Introduction to the Hamilton T1: Breath types and Modes

So you just got a Hamilton T1 ventilator and you’re like: what the hell do all these things mean? Pramp, RCexp, ETS, I:E, flow trigger, why is it like I’m learning mechanical ventilation all over again?????

If you’ve listened to my podcasts on mechanical ventilation or read my book on mechanical ventilation you know that the book is largely based on the ReVel and Zoll platforms and not on the Hamilton, which has become the standard platform across most of air medical.

Fret not, I’m here to help you with the transition.

When using a Hamilton vent, you shouldn’t change anything that you normally do before using a vent.

Meaning, step zero is: check the ETT size, depth, cuff inflation, and then perform a leak test (what the ReVel calls a circuit test).

From there, you’ll select your patients sex and their weight. This is awesome, because once you select the appropriate weight, the vent will tell you how many mL/kg your tidal volume is once you start ventilating them.

The main difference in this ventilator. The Hamilton is a pressure driven ventilator, unlike most older models of vents. What does this mean? It means that no matter what mode of ventilation you’re in, you are technically delivering a pressure breath. This is more physiologic for patients as you get a ton of flow at the very beginning of your breath and then the flow tapers off as your lungs begin to fill. In other ventilators, like the ReVel, you only get a pressure breath when you’re in a pressure mode. If you’re in volume control, you get a linear breath. As a reminder about how these two breath-types work:

In volume control, we put in a tidal volume of 500mL and an Itime of 1.0 seconds. The vent will calculate that 0.5L / 1 sec. Convert that to a minute by multiplying by 60, and you’re left with a flow of 30LPM for one second. As long as there isn’t significant leak in the circuit, the tidal volume will be 500mL. If you want to replicate this breathing pattern, try to box breath. Inhale slowly over four seconds. Now do that for four days straight. It might get a little irritating.

In pressure control, you simply put in a safe pressure for the lungs to see. Let’s say you have a PEEP of 8cmH2O and decide on a pressure control of 12cmH2O. You’ll still set up an I-time, but you’ll also set another time as well. If you simply blasted the patient with 12cmH2O over their PEEP right at the beginning of their breath, the patient would feel like we’re punching them in the lungs. Therefore, in pressure mode, there are two times that you set. You set the total length of the breath (I-time) and you set the amount of time that it’ll take the vent to go from the PEEP to the Peak pressure.

On the Revel vent, this was called the “rise time” and on the Hamilton it’s called the Pramp, which actually makes more sense. In the Hamilton vent, this is the only type of breath that the vent gives. Let’s keep our same numbers so far. So we have a PEEP of 8cmH2O, a Pcontrol of 12cmH2O, and an I-Time of 1 second. When the breath begins, the pressure control will be gradually added depending on your Pramp. If your Pramp is 100ms, the pressure in the circuit will go from 8 to 20 over the course of 100ms and then the pressure will be held in the circuit for another 900ms before allowing the patient to exhale. The fact that the lungs filled very early in the breath makes compliance better and allows for additional oxygenation during the inspiratory period (when compared to volume control). Regardless of what mode of ventilation you’re in, this is how all breaths are delivered in the Hamilton, which makes it a superior ventilator.

Modes of invasive ventilation:

CMV+ - think of this in the same way you think of AC/V

PCV+ - Think of this in the same way you think of AC/P

SIMV+

PSIMV+ - SIMV Pressure

ASV

If you’ve listened to my podcast or read the book, you know that I suggest starting on assist control volume for almost all adult patients. On the Hamilton this would be CMV+. While it’s not technically assist control volume, it’s the closest mode to the revels ACV.

But wait, if the Hamilton is a pressure driven ventilator, how can you choose a volume mode? That’s because what CMV+ really is, is PRVC (pressure controlled, volume regulated breaths). When you’re setting this mode of ventilation up, you select all the parameters you normally would (respiratory rate, I-Time, tidal volume, PEEP, and FiO2). However, you’ll also set something called the Plimit. This is the ceiling if you will. You’re essentially asking the vent to give a tidal volume of 480mL, but you’re telling it that you’re only willing to have Peak Pressure of let’s say: 35cmH2O. As long as the lung compliance is good and the resistance is normalish, the 480mL tidal volume will go in. However, if the patients lungs are garbage, the volume will be capped once the pressure exceeds your limit (35 in my example). Essentially in this mode you’re targeting a volume, but you’re making sure that the pressures don’t become unsafe in the process.

So when would we need to switch to PCV+? The answer is that we don’t ever really need to make that switch, clinically at least. In CMV+, you can control the tidal volumes that you want. If the pressures are a little high, you’ll adjust your tidal volume accordingly. In PCV+ you physically adjust the pressure control. If your tidals volumes aren’t where you want them (6-8ml/Kg), you change the pressure control up or down to get more or less tidal volume. The only difference between these two modes is which button you’re wanting to control. However, as both of them are actually delivered as a pressure breath, the patient does not feel the difference between the two modes. In the revel we would start patients on volume control and if their compliance and oxygen saturations were garbage we would switch to pressure control. In the Hamilton, there is no benefit to doing this, so you can simply stay in CMV+ as you’re already getting the compliance benefit of having the patient in a pressure mode.

As for the other modes of ventilation: SIMV+ and PSIMV+, there is a trigger warning coming. In my opinion, most of the people that use SIMV as their sole approach to mechanical do so for one of two reasons. The first is that that’s how their preceptor taught them to do it so this is the mode of veniltation that they’re most comfortable with and don’t want to learn other modes. While I understand this, I do vehemently disagree with it.

The second is different. These are the people at your shop who think they’re more clever than the rest of you. “I am more superior in my knowledge so I will choose a more complicated mode of ventilation to show you how smart I am”. Whenever I ask these people why they always choose SIMV, they tell me that the mode of best for the patient, that we can synchronize better with the patient, etc… All of this is completely untrue (as all modes of ventilation can synchronize with the patient), I ask them why they never use CMV or PCV.. They generally just keep repeating themselves and reply while looking down their nose. Even when presented with a body of literature to support no longer using the mode as a standard, you cannot get these people to change.

Remember, in medicine, we cannot be married to anything that we do. As more evidence presents itself, we must be willing to change or we’ll be the grumpy dinosaur that we all know. As early as 2005, we knew that SIMV extended intubation days for children and neonates and increased the rate of bronchopulmonary dysplasia. In adults with sick lungs, patients have longer extubation times, longer weaning, more failed extubations, and more total vented days on SIMV as compare to straight up pressure modes and ASV.

In non lung-injured patients,  in 2020 Godoi and his colleagues showed that SIMV and AC modes have no real difference in duration of ventilation.

So wait, in sick patients, SIMV has a lower mean airway pressure and a more side effects. In not sick patients, there’s no difference… So, in the transport world, why the hell do we use this mode at all? Good question.

Let’s recap what we’ve learned about the Hamilton so far. There are many modes of invasive ventilation on the Hamilton, but with it being a pressure driven vent, CMV+ is the easiest and most beneficial mode. SIMV is more of an ego mode, not a patientcentric mode. When we start a case, we will first assess the ET tube like we do on all patients:

Step zero:

assess ETT size, depth, and cuff inflation

select patient size and sex

select Mode: CMV+

Initial Settings adult:

Plimit 30-35cmH2O, RR 16, I:E 1:2.7 (TI 1second), Vt 8ml/kg (seen at bottom of screen), PEEP 8, FiO2 whatever (40-100) Controls  More: Pramp 70-100ms.