video explain how to do a full check.
How can I verify the anesthesia machine?
- Integrity of the system (visual inspection)
- Oxygen source
- Leak in the anesthesia machine
- Connection to the appropriate breathing circuit
- Valves and soda lime, if applicable
- Size of the respiratory bag
- High-pressure leak test
- Internal tubing (coaxial) tubing test, if applicable
- Pop-off valve
- Scavenging system
When should I use a mechanical ventilator?
Anesthesia ventilator typically delivers intermittent positive pressure ventilation (IPPV). It can be beneficial to any patient with respiratory issues, such as hypoxemia and hypoventilation. Under anesthesia, the main concern is hypoventilation. It is easily diagnosed by a capnometer. This is fairly common during anesthesia but often undiagnosed in the absence of a capnograph.
An end-tidal CO2 above 50 mmHg can be used as a threshold to ventilate a healthy patient. However, this should be adapted to the patient as some diseases may limit the use of ventilators (e.g., some cardiovascular diseases). Additionally, when using a ventilator, the anesthesia team can dedicate their time to monitoring the patient instead of concentrating on bagging it, improving efficacy and safety.
What temperature is acceptable under anesthesia?
Below 36°C, hypothermia can have harmful consequences for the patient, such as:
- Decreasing anesthesia needs, potentially causing an overdose
- Arrhythmia, bradycardia, decreased response to anticholinergic drugs
- Shivering increases oxygen consumption in the face of cardiovascular instability and inability to provide enough oxygen
- Reduced coagulation
- Decreased immune response
Clinically, the most common complication is a prolonged recovery caused by a relative overdose. In addition to being harmful to the patient, this also interferes with the efficacy of the anesthesia and surgery team by adding workload that could have been prevented.
Therefore, a core temperature above 36°C is recommended. However, be careful not to induce hyperthermia when keeping a patient warm. Continuous monitoring of core temperature is therefore advised.
Should complete monitoring be used for every patient?
Yes, complete monitoring that is adapted to the patient should be used. What “complete” is differs from one patient to another. Additionally, monitoring is not only about having the monitoring installed. It also means actively monitoring the patient and making sure every measured parameter stays in a normal range.
A retrospective study showed that the use of the pulse oximeter and the monitoring of pulse during anesthesia decrease the risk of mortality in cats (80,000 anaesthetic and sedation procedures) (Brodbelt, Pfeiffer and al. 2007). Other monitoring methods were not included in this study as they were rarely used at this time. In human medicine, the use of the pulse oximeter and a capnometer can prevent 93% of complications during anesthesia (Tinker, Dull and al. 1989).
Clinically, noninvasive monitoring, especially if they can be installed rapidly, should be used for every patient. These include pulse oximeter, capnometer, ECG, and noninvasive blood pressure monitoring. The other devices (invasive or longer to set up, such as invasive blood pressure monitoring) should be used when needed.
When should soda lime be changed?
Ideally, soda lime should be changed before its exhaustion. Different methods exist to know when soda lime is exhausted and are detailed in the documents available online (link). If the anesthesia workload is fairly constant, it is possible to change soda lime on a regular basis (for example, every Monday). If not, it should be replaced if rebreathing occurs, if color changes or after a given time.
Monitoring that soda lime is efficient should be done even if the soda lime is changed regularly, or if the color is normal, or even if it has just been changed. The only reliable monitoring to make sure the soda lime is effective is the capnometer. The other methods are not, and could appear normal in the face of exhausted soda lime. If the patient rebreathes CO2, soda lime should be changed even if it is white or if it has just been changed.
What is the risk of using an endotracheal tube that is too long?
Using an endotracheal tube that is too long increases dead space, resistance and the risk of bronchial intubation. The ideal length should be the distance between the teeth and the thoracic inlet.
Increasing dead space increases rebreathing, causing respiratory acidosis, which can be harmful to the patient.
If the tip of the tube goes too far, it could end up in a bronchus, impeding the ventilation to the other lobes of the lungs. This will cause severe hypoxemia, worsen by hypercapnia.
Finally, resistance is proportional to the length of the tube (among other parameters). This is rarely a concern compared to the other consequences (dead space and bronchial intubation) but could be harmful to a patient that is already exhausted.
What is dead space?
Generally speaking, dead space can be defined either as a volume of gas that is rebreathed without a change of composition, or the volume of respiratory tract that does not participate in gas exchange.
Different kinds of dead space have been defined:
- Anatomical dead space: it is the volume that is breathed and does not reach the alveoli (i.e., tracheobronchial tree and upper respiratory airway).
Physiological dead space: it is the volume that is breathed that does not participate in CO2 elimination.
- Mechanical dead space: it is the volume in the equipment (endotracheal tube, capnograph, breathing circuit) that is re-breathed without a change of composition.
- Anatomical and physiological dead spaces are nearly similar in normal healthy patients. They will differ in some pathological conditions, such as shunts (ventilation of alveoli without perfusion)
How can exposure to waste anesthetic gases be minimised?
To decrease exposure to waste anaesthetic gases, it is recommended to:
- Leak test all equipment
- Avoid mask induction and induction boxes
- Intubate and inflate cuff for all patients
- Connect breathing system and leak test ET-tube before turning on the vaporizer
- Use low-flow anesthesia
- Cap breathing system when not in use
- Ventilate the recovery area (15-20 air changes per hour and no recirculation of air in the hospital)
- Fill vaporizer at the end of day
- Carry out regular maintenance
It is also recommended to monitor waste anesthetic gases every six months on the premises.
How to choose between a Bain circuit and a rebreathing circuit?
The choice should be made based on the pros and cons of each circuit (see next question) but it can also be easily made based on the weight of the patient.
- Below 5 kg, a Bain circuit is often recommended because the patient might not be strong enough to breathe against the resistance of the rebreathing circuit.
- Above 10 kg, the rebreathing circuit should be used, as patients are generally strong enough to overcome the resistance of the valves and soda lime. Additionally, above 10-15 kg, the use of a Bain circuit is not economical.
- Between 5 and 10 kg is a gray zone. The patient might not be strong enough to overcome resistance. However, using a rebreathing system decreases oxygen, inhalant waste and temperature loss.
What are the differences between a Bain circuit and a rebreathing circuit?
- Faster change of anesthesia plane
- Less resistance to breathing
- Fresh gas flow
- 150-200 ml/kg/min
- Decreased O2 and inhalant consumption ($)
- Warm and moist inspired gas → decreased temperature loss
- Fresh gas flow
- Initially 100ml/kg/min then 20-30 ml/kg/min
- Minimum 200-300 ml/min
What should be the size of the breathing bag?
The size of the breathing bag should be enough for the patients to take a deep inspiration without emptying the bag. Usually, 5 to 10 times the tidal volume (which is 10-15 ml/kg in dogs and cats) is considered enough. Practically, a bag of 1 L per 10 kg BW (rounded up) can be used. For instance, for a 20 kg dog, a 2-litre bag should be enough.
What is the risk if the breathing bag is deflated? What about if it is over-inflated?
If the bag is deflated: there is not enough reserve in the breathing system for the patient to take a complete inspiration. Negative pressure will develop in the breathing system and the patient’s airway. This could cause pulmonary oedema, decreasing gas exchange efficiency. If the bag is deflated, fresh gas flow should be increased.
If the bag is over-inflated: this could mean that either the scavenging system does not work or the fresh gas flow is too high. The first hypothesis can be deadly for the patient if the pressure in the system increases. The first thing to do is, therefore, to check the pressure in the breathing system and verify what is wrong with the scavenging system (pop-off valve closed, active aspiration turned off, etc.) If the pressure is normal, it might just mean that the fresh gas flow is too high and it could be reduced. As long as the pressure in the breathing system is close to atmospheric pressure, there is no risk of having an over-inflated breathing bag. It can only be detrimental for the monitoring of the respiratory rate if the breathing bag is used to count the rate. On a Bain circuit, decreasing the fresh gas flow can cause rebreathing of CO2. It is therefore recommended to monitor inspired CO2 if fresh gas flow is decreased below 150-300 ml/kg/min on a Bain circuit.