Philips Respironics V60, User Manual

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Principles of operation
4-13
PPV mode (optional)
The PPV (proportional pressure ventilation) mode provides patient-triggered
breaths that deliver pressure in proportion to patient effort. Additionally a user-
settable backup rate activates machine-triggered, pressure-limited, and time-
cycled breaths in the case of apnea. In the PPV mode, patient effort
determines the pressure, flow, and tidal volume delivered by the ventilator. The
ventilator responds to patient effort, allowing the patient to determine when to
start and end a breath.Additionally flow and pressure change based on the
patient’s efforts throughout inspiration.
The physics behind PPV. Two forces oppose ventilation, resistance and
elastance.
Resistance is the impedance to air movement in the airways:
Pressure/Flow = Resistance
Airway resistance in healthy adults ranges from approximately 0.5 to
2.5 cmH
2
O/L/s.
Elastance is the elastic opposition to ventilation or the tendency of the lungs to
resist inflation (elastance is the reciprocal of compliance):
Pressure/Volume = 1/Compliance = Elastance
The compliance of lungs and chest wall for a healthy adult is approximately
0.1 L/cmH
2
O, resulting in an elastance value of 10 cmH
2
O/L.
The inspiratory muscles, therefore, must generate force to overcome the
resistance and elastance of the respiratory system. The proximal airway
pressure is the net result of this contraction of these muscles: it is the force of
the inspiratory muscle contraction minus both the pressure needed to generate
air flow (overcome respiratory system resistance) and the pressure generated to
inflate the lungs (overcome respiratory system elastance).
How PPV works. The delivery of a PPV breath is controlled by the maximum
elastance (volume) assist (Max E), maximum resistance (flow) assist (Max R),
and PPV % settings. The actual delivered assistance to overcome elastance is
the product of PPV % and Max E. The actual delivered assistance to overcome
resistance is the product of PPV % and Max R. In general, Max E should be set
relative to the respiratory elastance and Max R should be set relative to the
respiratory resistance, although you do not need to know the actual value of
either to apply PPV. You adjust assist levels to optimize patient comfort. The
resultant pressure support delivered in the PPV mode is the resistance assist
times patient flow plus the elastance assist times the patient volume. Because
the patient completely controls ventilatory output,
1
PPV may significantly
improve patient-ventilator synchrony and ultimately, patient comfort.
The PPV backup rate ensures that the patient receives a minimum number of
breaths per minute if the spontaneous breathing rate falls below the Rate
1 Marantz, S., Patrick, W., Webster, K., et al. “Response of ventilator-dependent
patients to different levels of proportional assist.” Journal of Applied Physiology, Vol.
80: 397-403, 1996.