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# Effect of Flow Rate on Inspiratory and Expiratory Time * Increased inspiratory flow rate (Figure 1-2) decreases inspiratory time and allows for a longer expiration time; conversely, if the flow rate is decreased, the inspiratory time increases and the expiratory time decreases. 45 LPM converts t...

# Effect of Flow Rate on Inspiratory and Expiratory Time * Increased inspiratory flow rate (Figure 1-2) decreases inspiratory time and allows for a longer expiration time; conversely, if the flow rate is decreased, the inspiratory time increases and the expiratory time decreases. 45 LPM converts to 45,000 mL by moving the decimal point 3 places to the right. 45,000 mL/minute is equal to 45,000 mL/60 seconds. 550 mL = X sec 45,000 mL / 60 sec = 750 mL/sec X = 33,000/45,000 = 0.73 sec It takes 0.73 seconds to inspire 550 mL. Tidal Volume (VT) = 1 breath every 4 seconds Length of each breath (TCT) Inspiration time = 0.73 sec Expiration time = 4.00 - 0.73 = 3.27 sec I:E Ratio = 3.27/0.73 = 4.47 Cycle time remains at 4 seconds # Effect of Changes in Resistance and Compliance (1 of 2) * The effect of increased airways resistance on the pressure-time waveform. * Pressures plotted on the pressure time scaler are calculated from the known parameters of airway resistance (Raw), Compliance (C), Inspiratory flow and the delivered tidal volume. FLOW OR VOLUME = PRESSURE / RESISTANCE Therefore, PRESSURE = FLOW X RESISTANCE This pressure is called the transairway pressure. To calculate this pressure, first convert the flow in LPM into liters per second by dividing by 60 Then multiply the flow by the airway resistance 10 cm.L/sec to give 5 cm.H2O. This is the transairway pressure which is the pressure required to overcome resistance as gas flows through. # Effect of Changes in Resistance and Compliance (1 of 2) * Once the gas molecules reach the airway, we now have to calculate the plateau pressure which is required to expand the lung against the elastic recoil of the lung. Static Compliance = Volume / Pressure If you solve the equation for pressure; Pressure = Tidal Volume / Static Compliance Tidal Volume = 550 ml Static Compliance = 50 ml/cm H2O Therefore, Plateau = 11 cm H2O. PIP = 16 cm H2O We added 5 cm H20 # What happens if airway resistance doubles to 10 cm If Airway resistance doubles to 10 cm., due to secretions, bronchospasm, or other obstructions, the PIP goes up to 21cm H2O, but Plateau remains at 11cm H2O.