Assisted Ventilation for Pediatric Patients
I General Concepts of Ventilation (Box 30-1)
A Alveolar ventilation: The measure of the adequacy of ventilation; CO2 removal is directly related to alveolar ventilation.
B Compliance: The measure of distensibility of the lungs and thorax expressed as the volume change in the lung per unit of pressure change. The higher the compliance, the greater the volume change in the lungs for a given change in pressure. With a reduction in compliance, a greater pressure gradient is required to move a given volume of gas into the lungs.
C Resistance: The measure of the tendency for the airways and lung tissue to resist the flow of gas expressed as the change in pressure per unit of gas flow: the greater the resistance to gas flow, the greater the pressure gradient necessary to deliver a volume of gas in a given time interval.
D Time constant: The relationship between compliance and resistance and pressure equilibration between patient and ventilator circuit, or filling and emptying of the lungs during inspiration and expiration. One time constant is the measure of the time necessary for alveolar pressure to equilibrate to 63% of a change in airway pressure; 99% pressure equilibration occurs in lungs with normal compliance and resistance in approximately five time constants.
II Continuous Positive Airway Pressure (CPAP)
1. Continuous distending pressure maintained throughout respiratory cycle.
2. Prevents alveolar collapse during expiration, and reduces the pressure required to open alveoli during inspiration.
3. Improves and maintains functional residual capacity (FRC) and ventilation-perfusion relationship.
1. The variable that a ventilator controls to effect inspiration
3. Mandatory and spontaneous ventilator-supported breaths are classified as pressure controlled or volume controlled.
B Pressure-control ventilation (PCV)
1. Pressure is the variable that the ventilator controls to effect inspiration.
2. The shape of the inspiratory pressure waveform remains consistent breath to breath as compliance and resistance change.
3. Flow pattern and delivered tidal volume (Vt) vary depending on changes in compliance and resistance.
4. Volume delivery primarily depends on the change in pressure from baseline to peak pressure, referred to as delta-P (ΔP).
5. As the lungs fill and lung pressure approaches the set pressure target or limit (circuit pressure), the inspiratory gas flow rate to the patient decreases or decelerates.
6. Time-cycled, pressure-limited (TCPL)
a. Most common type of ventilation used to support infants weighing <10 kg
b. Pressure-controlled, incorporating a continuous flow of gas through the breathing circuit, a pneumatic exhalation valve, and a timing mechanism
c. The breath rate setting determines a fixed time interval to trigger mandatory breaths.
d. During inspiration the exhalation valve closes; flow is diverted to the inspiratory limb; pressure builds to a preset limit; and the breath is cycled after a preset inspiratory interval.
C Pressure support ventilation (PSV)
1. PSV is used to support spontaneous breaths.
2. Triggered by patient inspiratory effort, the ventilator provides the inspiratory flow necessary to reach and maintain a set pressure support level (pressure limit) for the duration of inspiratory time.
3. Breaths are terminated (cycled) when inspiratory flow decreases to a predetermined level, usually a percentage of peak inspiratory flow rate.
4. The change in inspiratory flow rate depends on patient demand and lung mechanics.
D Volume-control ventilation (VCV)
1. Volume and flow, which are functions of each other, are the control variable.
2. A set Vt is delivered in an inspiratory time that is either directly set or set indirectly as the result of setting a peak inspiratory flow rate.
3. Volume is constant from breath to breath, and the pressure gradient required to deliver a given volume varies with changes in compliance and resistance.
1. All breaths are mandatory and are time triggered by the ventilator at an interval determined by the breath rate setting or patient triggered if the patient’s inspiratory effort meets the set-triggering criteria.
2. All delivered breaths can be either volume controlled (e.g., AC-VCV) or pressure controlled (e.g., AC-PCV).
V Indications for Mechanical Ventilation (Box 30-2)
VI Pediatric Ventilator Settings
1. The disease process and the size of the child are considered when selecting ventilator settings.
2. Manual ventilation is useful to assess the compliance of the lungs while observing the chest excursion and evaluating breath sounds.
3. Peak inspiratory pressure (PIP) and positive end-expiratory pressure (PEEP) observed on an airway manometer during manual ventilation are noted and used as a guide to select the initial ventilator settings.
1. SIMV volume or pressure controlled
