“It’s Time to Vent”
Written By Rozalyn Hesse, MD; Edited by Timothy Khowong, MD
Introduction:
Emergency medicine places a heavy emphasis on mastery of any difficult airway, however what happens after your respiratory failure patient is successfully intubated? Ventilator management is also a crucial aspect of ER care; understanding how to optimize mechanical ventilation has been shown to decrease ED mortality, duration of ventilation, and hospital stay. Unfortunately, it’s not uncommon for ER physicians to be flummoxed by an alarming vent. Let’s discuss ventilator basics and some fundamental strategies to approach a crashing patient on the ventilator.
Ventilator Breaths:
Let’s go over the three types of breathes that the ventilator can deliver to a patient—these are called controlled, assisted, or supported breathes.
Controlled breaths means that the patient is doing nada in terms of breathing, these breaths are completely controlled by the machine from start to finish. The patient is not even signaling to the ventilator that they want a breath (for example, due to deep sedation or brain death). If a respiratory rate of 12 is set, that means at second 5, the machine will initiate a breath and will completely control the breath from beginning of inhalation to end of exhalation.
Assisted breaths means that the patient is at least triggering a breath. They are telling the ventilator, “Hey, I do want to breathe… so could you do that for me?”. The sudden change of negative pressure will be sensed by the ventilator, and it will trigger the machine to deliver a ventilator controlled breath.
Supported breaths are breaths that just need a little help. Not only is the patient triggering the breath, the patient is even providing some of the work with breathing. The ventilator will simply assist with some preprogrammed amount of support.
A ventilator will completely breathe for a patient that has no interest in ventilating, like a human drags this cat that has zero interest in walking
Aww, this cat is telling her human that she wants up, however she cannot do any work. Similar to a patient that has the drive to breathe but cannot actually ventilate
Almost there! This cat wants to jump and even made a noble leap. Unfortunately, it’s not quite enough. He probably just needed a little support.
Important Ventilator Variables:
Now let’s talk about the different variables involved with mechanical ventilation
Respiratory Rate: This refers to the number of breaths that must be given per minute. These breaths may be triggered by the patient or initiated by a ventilator, based on the rate and mode that the ventilator is set. For example, if a respiratory rate is set at 12, the ventilator will trigger a breath at second 5. However, if the patient initiates a breath before that, the ventilator will count that breath.
Positive End-Expiratory Pressure (PEEP): We know that PEEP is essential for increasing alveolar recruitment. It is the pressure at the end of each breath that is necessary to prevent complete alveolar collapse. PEEP is typically set at 5 cm H2O but can be set higher, particularly in atelectasis or fluid filled alveoli.
FiO2: Initially, the fraction of inspired oxygen is set at 100%, BUT DO NOT LEAVE IT AT THAT. Start titrating FIO2 down to below 60% as soon as possible to prevent free radical damage. You should be using the PaO2 on your ABG to tell you the patient’s oxygenation to fix your FiO2.
Tidal Volume (Vt): Tidal volume is the minimum volume delivered with each breath. This is typically between 4 and 8 cc/kg, IBW. IBW is controlled by the patient's height. The formula is as follows, but don’t feel bad if you have to reference it via MDcalc.
Men: 50 + 2.3 (height -60)
Women: 45.5 + 2.3 (height -60)
Peak Inspiratory Pressure (PIP): This refers to maximum pressure needed to deliver a breath during inspiration. This pressure is a result of airway resistance from the endotracheal tube and large airways.
Plateau Pressure: After inhalation and after peak inspiratory pressure has been met, there is a plateau pressure. This is the pressure applied to the small airways and alveoli. The plateau pressure can be easily calculated by performing an end inspiratory pause on the vent.
Inspiratory to Expiratory Ratio (I:E ratio): Denotes proportion of breath cycle that is devoted to inspiratory and expiratory phase. A normal I:E ratio is 1:2 or 1:3, however special consideration should be taken with patients with COPD or asthma. These patients require a longer respiratory phase to fully expire air due to obstructive bronchospasms.
Ventilator Modes
Volume Assist/Control: This mode is designed to provide a preset volume of breath. If the tidal volume is set at 400 cc, the patient will either trigger the ventilator or the ventilator will completely initiate a breath itself based on the set respiratory rate to give the patient 400 cc of air. Although the volume is controlled, the pressure is not. Therefore the patient is at risk of lung injury if volumes are set too high and the patient has poor lung compliance.
Pressure Assist/Control: This mode is designed to provide a breath based on pressure. This means that the breath (either triggered by patient or ventilator) will cycle from inspiration to exhalation phase after a set pressure is reached. Therefore the tidal volume is variable and is dependent on the compliance of the lung. For example a more elastic lung will receive a higher volume from a preset pressure, while a more stiff lung will not obtain the same amount of volume from the same pressure.
Pressure-support ventilation (PSV): This can be seen as a minimal ventilator support mode. Indeed, this mode is often used to see if a patient can actually be extubated. In this mode,a minimum amount of pressure support ( typically 5 cm H20) is set. No minimum respiratory rate is set. The patient is expected to trigger all breaths and to have adequate respiratory strength to breath an acceptable tidal volume. All they should need is a small amount of support to overcome the intrinsic airway resistance of that little ET tube.
Volume Synchronized Mandatory Ventilation (SIMV) + PS: Bring your mind all the way to the beginning of this post where we spoke about the types of ventilator delivered breaths. In this mode, the patient will actually receive all three of those breaths—Controlled, assisted, and pressure supported. For example in this mode, one needs to set the RR, Vt, and PEEP. Say you set the RR at 12 and Vt as 400. If the patient does not trigger a breath at the 5 second mark, the ventilator will deliver a controlled breath. If the patient actually triggers the vent to take a breath at or near the 5 second mark, the ventilator will deliver an assisted breath. Additionally, if the patient wants to breathe in between those 5 seconds then the ventilator can provide a pressure supported breath.
Pressure Regulated Volume Control (PRVC): This mode is similar to volume assist/control, in that there is a preset tidal volume. The patient will either trigger the ventilator to give an assisted breath, or a controlled breath will be given based on respiratory rate. The main difference is that this mode is cognizant of high pressures and will prevent barotrauma by switching out of the inspiration phase to cycle to exhalation if pressure gets too high, even if the preset tidal volume has not been met.
Trouble-shooting the ventilator
Lastly, let's round up this post by discussing an algorithm to troubleshoot a ventilator.
A common mnemonic for the deteriorating vented patient is DOPES
Dislodgment
Obstruction
Pneumothorax
Equipment failure
Stacking
Dislodgment. The ET can become dislodged too shallow or too deep. This can occur with any kind of manipulation of the patient such as during transportation. An under-sedated patient may also move the tube with their tongue. This can be assessed by observing the depth of the tube at the lip-line or by direct visualization with laryngoscopy. If the cuff is observed above the vocal cords the tube can sometimes be pushed deeper.
Obstruction. The ET tube can become obstructed by mucus plugging, kinking, and biting. If the patient is not obviously biting on the tube one can further evaluate by passing a suction catheter through the tube. Inability to pass the catheter is concerning for obstruction. If this cannot be fixed by suctioning or repositioning, the patient may need to be re-intubated with a new tube.
Pneumothorax. Suspect pneumothorax if there are asymmetric breath sounds and a patient is becoming more hypotensive and hypoxic on the vent. This can also be seen XR, however a quick POCUS can also be diagnostic. Try to look for the absence of lung sliding with the linear probe. Don't forget: if you see B lines, there is no pneumothorax. The treatment is needle decompression followed by a de-compressive chest tube.
Equipment failure. This refers to problems such as a leak in the circuitry. Gurgling from the patient’s mouth could indicate that there is a leak around the ET tube cuff. One should also visually observe for leaks all the way from the tube to the machine.
Breath stacking. This refers to a vented patient not fully exhaling air before another breath is initiated. One can evaluate for this by looking at the flow curve on the screen of the vent. Normally the flow curve must always return to 0 or midline. If the flow curve does not return to 0 before inspiration occurs, there is breath stacking. To fix this, disconnect the patient from the vent. You may hear a nice gush of air from the tube. You can even press on the chest with your hands to facilitate full exhalation.
Summary:
It is important for ER docs to understand the core basics of a ventilator. It is not a mystical box that delivers breaths to your intubated patient. By knowing key variables and modes involved with the ventilator, one can optimize oxygenation for their intubated patient and prevent serious harm.
References:
Lodeserto, F. (2020, June 29). Simplifying mechanical ventilation – part I: Types of breaths. REBEL EM - Emergency Medicine Blog. Retrieved September 7, 2022, from https://rebelem.com/simplifying-mechanical-ventilation-part/
Miller, E. (2017, June 9). Taking ownership of the ventilator – how to manage and troubleshoot. emDOCs.net - Emergency Medicine Education. Retrieved September 7, 2022, from http://www.emdocs.net/ventilatormanagement/
Morgenstern, J., & Morgenstern, J. (2019, February 17). Breath stacking FIRST10EM. First10EM. Retrieved September 7, 2022, from https://first10em.com/post-intubation/breath-stacking-first10em/
Scott, W. D. (n.d.). Managing initial mechanical ventilation in the emergency department. Emcrit. Retrieved September 7, 2022, from https://emcrit.org/wp-content/uploads/2010/05/Managing-Initial-Vent-ED.pdf