Some of the most common questions I am asked involve the anesthetic ventilator. I hear everything from "How do I use it?" to "I am afraid of it," to "We have one, but no one knows how to use it." What a shame! I am here to tell you that it is not the big scary machine that it is perceived to be and that once you understand a few basic concepts, you will never again want to anesthetize a patient without one.
During anesthesia, you have probably assisted your patient's respirations by squeezing the reservoir bag. This is called manual ventilation. Mechanical ventilation is just like continuous manual ventilation except that the ventilator is doing the work of squeezing the bag.1 It is like having another technician with you to make sure that your patient is breathing so that your hands are free to do other things.
We are not talking about the ICU long-term ventilator (that thing is scary). If a patient is subjected to greater than 60% oxygen for more than 12-24 hours they are at risk for oxygen toxicity.2 This is a state where lung damage occurs due to an increase in free radical formation.2 The long-term ventilator uses a variable oxygen concentration and can be used for an extended period of time. It is typically used on patients that are in respiratory failure due to some type of pulmonary disease or trauma.
Our friend the anesthetic ventilator is to be used with 100% oxygen for a limited duration and typically on normal lungs. Anesthetic respiratory failure is usually not due to lung pathology, but to the patient's inability to ventilate normally (e.g. during a thoracotomy), or due to hypoventilation or a decreased ventilatory drive due to anesthetic drugs.
Why use an anesthetic ventilator?
An anesthetic ventilator is useful because it can ventilate a patient better than the patient can ventilate itself.3 Analgesics, surgical positioning, decreased body temperature, and diaphragmatic impairment are all risk factors for inefficient ventilation to which our anesthetized patients are likely to be exposed.3 An anesthetic ventilator is useful in patients with an abnormal respiratory system. This includes the obese, debilitated, pneumonia, or head trauma patients, and patients undergoing a thoracotomy or diaphragmatic hernia repair.1 Remember that there is a difference between breathing and ventilation. Breathing refers to the lungs taking in and expelling air. This does not mean that adequate ventilation is occurring. Ventilation takes gas exchange into account. Simply stated, adequate ventilation means that oxygen is getting across the lungs and into the blood and that carbon dioxide is getting out of the blood by being exhaled via the lungs.
A tiny bit about gas exchange
Using an anesthetic ventilator properly prevents hypoxemia and efficiently eliminates carbon dioxide. Let's review some basics about oxygen and carbon dioxide:
Hypoxemia means that the blood is deficient of oxygen. All tissues in the body need oxygen to live and rely on effective ventilation to bring oxygen across the lungs, into the bloodstream, and to the cells. Some causes of hypoxemia are: blood shunting past the lungs, a ventilation and blood perfusion mismatch, hypoventilation, low inspired oxygen, and low perfusion.2 It is important to remember that hypoxemia does not equal cyanotic mucous membranes.2 This is especially true for a patient that has been delivered 100% oxygen.
PaCO2 refers to the amount of carbon dioxide in arterial blood. Carbon dioxide is a byproduct of metabolism. Our cells make it, push it into the blood stream, and rely on effective ventilation to take the gas across the lung membrane and away from the body. Anesthetized patients are prone to have an elevated PaCO2 level. One reason for this can be decreased elimination of carbon dioxide. Failure to "blow off" carbon dioxide can be due to hypoventilation (from drugs given or central nervous system depression), muscle weakness (from neuromuscular blockers or hypokalemia), or lung disease impairing the diffusion of carbon dioxide across the membrane.2 Rebreathing of carbon dioxide will also cause an elevated PaCO2. Some causes of this may be exhausted soda lime, excessive dead space in the breathing circuit, a misbehaving one-way valve on the anesthesia machine, excessive endotracheal tube length, or a relatively large end tidal CO2 monitor attachment.2 Finally, the body may be making too much carbon dioxide in a hyper metabolic state such as is the case with malignant hyperthermia,2 but this is rare.
What is the difference between my patient's spontaneous breathing and the positive pressure ventilation that the ventilator delivers?
During a spontaneous inspiration, intrathoracic pressure becomes negative. This is what allows the lungs to inflate, and during this time of negative pressure the volume of blood returning to the heart increases.2 The opposite is true during positive pressure ventilation. During inspiration, the ventilator is pushing air into the lungs to expand them. This creates a positive pressure in the thorax which can collapse large vessels and decrease the volume of blood returning to the heart.2 If the inspiratory phase is too long, there may not be enough time during expiration for the heart to fill. This causes a decrease in cardiac output. During positive pressure ventilation, the inspiratory phase should be less than two seconds and the inspiration to expiration ratio (I: E) should be from 1:2 to 1:4.2 The anesthetic ventilator at DoveLewis has the I: E ratio pre-set, but I think that it is important to understand these physiological differences.
Are there risks in using positive pressure ventilation?
Positive pressure ventilation does have risks. A pressure that is set too high can rupture alveoli in the lungs, which may result in a pneumothorax or pneumomediastinum. If positive pressure is maintained throughout the respiration cycle, cardiac output may be reduced, resulting in a drop in blood pressure and perfusion. Hyperventilation may be induced by setting the respiratory rate at high levels. This may lead to excessive amounts of carbon dioxide being exhaled resulting in respiratory alkalosis. In addition, there are certain cases where using positive pressure ventilation is contraindicated. A compromised airway should not be subjected to undue pressure. This includes patients with an untreated pneumothorax and those that have recently had esophageal surgery.5 Patients with increased intracranial pressure should also not be subjected to positive pressure ventilation.5
Types of anesthetic ventilators
There are several different ways that a ventilator can deliver a breath to a patient.
- Pressure cycled-delivers volume until a preset pressure is reached.
- Volume cycled-delivers a preset volume regardless of how much pressure is necessary to deliver it.
- Time cycled-breaths are given at fixed intervals regardless of the patient's spontaneous efforts.
Most veterinary anesthetic ventilators use a combination of these delivery systems. The most common anesthetic ventilator is the pressure controlled and time cycled ventilator3, like the one we use here at DoveLewis.
Prior to placing a patient on the ventilator, make sure you leak test not only the anesthetic machine but the ventilator as well. Be aware of normal values for your patient and how to set parameters on your machine.
Tidal Volume (TV) - amount of gas delivered during one inspiratory phase.
TV = 10 - 15 ml/kg body weight 2
Example: a 5 kg cat would take a volume of 50-75 ml of air per inspiration (tide)
Respiratory Rate (RR) - number of breaths the patient receives in one minute.
RR = 8 - 12 breaths per minute in an anesthetized patient
Minute Volume (MV) - amount of gas the patient inhales during one minute
MV = TV x RR
Normal minute volume should be 150-250 ml/kg/min
Pressure - refers to the amount of pressure that the delivered gas puts on the airway.
Normal pressure = 15 - 20 cm H2O
May need to be increased to 30 cm H2O in the case of the thoracotomy1 where the negative thoracic pressure has been removed or in cases where abdominal contents are putting pressure on the diaphragm.
Gaining control of ventilation
You have set your parameters, attached your patient to the ventilator, and turned it on. It will now take some time for the patient to stop spontaneous breathing and allow the ventilator to take over. A rate of 12-16 breaths per minute may be needed for 3-5 minutes before the patient's own efforts disappear.1
Monitoring the mechanically ventilated patient
Once you have your patient on the ventilator set at the parameters you estimated, it is time to monitor ventilation and fine-tune your settings.
The most commonly used anesthetic monitoring device may arguably be the pulse oximeter. This great device is easy to use and measures the oxygen saturation of hemoglobin in the blood and reports a pulse rate. It does not measure arterial carbon dioxide (PaCO2) and therefore does not measure the ventilation status of the patient. (See following examples.)
Capnography is considered to be the gold standard in anesthetic monitoring. A capnometer samples the gas at the endotracheal tube and tells you the amount of carbon dioxide present in the patient's exhaled breath. This value is known as the end tidal carbon dioxide or ETCO2. A capnograph will display the amount of CO2 present at all times in graph form so that you can see the rise and fall of CO2 as the patient inhales and exhales. Abnormalities in this graph will alert the anesthetist to problems that may arise such as the patient taking spontaneous breaths or an obstructed endotracheal tube. The normal upper end value of end tidal carbon dioxide (ETCO2) in the anesthetized patient is 60 mm Hg as long as the patient does not have any intracranial pathology.2 A limit of 25 mm Hg on the low end should be addressed2 (usually due to the patient blowing off too much CO2 due to an excessive RR or TV) as a low ETCO2 can lead to decreased delivery of blood flow and oxygen to the brain.2 Capnography is a good monitor of ventilation status, but is not always a good substitute for blood gas evaluation. End tidal carbon dioxide (ETCO2) is not equal to arterial carbon dioxide (PaCO2), and a capnometer can underestimate this value in cases of excessive dead space and during a thoracotomy.2
Blood gas analysis is beyond the scope of this article, but we will briefly discuss the normal values to get an understanding of why these different monitoring forms are not equal.
Example 1.2 Normal values of an arterial blood sample of patient breathing room air
|PaCO2 (arterial carbon dioxide)||35-45 mm Hg|
|PaO2 (arterial oxygen)||80-110 mm Hg|
|PaO2||4-5 x percent of inspired oxygen|
Example 2.2 Normal values of an arterial blood sample of patient on 100% oxygen
|PaCO2 (arterial carbon dioxide)||35-45 mm Hg|
|PaO2 (arterial oxygen)||500 mm Hg|
In the case of a patient receiving 100% oxygen, the arterial oxygen level would have to drop significantly in order for the pulse oximeter to demonstrate a concerning value. This makes the pulse oximeter a poor monitor of ventilation status.
Example 3.2 Hypoventilated patient under anesthesia on 100% oxygen (Normal values in parentheses)
|PaCO2 (arterial carbon dioxide)||114 (35-45 mm Hg)|
|PaO2 (arterial oxygen)||454 (500 mm Hg)|
In this case the pulse oximeter would probably be giving you a great reading while your patient was likely in respiratory acidosis. If this patient had excessive dead space to overcome, your capnometer which measures ETCO2 may underestimate the severely high PaCO2.
Giving control back to the patient
When it is time for the patient to come off of the ventilator, most patients will make the transition back to spontaneous respirations with little to no trouble at all.3 Most of the time I am able to simply turn the ventilator off, put the reservoir bag back on, move the scavenger hose back to the pop-off valve, and the patient is already breathing on its own. If not, I wait twenty to thirty seconds and give them a manual breath. Chemoreceptors in the brain tell the patient to breathe when carbon dioxide begins to build up.3 Patients that are older, debilitated, on neuromuscular blockers, on high levels of opiates, or deeply anesthetized might need a little more help to regain control than simply turning them off of the ventilator. Weaning a patient off of the ventilator involves gradually reducing the respiration rate to about five breaths per minute while monitoring the patient for spontaneous breaths.1 This allows for time to let carbon dioxide build up enough to trigger the chemoreceptors to tell the patient to breathe. When this happens the patient can be taken off of the ventilator but may still require assisted breaths where the anesthetist squeezes the reservoir bag during inhalation.1 This process may take five minutes, but eventually the patient will be able to regain a normal respiratory rate and tidal volume.