@CPSolvers @thecurbsiders @cardionerds @justinberk @MikeRoseMDMPH @JudithBVick @BBroderickMD @TijanaTuhy @AlexHorneMD @SSRaj017 @reverendofdoubt Since you guys asked for it - @WApplefeld and @Sam_Brusca present “An Introduction to Mechanical Ventilation: A Tweetorial Series”
In this series, we hope to clearly illustrate our approach to the physics, physiology, and practical application of mechanical ventilation.
Here is chapter 1: A Physics Primer 1/x
Here is chapter 1: A Physics Primer 1/x
Imagine a garden hose. The flow of liquid develops from the high pressure region to the low pressure region, with the nozzle (and potentially your thumb) acting as a resistor. 2x 
You’ve seen this before - in your backyard spraying your siblings… but also in physics when you first learned Ohm’s law: a voltage difference drives current flow across a resistor. 3/x
In medicine and physiology, we apply this equation to a wide variety of situations, including pulmonary vascular resistance, mean arterial pressure, and air movement into and out of the lungs! 4/x
In these cases, Ohms law is adapted so that a pressure difference (ΔP) drives a flow (F) across a resistor (R): ΔP = F*R. 5/x
During a ventilator inspiration, a pressure difference is established between the airway (Paw) and the alveoli (Palv). This pressure difference is either caused by (or generates) a flow (F) through a resistor (R). 6/x
At the start of a breath, when the alveolar pressure is low and the airway pressure generated by the vent is high, flow of air will rush into the alveoli past the resistive elements of the large and small airways. 7/x
Let’s picture a situation where we stop all flow. B/c there’s no flow, there’s no pressure gradient & Paw is equal to Palv. B/c these 2 pressures are equal, the pressure in the airway (read on the vent as the plateau pressure, Pplat) is dictated entirely by alveolar mechanics 8/x
Alveolar pressure during flow cessation (Palv = Pplat) can be thought of like a tied off balloon. What determines the pressure in this balloon? 9/x
3 things determine the pressure in the alveoli: 1) the pressure in the alveoli before inflation began (PEEP), 2) the volume you deliver to the alveoli (tidal volume), and 3) the ease with which the lung inflates (compliance) 10/x
Compliance is sometimes tricky to understand. It’s a change in volume for a given change in pressure. Picture a tire (low compliance) and a Safeway bag (high compliance). 11/x
A tire take a tremendous amount of pressure to inflate. Conversely, a Safeway bag will increase its size with only small changes in pressure. 12/x
Combining the dynamic and static components described above here gives us the “Equation of Motion”, which is one of the two most important concepts in mechanical ventilation 13/x
Equation of Motion- the pressure in the airway is dictated by the flow (F) delivered, intrinsic resistance (R) of the respiratory system, volume (V) in the alveoli, compliance (C) of the respiratory system, and PEEP: Paw=F*R+V/C+PEEP in a paralyzed relaxed pt. 14/x
We will address what happens in a Pt who is making respiratory efforts at a later date. But for now, let’s consider the different parts of this equation. Different modes of ventilation allow us to directly change certain variables and indirectly influence others. 15/x
On the vent, we can choose to set the PEEP and either the Airway pressure and time over which it is delivered (pressure control) OR the volume and flow (volume control). Resistance & compliance are properties of the respiratory system itself and are dictated by the patient. 16/x
Throughout this series, our goal will be to dissect this equation and provide you with a framework for understanding mechanical ventilation that is both physiologically accurate and clinically useful. Please follow along as we release installments. 17/x
