Patient Ventilator Interactions

https://doi.org/10.4187/respcare.09316

The mathematical model describing this is the Equation of Motion for the Respiratory System. There are many forms of this equation, but a simple version is expressed as:

Pmus + Pvent = EV + R V (1)

where Pvent is the inspiratory pressure generated by the ventilator (above PEEP), Pmus is the inspiratory pressure generated by the respiratory muscles, E is elastance (cm H2O/mL), V is volume (mL), R is resistance (cm H20/L/s), and V is flow (L/min), all measured relative to their end-expiratory values

Dominant load can either be resistive (R * V) or elastic (E * V)

When assessing load or patient-ventilator interactions, attention should be focused to the waveform opposite the control variable. e.g. Thus, in VC you see the load and patient interactions in the pressure waveform

What to do if we can’t get a plat?

Use square wave-form, ACVC

At the end of inspiration, airway pressure is the result of both resistive and elastic loads. At the beginning, it’s all resistive.

If the flow stops (manual or preset inspiratory hold) the resistive load is zero (R􀀄0 = 0) and only the elastic load remains, this is the classic way taught to evaluate the loads. You now can identify this with or without an inspiratory hold: the higher the resistive load, the higher the initial step up in pressure, and if there is an end inspiratory pause, the greater the fall from the peak to the plateau pressure.

In square waveform - higher pressure slope = higher elastance. If slope steepens, suggests over-distention OR effort (Pmus)

It is not possible to reliably discern the elastic or resistive loads while the Pmus occurs

The respiratory system has 2 time constants, inspiratory and expiratory, as compliance, and especially resistance, are usually not the same during inspiration and expiration

Synchrony means a simultaneous action, development, or occurrence. In mechanical ventilation, it refers to the timing of Pvent and Pmus signals in relation to each other.

Work of breathing refers to both the patient and ventilator work, and their relation

Dyssynchrony types:

Problems with trigger

-Late trigger - Pmus (drop in baseline pressure) occurs before vent -Early trigger - flow start and then negative deflection during early inspiration. -False trigger - inspiration triggers by a non Pmus signal (e.g. secretions or fluids in the tube -Failed trigger - downward deflection that does not reach 0 . Can occur at any phase of the expiratory limb. Also may occur if there is intrinsic peep

Problems with inspiration

-Work shifting. CPAP - all Vt is PMus. When Pvent and Pmus working together, some work is done by each. VC or that use adaptive targeting schemes, the relationship is inverse (i.e. as the patient does more work, the ventilator does less work and total work remains constant). Can be driven by hypoxemia, metabolic acidosis, J fiber etc. -> requires sedation

Problems with cycle (start of expiration)

-Late cycle - inspiration ends after a delay in the Pmus peak (aka neural inspiratory time is shorter than actual inspiratory time) -Early cycle -inspiration starts before the Pmus peak (aka neural inspiratory time is longer than actual inspiratory time) - often shows up as ‘double triggering/breath stacking’; or if the ventilator cycles from a non patient signal.

Problems with expiration

-Expiratory work (a negative Pmus, deforms waveform away from baseline) - can occur due to pursed lip breathing in COPD, acidosis, anxiety.

Patterns on the ventilator

-Multiple trigger (many ventilators have refractory period) - can be due to early cycle, early trigger, and false trigger. -Tidal volume discrepancy - leak from the circuit, airway, or lung, active exhalation during inspiration, air trapping, or flow sensor malfunction.