Thread by @WApplefeld

@CPSolvers @thecurbsiders @cardionerds @justinberk @MikeRoseMDMPH @JudithBVick @BBroderickMD @TijanaTuhy @AlexHorneMD @SSRaj017 @reverendofdoubt By request- Introduction to mechanical ventilation with @sam_brusca and @wapplefeld part 2: Ventilator Modes.

#KingofVentilators 1/

In the last tweetorial, we discussed the Equation of Motion: the equation that underpins the physiology and physics of mechanical ventilation. 2/

It states that in a relaxed pt, the pressure in the airway is equal to the resistance of the airway times the gas flow through that airway + pressure in the alveolus. The alveolar pressure is determined by the respiratory system compliance, volume in alveoli, & PEEP. 3/

Written simply

Paw = F * R + V/C + PEEP.

4/

Today we are going to talk about modes of ventilation and how each mode manipulates a different variable in the equation of motion to help you achieve your goals of oxygenation and ventilation. Specific goals will be covered in a later session. 5/

As we begin our discussion, we must first outline some important ventilator terms. See below for definitions of a few frequently used ventilator buzz words. 6/

The 1st mode of (modern) ventilation is called CMV or “controlled mechanical ventilation.” In this mode, the physician sets a tidal volume, FiO2, respiratory rate, and peep. The machine will deliver a set volume at the prescribed rate, regardless of what the patient is doing. 7/

This mode isn’t often used in the ICU, as it can be quite uncomfortable for patients. If the pt tries to exhale while the machine is set to deliver a breath, the machine will continue delivering the breath anyway, resulting in significant dyssynchrony. 8/

Similarly, if the pt attempts to breathe faster than set rate, they won’t get additional breaths. Instead, they are forced to inhale against closed valves. This is uncomfortable and can induce lung damage with large negative pleural pressures (high transpulmonary pressures) 9/

Though RARELY used, CMV is worth knowing about b/c it is sometimes used in prehospital transport setting. Additionally, CMV could become important in a disaster/ pandemic surge setting, where low-tech vents are utilized (like this single use, portable vent displayed below) 10/

To improve patient-ventilator synchrony, we need to account for patient-initiated breaths. How do we do this? We have 2 options: 11/

1) We can treat them as excess spontaneous breaths beyond the mandatory breaths that we have already set (SIMV) vs 2) We can give each patient attempt (aka trigger), the ventilator’s full support (Assist Control). 12/

If the pt’s respiratory drive is slower than the set respiratory rate (RR), then both of these modes are equivalent. Today we will focus on Assist control (AC), which is the most commonly used ventilator mode. 13/

In AC, baseline respiratory rate is set. If pt makes no effort to breathe, they will get this # of breaths in 1min. BUT, if pt does attempt to trigger breaths, vent will deliver the same set tidal volume (ACVC) or pressure (ACPC) as is delivered during a non-triggered breath. 14/

In other words, all breaths - be they Pt or machine triggered - are treated the same: full support. Let’s look at two examples of AC. 15/

Now we have to decide how that support is delivered. If you prescribe a pressure, you are in assist control pressure control (ACPC). If you prescribe a volume, you are in assist control volume control (ACVC) 16/

Depending on if you prescribe ACVC or ACPC, different variables will be set on the ventilator vs monitored for airway mechanics and patient synchrony. We can again look to our equation of motion to illustrate this. 17/

In Pressure control, you set the Paw and an I-time. The I-time is the time over which the inspiratory pressure is applied 18/

The pressure gradient between the Paw and Palv drives flow. At the start of a ACPC breath, flow is the highest because the pressure gradient is highest. 19/

As the breath continues and Palv gets closer to Paw, the gradient decrease and flow decreases. 20/

When the alveolar pressure equalizes to the airway pressure, no more flow is delivered. So in ACPC, if during the I-time flow drops to zero, you know that Palv is AT LEAST = Paw. 21/

Importantly, as flow is not set by the ventilator, patient effort can augment flow, and thus the patient can entrain more volume in the alveoli than is dictated by alveolar compliance and the Paw alone. It is a misnomer, that a plateau pressure cannot be > Paw in ACPC. 22/

How do we monitor respiratory mechanics during ACPC? Recall that Flow over a period of Time is equal to a volume. More flow over that time means more volume delivered to the patient. If resistance increases, less flow will make it from the airway to alveoli and TV will fall. 23/

Recall also that Palv is equal to Volume/Compliance + PEEP. If Compliance drops, the gradient between Paw and Palv will be smaller and less flow will be delivered. Tidal volume will again fall. 24/

Think of increased resistance like a milkshake with @Oreo chunk gumming up the straw! You get less of the volume for the same pressure change. Alternatively, decreased compliance is like trying to inflate a tire – you need a high pressure gradient to deliver the same volume. 25/

This is the big problem with ACPC: changes in resistance or compliance will influence how much tidal volume is delivered. 26/

How do you increase TV? 1) Increase gradient between Paw & Palv by increasing the pressure above PEEP vs 2) you can lengthen I-time provided Paw hasn’t yet equalized to Palv (identified as inspiratory flow dropping to zero). Below, lengthening the I-time would be effective. 27/

Now lets transition to assist control, volume control. In ACVC you set the flow and the volume. The resistance and compliance of the airway determine how much pressure will result from the delivery of that volume at that flow.
28/

Recall that flow is Volume delivered over a set time. In ACVC, there are two ways of delivering flow 1) a constant flow (sometimes called “square wave flow”) and 2) a flow that is high at the start of the breath but decreases over the I-time (“decelerating ramp”). 29/

Here's Square Wave flow: the flow starts at a prescribed level & continues at that rate until end of the breath. If the TV=500cc and the flow rate=30L/min square wave, then 0.5L will be delivered at 0.5L/sec so the inspiratory phase will last 1 second. TV = F*time 30/

In Decelerating ramp, the flow starts high at the start of the breath and decreases as the inspiratory phase continues. This is done because it is thought to be more comfortable for the patient. 31/

Here you set the tidal volume and the peak inspiratory flow rate (Vmax). Because flows taper off with time, you need higher initial flows or a longer time to get the same amount of volume compared to square wave flow. Here TV = 0.5*F*time 32/

Below, we have a tidal volume of 500cc, and a peak flow rate of 60L/m (1L/s). Because this is decelerating ramp, we aren’t delivering flow at 1L/s during the whole breath, just at the start before flow tapers off. So to get 0.5L in, we will still need 1second. 33/

Of note, ACVC decelerating ramp has the same breath architecture as ACPC. You can do the same thing on both. So what mode you choose will often be guided by institutional preference. 34/

In ACVC, both with square wave and decelerating ramp, the flow entering the alveoli works to increase Palv and Pairway because Palv contributes to Paw. 35/

Palv is determined by the compliance of the respiratory system (how tight the balloon is), volume, and PEEP. Paw is determined by Palv as well as the resistance and how fast the flow is. This is reflected in the waveforms, but we will discuss this at a later time. 36/

Finally, lets briefly discuss spontaneous modes: Pressure Support, CPAP, NAVA, PAV, etc. Here, I-time & delivered TV are variable & dependent on pt. The patient controls how long they inspire & how much flow to take in. We’ll address them in more detail when we get to weaning 37/

They are designed to “support” patients as they prepare for ventilator liberation. The most common spontaneous mode is Pressure support (PS). In PS, when a patient triggers a breath, the ventilator will deliver a set pressure above PEEP. 38/

The vent will sense when patient’s inspiratory flow falls by certain % at which point the ventilator will no longer deliver that amount of pressure. Thus, the negative pleural pressure generated by the patient as well as the length of their sustained inspiration determine TV 39/

Takeaways:

AC modes deliver full support to both time-triggered & patient triggered breaths. Full support can be prescribed based on a set pressure & I-time (ACPC) or a set flow rate & tidal volume (ACVC)... 40/

With both ACVC and ACPC, the dependent variables in the equation of motion must be monitored for changing lung mechanics and patient synchrony. 41/

In paralyzed patient with constant mechanics, you can switch between ACPC & ACVC with no effect on minute ventilation. Tidal volume, respiratory rate, & plateau pressure will be identical. Choosing between these modes is based on institutional preference & provider comfort 41/

We also want to direct everyone to an incredible resource put out by @megan_acho and Burton Lee – true masters of the vent. This video on @ATSScholar covers ventilators for non-intensivists atsjournals.org/doi/abs/10.341… and we can’t recommend it highly enough!! 42/

Next time: PEEP! with @WApplefeld and @Sam_Brusca (we promise)