The Ventilator Book by Owens William

Author:Owens, William [Owens, William]
Language: eng
Format: epub, azw3
Publisher: First Draught Press
Published: 2018-05-12T16:00:00+00:00

VCV delivers a constant flow of gas during inspiration until the target tidal volume is reached. The waveform on the ventilator is referred to as a “square top” inspiratory flow.

Patients with COPD or asthma often like this— they typically have a high degree of air hunger and getting the tidal volume in quickly helps alleviate it. Other patients, however, find constant inspiratory flow to be uncomfortable, like drinking from a fire hose. Modern ventilators may permit the clinician to select a decelerating flow instead, which usually improves patient comfort. This is discussed in more detail in the chapter on “Trigger and Flow.”

Pressure Assist-Control

In pressure assist-control ventilation (PCV), the physician sets a rate and a driving pressure. This is the change in pressure that occurs during either a machine-administered or patient-triggered breath (remember, in assist-control the patient can breathe above the set rate). In addition, an inspiratory time (I-time) must be selected. In VCV, the ventilator turns off the flow once the target tidal volume has been reached. In PCV, the ventilator goes up to a set pressure and will hold it as long as it’s told before it turns off the flow—this is the I-time.

The ratio between the I-time and the expiratory time is known as the I:E ratio. In spontaneously breathing people, this ratio is usually between 1:2 and 1:4. In other words, breathing in takes about one second, and between two and four seconds is spent exhaling. When setting the ventilator, pay attention to the total time for each breath. If the rate is 20 breaths per minute, then it’s 3 seconds per breath (60 seconds divided by 20). If the I-time is one second, then the expiratory time is two seconds—hence, a 1:2 ratio. If the rate is 15 breaths per minute and the I-time is one second, then the I:E ratio is 1:3 (60 seconds divided by 15 breaths is 4 seconds per breath).

Once the I:E ratio is 1:1 or higher, it’s known as inverse-ratio ventilation. If the vent rate is 20 and the I-time is set at 2 seconds, then the I:E ratio is 2:1. Try breathing in for two seconds and then out for one— this is uncomfortable. There are special situations where inverse ratio ventilation is useful (such as severe ARDS), but it requires a great deal of sedation. In general, keep the I:E ratio between 1:2 and 1:4.

The driving pressure is the pressure change during the breath. What this means is that when it’s time for a breath, the pressure rises from whatever it is at end-expiration to a peak pressure, holds for the I-time, and then goes back to the end-expiratory pressure. For example: a patient has a set rate of 15, a driving pressure of 20 cm, an I-time of 1.0 seconds, and a positive end-expiratory pressure (PEEP) of 10 cm. 15 times a minute, the pressure will rise from the PEEP of 10 to a peak pressure of 30 cm (peak – PEEP = driving pressure). The pressure will hold at 30 cm for one second and then return to 10 cm, the PEEP.

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