At its core, the ventilator is a simple device: a mechanical bellows. There are three questions to ask when dialing in the settings: target, trigger, and termination. Well, maybe more than three questions, but those will get you started.
Rob Orman MD and Haney Mallemat MD
PEARLS
SIMV is a good option in patients with overdose who are expected to wake up.
Patients with angioedema maintain their ventilatory drive and pressure support is a good option.
You can dial up the PEEP and FiO2 in synchrony if you are having trouble oxygenating the patient.
CASE 1
A patient has a polypharmacy overdose and is hypoventilating. The patient needs intubation for airway protection. How should you set the ventilator?
You expect this patient to wake up at some point. You could consider SIMV. This will guarantee them a certain amount of breaths and a certain tidal volume. You can apply a little pressure support as they are waking up. You can reassess and gradually decrease the respiratory rate on the ventilator to transition them from full support to patient controlled ventilation.
CASE 2
The patient has angioedema. The intubation is for airway protection. They will be breathing on their own but need something in between their airway and their lungs to prevent closing of the airway.
This is a situation for pressure support ventilation. They are able to determine their respiratory rate on their own. Dial the pressure support up or down for comfort and the best tidal volumes.
What about in the post-intubation period? The patient will be paralyzed initially. You won’t put them on pressure support immediately. You can start them on SIMV or assist control. Have the nurses notify you when the patient starts waking up so you can dial down the settings to pressure support.
Pressure support is a pressure mode of ventilation but flow is determined by the patient. The patient determines when the breath is initiated or terminated. The ventilation will terminate when that person drops their respiratory drive by a certain percentage.
CASE 3
A common reason for intubation is septic pneumonia. The patient has metabolic and airway protection problems. What is your setting?
Use assist control in this situation. You want to take away their work of breathing. The patient is using a tremendous amount of energy expenditure to keep their respiratory rate up and you are taking that away to improve cardiac output to the rest of their body. The patient will be sedated and placed on assist control. Assist control can be set to tidal volume or pressure.
Tidal volume should be 6-8cc/kg of ideal body weight. Flow rate should be started at 60L per minute. If the patient is comfortable, you can dial up as needed. Initial respiratory rate should be 16-18. If you are intubating in the setting of acidosis, you do not want to set the rate to 10 or 12. Watch the patient to see what they are doing before intubation. If they are really working to breathe and tachypneic, they may need this respiratory rate to maintain their acid-base status. Dropping the respiratory rate risks worsening acidosis. This is why you don’t want to intubate the DKA or aspirin overdose patient.
Get a blood gas in 20 minutes and adjust as needed.
How much PEEP should you give? PEEP is a good thing and helps open up the alveoli. This is especially useful in patients with pneumonia or shunting of blood due to collapse of lung tissue. There are only two things you can do for type I respiratory failure; increase the FiO2 or increase the PEEP. If you have patient with a lot of alveolar collapse, start with a higher PEEP such as 7-8 or 10. If you have an obese or pregnant patient, also start with a higher PEEP. You can use PEEP tables if you are having trouble oxygenating the patient. You can dial up the PEEP and FiO2 in synchrony to oxygenate the patient.
Can you damage the lungs with too much PEEP? You have to be careful as you are titrating PEEP. Too much intrathoracic pressure will decrease the great vessels, decrease venous return and can lead to hypotension. Too much pressure will cause injury to the lungs due to overdistention. Use a PEEP table and work with your respiratory therapist. These can spare you from having too much FiO2 which can cause harm to patient.
If you have a patient who is not oxygenating well immediately post-intubation and you have them on a higher level of oxygen therapy, you need to consider going higher on your PEEP. You can increase the PEEP by 2-3 cm at a time and watch what happens to the oxygenation. You can consider doing a recruitment maneuver with the assistance of your respiratory therapist. This helps pop open the alveoli by using high levels of PEEP.
How can you tell if the level of PEEP is causing harm? If the blood pressure starts to drop, it is a bad thing. Increasing pressures in the lung are also bad.
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ARDS Network PEEP Table Oxygenation goal: PaO2 55-80 mmHg or SpO2 88-95% Use a minimum of PEEP of 5cm H2O. Consider use of incremental FiO2 combinations such as below to achieve goal. |
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|
Lower PEEP/higher FiO2 |
||||||||||||||
|
FiO2 |
0.3 |
0.4 |
0.4 |
0.5 |
0.5 |
0.6 |
0.7 |
0.7 |
0.7 |
0.8 |
0.9 |
0.9 |
0.9 |
1.0 |
|
PEEP |
5 |
5 |
8 |
8 |
10 |
10 |
10 |
12 |
14 |
14 |
14 |
16 |
18 |
18-24 |
|
Higher PEEP/lower FiO2 |
||||||||||||||
|
FiO2 |
0.3 |
0.3 |
0.3 |
0.3 |
0.3 |
0.4 |
0.4 |
0.5 |
0.5 |
0.5-0.8 |
0.8 |
0.9 |
1.0 |
1.0 |
|
PEEP |
5 |
8 |
10 |
12 |
14 |
14 |
16 |
16 |
18 |
20 |
22 |
22 |
22 |
24 |
|
Adapted from NIH NHLBI ARDS Clinical Network Mechanical Ventilation Protocol Summary |
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When should you obtain the first ABG? This is an area of debate. Mallemat recommends 15-20 minutes after intubation. Don’t forget that you have a pretty good saturation monitor that reflects the oxygenation level. In most cases of intubation, the oxygenation will be a 100% and the ventilator will be set to 100%.
When you have blood gas results, you can look at the PaCO2 and compare to end-tidal CO2 if available. You will see a gap or gradient between the end-tidal CO2 and PaCO2. If you are cognizant of the gap and the hemodynamics of the patient are unchanged, you can follow the end-tidal CO2. For example, if the PaCO2 is 45 and the end-tidal CO2 is 40, the gap is 5 and you can make changes to the ventilator and follow the end-tidal CO2.
Do you need to obtain repeat blood gases or can you follow the pulse oximetry and end-tidal CO2? You can follow the pulse oximetry. If you are resuscitating a patient, you may change the cardiac output which will affect the end-tidal CO2. Mallemat will repeat an arterial blood gas during the acute resuscitation phase to make sure he is on the right track. If the patient is stable, you can abandon blood gases unless something changes.
Oxygen; less is more. It is ok to be at 100% when you are first intubating the patient. You need to titrate down the oxygen as quickly as you can. The goal is less than 60% of FiO2. Mallemat targets oxygen saturations anywhere between 88 and 94%. Extrapolation of cardiac arrest literature shows that too much oxygen is harmful. A PEEP table can help you titrate.
