One of the most intimidating parts of being in the ICU is hearing something like “PCV, 15 on 10, FiO2 70%” What does that mean?!?! While this post is by no means going to go over the intricacies of ventilation, it should hopefully be enough of a primer to understand what’s being talked about on rounds.
The key to understanding modes of ventilation is knowing what you can control on a ventilator. The major variables are:
Trigger – time based (control) or patient initiated (spontaneous)
Delivery – Pressure OR Volume
Respiratory Rate – important in control modes
PEEP (positive end expiratory pressure)
FiO2 – From 21% to 100%
There are many others (flow rate, i:e ratio, cycle trigger, etc but I won’t get in to those for now)
In a spontaneously breathing patient, a ventilator breath is triggered by patient initiation (usually through a change of flow or pressure in the circuit when the patient breathes in). If a patient becomes apneic, a backup set rate kicks in.
In a controlled mode of ventilation, a breath is delivered periodically based on rate. If the respiratory rate is set at 20, a breath is triggered every 3 seconds.
Once a breath is triggered, the ventilator can either deliver a fixed volume of air (tidal volume) or a continuous flow until a pressure is reached (inspiratory pressure). Only one of the two can be set (since they are dependent on each other).
The positive end-expiratory pressure level is the pressure maintained in the airways between breaths. It is usually at least 5 cm H2O and may be set higher in certain conditions (i.e. ARDS). This pressure is meant to hold airways and alveoli open to prevent collapse
This is simply the percentage of inhaled air that is oxygen and can be room air (21%) to 100% oxygen. This should be titrated to the lowest level that gives you an acceptable arterial oxygen saturation.
Now let’s go through some examples..
ACVC Vt400 PEEP10 FiO2 50% RR15
– In this mode, a tidal volume of 400 ml is delivered every 4 seconds. PEEP is maintained at 10 and FiO2 is 50%
PSV 10/8 FiO2 30%
– In this mode, breaths are triggered by patients and an inspiratory pressure of 10 is delivered with a PEEP of 8 and FiO2 30%
– Note that in some institutions this same mode may be described as 18/8 (where the nomenclature is Ppeak/PEEP instead of Pinsp/PEEP)
CPAP 5 FiO2 60%
– In this mode, all breaths are patient triggered and there is no additional ventilatory support of these breaths above the set PEEP of 5
To the veterans out there, look out for some more advanced ventilatory modes and discussion to come!