The Dreaded Cold Read
Does anyone ever send you a cold read? A cold read is when you get a piece of information, but really no context. An example of this would be the ECG or lab panel that you get sent you with the predictable question: "What would you do?" This is a difficult question to answer, because the person is generally asking you to make a treatment decision based on only one part of the story (whatever they sent you). This really isn't the way we make informed treatment decisions in real life situations - we take the whole patient into account. When we get an ECG, we like to know if the patient has chest pain, a cardiac history, if they're diaphoretic, if there is pulmonary edema, and so on to infinity. How does this apply to the ventilator?
Whenever discussing ventilator strategies, I feel like the discussion is a bit of a cold read whenever the waveforms are not taken into account. The waveforms tell you what the flow, volume, and pressure are doing over time - and they're extremely helpful in managing a patient. I realize that some of you do not have waveforms on your ventilators. If you don't have waveforms on your ventilator, when you do retrievals from outlying hospitals, examining the waveforms on their ventilator will help you set up a care plan for the transport. If you do have waveforms on your ventilator, you've very fortunate - because they can help you set up a precise strategy to fine-tune your settings.
Once you see a picture like this, which is a screenshot of a ventilator someone sent to me, things start to make a lot of sense. Waveforms will often give you the exact reason a vent is acting a certain way. We will return to this example in a later post.
In this blog we are going to look at the basics of waveforms. We are not going to worry about the actual values just yet. Instead, we are going to look at the direction of the waveforms of a negative pressure breath, and a positive pressure breath.
Paw = Airway Pressure. This is a live look at the pressures inside the airway.
Flow = The speed of the the air traveling in and out of the lungs (measured in L/min).
Volume = The volume (either liters or millimeters) traveling in and out of the lungs)
Negative Pressure Breathing
Green = Inhalation
Red = Exhalation
Paw: Notice how the pressure drops during inhalation. This is caused by the chest expanding, which in turn creates a negative space in the chest which allows air to flow in. As the Paw reaches the most negative inflection point, this is the peak negative airway pressure. The pressure returns to baseline when the alveolar pressure reaches atmospheric pressure again (when pressures are equalized, the INflow stops). The pressure then rises as the patient exhales. This pressure compresses the volume in the chest, and as long as the airway is open, the volume will then leave the thoracic cavity. The pressure returns to baseline when the alveolar pressure reaches atmospheric pressure again (when pressures are equalized, the OUTflow stops).
Flow: As the patient creates the negative pressure and space in their chest, air starts to flow in. When the flow is at the fastest point (farthest away from baseline) this is the maximum inspiratory flow rate. The same is true when the patient begins to increase the pressure in the chest and compress the volume in the thoracic cavity - the flow reverses air starts to leave the chest. You will notice that the flow starts out very fast and then gets slower. This is because it is very easy for the lungs to empty the larger airways (the first to be cleared) and then it takes a little more time for the smaller airways to clear out their volume, since the resistance is higher flowing through the smaller, more distal airways (all of this assuming healthy lungs). Of all of the waveforms to watch, my opinion is that the flow waveform is usually the most helpful when troubleshooting / changing settings.
Volume: You'll notice the volume waveform is a little different than our other two waveforms. This waveform usually only exists above the line. The volume waveform will rise when volume enters the lungs, and then return to baseline as the volume leaves the lungs. Some ventilators will have the ability to change the display color of the inhaled vs. exhaled volume as I have illustrated in the picture above.
Positive Pressure Breathing
Green = Inhalation
Red = Exhalation
Paw: The biggest change from a spontaneous, non-ventilated patient, to a ventilator delivered breath, is the change in airway pressures during inhalation and exhalation. We start at a baseline pressure, and then the pressure increases all the way up to the peak inspiratory pressure (PIP / Ppeak). After the ventilator reaches its peak inspiratory pressure, exhalation then begins as pressure lowers all the way back down to the baseline. Notice that the baseline has moved down, since we spend almost all our time in the positive pressure range.
Flow: In this illustration I have not changed the flow pattern, we will address that in subsequent blogs. Different types of breaths will appear differently on the inspiratory flow waveform (square, decelerating, accelerating, and the once pictured is sine). How you set your inspiratory time will also change how quickly the flow goes in. The appearance of the exploratory waveform cannot be directly changed with ventilation, as it is a passive function of the diaphragm and chest. Well... at least that's mostly true. There are things that impact the way the exhalation waveform look, but they are mostly things like patient position, PEEP, expiratory time, and certain disease processes.
Volume: The shape of the volume waveform will reflect the the time of inhalation and exhalation, and how much pressure is exerted during these phases. The volume should also reflect the prescribed dose (volume mode) or how much volume a pressure is delivering (pressure mode).
Notice a difference? In this illustration we've added PEEP.
Paw: When we add PEEP we can see that the pressure waveform baseline has moved up from neutral.
Flow: Flow in this illustration has remained unchanged.
Volume: The volume that the ventilator is holding has increased from baseline. This is because the expiratory reserve volume has increased since the addition of PEEP. Your ventilator may or may not read this volume in-between breaths.
When we change settings on a ventilator, we are constantly re-writing the waveform - and so is the patient when they respond to the changes we make. You can think of the waveform as a constantly evolving storyline shared by you and the patient. As we continue in this series, we'll get into more and more specific examples of changes made on each side.