Wednesday, June 22, 2011

Performing a T-Piece Trial with the Ventilator

Utilizing the Pressure/Volume Loop to determine the appropriate pressure support level during  a simulated T-piece trial.

Many institutions perform spontaneous breathing trials with the ventilator using a small amount of PEEP (~ 5 cmH2O) and automatic tube compensation or a small amount of pressure support (~ 5 to 10 cmH2O). However, occasionally a traditional T-Piece trial may prove to be more beneficial [1].

Utilizing pressure support during a spontaneous breathing trial to off-set the work of breathing related to the artificial airway may lead to false positives, due to over compensation.

Why is this?

  1. Pressure support is a fixed amount of pressure delivered to the patient.

  1. Conversely, the patients inspiratory flow demand is variable.

  1. The patient has a specific artificial airway, either a tracheostomy tube or endo-tracheal tube, with a specific size.

  1. Each artificial airway (based on type & size) has a different resistance coefficient for a fixed inspiratory flow rate.

  1. So when inspiratory flow changes the resistance alters, which modifies the inspiratory work.

Note- This is why some institutions have built into their weaning protocols different pressure support levels for different sized airways.

Example:  A patient who has a 7.5 E.T.T. for an artificial airway, with an inspiratory flow rate of 45 lpm. At this specific flow rate (45 lpm) a set pressure support of 5 cmH2O will compensate for the resistance of the airway. Conversely, if the flow rate is much lower then it will over compensate, and if the flow rate is much larger the fixed pressure support of 5 cmH2O would under compensate.

Since the patient’s flow rate is not constant how does one set the pressure support level not to over compensate?

Performing a T-Piece Trial with the Ventilator

First, if the ventilator has automatic tube compensation the operator can use this without pressure support. Automatic tube compensation continuously tracks the flow and will titrate the pressure automatically to off set the work created by the artificial airway. If the operator sets the PEEP to zero (ZEEP trial), this is basically a T-Piece trial through the ventilator.

If your ventilator doesn’t have automatic tube compensation one may still perform a simulated T-Piece trial with the machine.

Here are the steps:

  1. Assess the patient if they meet criteria for weaning (per institution).

  1. If the patient meets criteria place them on a moderate amount of pressure support (~ 5 to 7 cmH2O) and low PEEP (5 cmH2O).

  1. Drop the PEEP to Zero (0) to see if the patient tolerates the decrease in mean airway pressure. If patient has a history of congestive heart failure assess lung sounds to assure patient does not develop pulmonary edema.

  1. Set the ventilator waveform screen for a Pressure/Volume loop. Set the loop so pressure is on the X axis & Volume on the Y axis.

  1. Titrate the pressure support when viewing the P/V loop to obtain a thin “flute” with counter clockwise movement (as pictured above). If the loop is too wide the pressure support setting is too high. If there is clockwise movement the pressure support level is too low.

This is a more cumbersome way to perform spontaneous breathing trials; however it is more precise, decreasing the potential for over compensation leading to false positive results.