As you head into the critical care environment, you’ll be learning a lot about ventilators and taking care of patients with artificial airways. Before we get started talking about ventilator basics, let’s review a few core concepts.
- Oxygenation: The process of adding oxygen into the body. Think of this as the chemistry component.
- Ventilation: The process of inhaling and exhaling. Think of this as the physiology component.
- Hypoxemia: A low oxygen level in arterial blood; normal is PaO2 80-100 mmHg.
- Muscles of respiration: the diaphragm and intercostal muscles are the main muscles of the respiratory system, especially when the patient is at rest and under no duress. When the patient is working hard to breath, then you may see accessory muscles come into play to aid respiratory function. These are the external intercostal muscles, the scalenes and sternomastoids (during inspiration) and the abdominal muscles and internal intercostal muscles (during expiration).
- Negative pressure ventilation: Under normal circumstances, humans breathe utilizing negative pressure ventilation. When the diaphragm drops and the intercostal muscles pull the ribs outward, pleural pressure decreases below atmospheric pressure. This negative pressure draws air into the lungs. As the diaphragm moves up, pleural pressure increases which pushes air out of the lungs. The air comes IN during that period of negative pressure.
- Positive pressure ventilation: When a patient is on a ventilator or even BiPAP, they will be breathing via positive pressure instead of negative pressure. Instead of air being drawn into the lungs, air is forced into the lungs through an ET tube or non-invasive device such as BiPAP.
A brief look at ventilator anatomy
There are several different types of ventilators, but they all essentially have the same components in common.
- A screen that shows a waveform, respiratory rate and a few other pieces of data such as peak pressures and end tidal carbon dioxide.
- Buttons, dials and knobs for adjusting the ventilator settings. These will be managed by the respiratory therapist and allow for adjustments in things like FiO2 and PEEP (we’ll talk about PEEP in a moment).
- A button to push that gives patients a minute or two of FiO2 at 100%.
- Tubing that connects air and oxygen to the ventilator.
- Tubing that delivers oxygen (but usually a mixture of air and oxygen) to the patient.
- An arm that supports the ventilator tubing.
Ventilator basics: settings totally simplified
Ventilators are complex pieces of equipment with a lot of variables that can be adjusted based on what the patient needs. That’s why respiratory therapists go to school for a couple of years, to learn the intricacies of ventilator and respiratory management. As a nursing student, the three most important settings you need to understand are FiO2, PEEP and rate. Notice I didn’t say they are the only things to understand, just the main ones! If you end up working in ICU, you’ll get deeply immersed in all things mechanical ventilation, trust me!
What is FiO2?
FiO2 is the fraction of inspired oxygen. Essentially, it’s how much oxygen we’re giving to the patient. Recall that atmospheric oxygen is 21%, so your FiO2 will always be above that. The key with FiO2 is that we want to give the minimum amount the patient needs to maintain an adequate oxygen level in the blood. High oxygen levels cause oxygen toxicity which can lead to central nervous system complications, fibrous lung tissue, and retinopathy (especially in preterm infants).
What is PEEP?
PEEP stands for positive end-expiratory pressure. In other words, it’s the amount of pressure in the alveoli at the end of expiration. Note that a patient can have physiologic PEEP caused by air trapping that occurs in conditions such as asthma and COPD (also called intrinsic PEEP). Today we’re talking about artificially raising the PEEP in order to improve gas exchange (you may hear this referred to as extrinsic PEEP). With airway pressures raised above atmospheric pressure, the alveoli essentially stay open longer, which gives us more surface area for gas exchange.
PEEP also helps us with something called “recruitment.” Remember the pores of Kohn from your A&P class? Those were the little “doorways” that exist between alveoli. Having higher pressures at end expiration enables us to essentially open those doorways and access neighboring alveoli. In other words, we are “recruiting” the neighboring alveoli for gas exchange through the pores of Kohn. Pretty cool, right?
Another great thing about PEEP is that its ability to improve gas exchange enables us to maintain adequate PaO2 at lower FiO2 levels. This, in turn, reduces the risk of developing oxygen toxicity. PEEP levels range from a low of 3-5 cm H2O all the way up to 24 cm H20 in patients with acute respiratory distress syndrome (ARDS). (link to ARDS.net)
Now, before you go thinking that applied PEEP is the answer to all your oxygenation problems, it does come with risks of its own. PEEP, especially higher levels, puts the patient at risk for barotrauma such as pneumothorax, interstitial emphysema and pneumomediastinum. So if your patient is on high PEEP settings and their SpO2 level plummets, you’re going to run in their with a stethoscope to see if you can hear lung sounds on both sides. It’s very possible they experienced a pneumothorax. Call the RT, the MD, get a stat chest x-ray, and anticipate setting up a chest tube!
The respiratory therapist or MD will determine the correct respiratory rate for the patient based off their unique physiologic needs. For example, if the patient is acidotic because they’re retaining their CO2, we can try increasing the respiratory rate to help the patient “blow off” their CO2. Some ventilator modes will allow a patient to initiate breaths, so it’s always possible that a patient could breathe above the rate set by the therapist. If that occurs, and we really need the patient to breathe at the prescribed rate, we usually sedate them in order to achieve this.
Ventilator basics: mechanical ventilation modes
In addition to rate, FiO2 and PEEP, the MD or respiratory therapist will adjust how the ventilator delivers oxygen to the patient. We call this the “ventilator mode.” Modes can be broadly categorized as pressure controlled and volume controlled.
In volume-controlled modes, we are delivering a certain amount of volume to the patient. The most common of these is a mode called Assist Control Ventilation or ACV. In this mode the patient triggers the ventilator for each breath and the ventilator “assists” the patient by delivering a controlled/pre-determined volume to the patient. Volumes are typically calculated at 8ml/kg IBW (ideal body weight). It’s important to note that even though the patient is triggering the breaths, we will set a backup rate on the ventilator so that if the patient doesn’t meet that minimum rate, the ventilator will kick in and deliver a breath.
In these modes, we will inflate the lungs to a certain pressure. One of the advantages is that it may have less risk of barotrauma than volume-controlled modes. One such mode, called Pressure Support Ventilation (PSV) is used a lot during the weaning process and the patient initiates each breath. That extra pressure support helps overcome the difficulty of breathing thorugh an ET tube of tracheostomy tube. The bigger the pressure, the bigger the breath!
CPAP is another mode used in the weaning process, or with patients with a chronic tracheostomy for airway management. The continuous positive airway pressure provided in this mode provides the patient with some pressure support while allowing them to control their respiratory rate and volume. A lot of times, we’ll have a patient who does CPAP all day and then “rests” on a more supportive ventilator mode at night.
High-frequency ventilator modes
Two high-frequency modes you may see are oscillatory and percussive. They both require a special ventilator that can deliver a high number of breaths per minute, (sometimes up to 800) at very low tidal volumes. In the adult population, you’ll see these modes used most typically in patients with ARDS or ventilator-induced lung injury. One thing to understand is that this breathing pattern is highly abnormal and patients typically have to be paralyzed with deep sedation in order to tolerate it.
Ventilator basics: two alarms you must know
You’ll soon learn that ventilators (and everything else in the critical care setting) come with a lot of sound effects. There are two alarms I want you to be hypervigilent about…the high-pressure alarm and the low-pressure alarm.
The high-pressure alarm will sound when pressures within the circuit are too high. This is often due to the patient coughing (which increases intrathoracic pressure), but it can also be due to an occlusion in the ETT. When your high-pressure alarm goes off, it may be that the patient needs suctioning. Don’t forget to preoxygenate and limit each suction pass to less than 10 seconds! Also, be aware that we don’t want to over-suction as it can cause trauma to the airways and limit suctioning only to times when it is actually needed (versus on a schedule).
The low-pressure alarm typically sounds because the tubing has become disconnected somewhere (which has caused a dramatic loss of pressure in the circuit). This is usually at the connection between the ETT and the vent tubing, but it can be at the ventilator itself. With tracheostomies it is almost always at the connection to the patient, so start your visual assessment there and then move toward the machine. Patients with tracheostomies “pop off the vent” waaaay more often than patients with ETT, mainly because they’re awake and moving around!
Another reason you could have a sudden low-pressure alarm is that your patient just self-extubated. I remember when I was a new-ish nurse, hearing that alarm and rushing in to my patient’s room to see her waving the ET tube in her hand. I got help in there STAT, we got her reintubated and everything was fine. It was scary as heck, though!
When your patient is on a ventilator, I want you to always think about four things: the mode of ventilation, how much FiO2 they’re receiving, their PEEP setting, and their respiratory rate. As you get more acquainted with critically ill patients, you’ll learn that this information can tell you a lot about your patient as well as was the ventilator can be adjusted to compensate for imbalances.
You’ll also see how the ventilator changes are made as you begin the process of liberating your patient from the vent. The FiO2 comes down, the PEEP comes down, and the RT will try them on a mode that allows the patient to control their respiratory effort (read more about ventilator weaning here). You’ll see how sedation affects their respiratory rates, how coughing affects the high pressure alarm, and how different modes can increase ventilator synchrony and patient comfort. And please don’t forget that the respiratory therapist is a WEALTH of wonderful information, so don’t be shy. I’ve had the pleasure of working with incredible therapists who love to teach and impress me every day with their in-depth knowledge of airway and ventilator management.
Get this on audio in Episode 83!
I hope this brief introduction has helped you understand ventilator basics and made the critical care environment a little bit less intimidating. Have a specific question or want to share your experience? Let us know in the comments below!
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