Mechanical Ventilation Flow Patterns




What is the difference between constant, decelerating and sinusoidal flow waveforms?

Flow of gas is calculated in liters per minute. Flow commences at the beginning of a breath and stops at the end of the breath. Gas flows into the lungs in inspiration and out of the lungs in expiration. The pattern of expiratory flow is more or less the same for different modes of ventilation, as long as the expiratory phase is long enough to prevent gas trapping. The normal flow pattern of gas moving in and out of the lungs is sinusoidal. In volume control ventilation a variety of different wave patterns can be used. In clinical practice, constant and decelerating flow patterns are used; the latter is preferred. In constant, decelerating and sinusoidal flow patterns, the inspiratory flow rate is equal to the peak flow rate, but the mean flow rate is higher in constant flow patterns rather than the other two. This suggests that this pattern will cause more shearing injury to the lung parenchyma. Therefore a decelerating flow pattern is probably the most effective flow pattern it ensures peak flow early in inspiration, while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it.



Flow Patterns in Volume Controlled Ventilation. In Pressure Control, the flow pattern is always decelerating. Note that tidal volume is the same in all three patterns, but that airway pressure is highest in constant flow. Decelerating flow is associated with the highest initial flow rate, thought to influence overall gas distribution, and is generally preferred.




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