This is Part 2 of a 5 part series on fundamental concepts in ventilator management. See Part 1 is here.

This section has a lot of equations in it, but don’t get wrapped up trying to commit them to memory. I’ve written them out purely to demonstrate how different variables relate to one another. Focus on the concepts and use the equations as an aid to visualizing the relationships.

Lung compliance ($C$) is defined as the tidal volume ($V_t$) divided by the pressure ($\Delta P$) required to deliver that tidal volume:

$C=\frac{V_t}{\Delta P}$

This equation can be rearranged to create two new equations that define how much gas is delivered.  For pressure control ventilation:

$C \times \Delta P=V_t$

For volume control ventilation:

$\frac{V_t}{C}=\Delta P$

For these equations, lung compliance is essentially constant. This means that driving pressure and tidal volume are chained together. In other words, you can deliver the correct volume of gas no matter which mode you choose.

You must also understand that these equations are only valid under static conditions, meaning that flow must be zero. Hospital vents (and a limited number of transport vents) have the capability to measure compliance under static conditions. This is done by performing an “inspiratory hold maneuver.” During an inspiratory hold maneuver, the vent delivers a fixed volume of gas then terminates the flow without allowing the patient to exhale. This ensures that the pressure at the vent is the same as the pressure in the alveoli.  This allows you to calculate static compliance ($C_{stat}$), which directly measures the stiffness of the alveoli.

$C_{stat}=\frac{V_t}{P_{plat}-P_{EEP}}$

*Positive end-expiratory pressure ($P_{EEP}$), plateau pressure ($P_{plat}$)

Note that $P_{plat}-P_{EEP}$ is the driving pressure that the alveoli see. For the most part, however, we can’t measure $P_{plat}$ in the prehospital setting. However, we can measure dynamic compliance ($C_{dyn}$). This is calculated the exact same way as $C_{stat}$, with the exception that gas is still flowing during the measurement:

$C_{dyn}=\frac{V_t}{P_{IP}-P_{EEP}}$

*Peak inspiratory pressure ($P_{IP}$)

$C_{dyn}$is a measure of the effort required to push air into the pulmonary system. There are two main factors that affect how easy or difficult it is to push air into the lungs: $C_{stat}$  (how stiff the alveoli are), and airway resistance ($R_{AW}$).

$R_{AW}=\frac{P_{AW}-P_{alv}}{flow}$

*Alveolar pressure ($P_{alv}$), air flow ($flow$)

This equation is not particularly useful to us, until we rearrange it:

$P_{AW}-flow\ \times \ R_{AW}=P_{alv}$

Hopefully you can see now why an inspiratory hold maneuver allows us to directly measure alveolar pressure: when flow is zero, the $flow\ \times {\ R}_{AW}$ term also becomes zero.  If we rearrange the equation one more time:

$flow\ \times \ R_{AW}=P_{AW}-P_{alv}$

It becomes apparent that for any given pressure differential ( $P_{AW}-P_{alv}$ in the equation, but it applies to all pressure differentials), flow and resistance are inversely related.  If you raise the resistance, flow decreases and vice versa.  Note that if $P_{alv}$ is higher than $P_{AW}$ it implies that $flow\ \times \ R_{AW}$ is negative.  There is no such thing as “negative resistance”, which means that flow must be negative.  In other words, the patient is exhaling.

Now that we understand $C_{dyn}$, what can we use it for?  The simple answer is, “not much of anything.”  A low $C_{dyn}$ tells you that the patient is difficult to ventilate, but generally you can figure this out by looking at the patient.  The problem with $C_{dyn}$ is that it doesn’t tell you if the patient is difficult to ventilate because of increased airway resistance or if they are difficult to ventilate because their alveoli are stiff.  Figuring out which (or both) the patient has is essential to setting the vent correctly.

There is a way around this, however.  We can identify the presence of clinically significant $R_{AW}$ through looking for “shark fins” on capnography.  The rising phase of the capnogram is a decent indicator of $R_{AW}$ because it measures how long it takes gas to move from the deepest portions of the lung out to the capnography sensor.  If it takes an extended time for air to move from deep in the lung to the sensor, this suggests that expiratory flow is restricted, and that $R_{AW}$ has risen to the point that it now alters our clinical decision making.  If the rise time of the capnogram is normal, this suggests that $R_{AW}$ is clinically insignificant, and therefore that $C_{dyn}$ is probably a good reflection of $C_{stat}$.

Keep in mind that absence of a shark fin is not proof of normal $R_{AW}$; shark fins are indicative of decreased expiratory flow.  If you refer to the resistance equation above, normal expiratory flow in the presence of elevated $R_{AW}$ can occur if there is a dramatic rise in the $P_{AW}$ to $P_{alv}$ pressure differential.  The two occurring together would be very unusual, and the methods to identify this rare occurrence are beyond the scope of this tutorial.  Just be aware that a normal capnogram is not a definitive rule out for increased $R_{AW}$.

At this point, you should have a solid understanding of the actual mechanics that happen during the delivery of a breath.  In the next section, we’ll discuss picking appropriate settings for our patients and why those settings are appropriate.