Basic Pulmonary Mechanics during Mechanical Ventilation Equation of Motion dP = R x Flow + dV / C st

Points of Discussion Equation of motion Airway pressures Compliance Resistance Pressure-Time Flow-Time Pressure-volume loop Flow-volume loop

Spontaneous Breathing Exhalation Inspiration

Precondition of Inspiration Pb Pa Gas Flow Pa < Pb Spontaneous breath Pb > Pa Mechanical ventilation

Lung Mechanics resistance = Dpressure / Dflow transairway pressure transrespiratory pressure transthoracic pressure compliance = Dvolume / Dpressure volume The equation of motion says that the pressure necessary to deliver a breath has two components; the pressure to overcome elastic recoil of the lungs and chest wall and the pressure to cause flow through the airways. The left hand side of the equation can be expanded to show that ventilating pressure may be made up of muscle pressure and/or airway pressure generated by the ventilator. The right hand side of the equation can be expanded to show that elastic recoil pressure is the product of elastance times volume while resistive pressure is the product of resistance and flow.

Tube + Spring Model Resistive Forces Elastic Forces

“The Feature of the Tube” Pressure Difference = Flow Rate x Resistance Airway Resistance “The Feature of the Tube” R = D P D F Pressure Difference = Flow Rate x Resistance

Pressure Difference = Volume Change / Compliance Cs = D V Volume Pressure D V D P D P Pressure Difference = Volume Change / Compliance

Compliance and Resistance Cs= D V D P R = D P D F In the ventilator circuit, the peak inspiratory pressure is a combination of the resistance related pressure and the compliance related pressure. An end inspiratory pause eliminates the flow related pressure and thus reflects on the compliance related pressure.

DYNAMIC CHARACTERISTICS: Equation of Motion DYNAMIC CHARACTERISTICS: dP = dV / Cdyn RESISTANCE: dPresistive = R x Flow STATIC COMPLIANCE: dPdistensive = dV / Cst dP = dPresist. + dP dist. dP = R x Flow + dV / C st

Equation of Motion dP = R x Flow + dV / C st

Components of Inflation Pressure PIP } Transairway Pressure (PTA) Paw (cm H2O) Pplateau (Palveolar) Inspiratory Pause Expiration Begin Inspiration Time (sec) Begin Expiration

Exhalation Valve Opens PIP } Transairway Pressure (PTA) Exhalation Valve Opens Paw (cm H2O) Pplateau (Palveolar Expiration Begin Inspiration Time (sec) Begin Expiration Paw (cm H2O) Time (sec) PIP Inflation Hold (seconds) Distending (Alveolar) Pressure Airway Resistance Expiration Begin Inspiration Begin Expiration

Spontaneous vs. Mechanical Inspiration Paw (cm H2O) Spontaneous Expiration Expiration Inspiration Time (sec)

PIP vs Pplat Paw (cm H2O) Time (sec) High Raw Normal High Flow Low Compliance PPlat Time (sec)

Lengthen Inspiratory Time Increase peak pressure Mean Airway Pressure Lengthen Inspiratory Time Increase peak pressure Increase PEEP Increase Rate Increase Flow Another helpful feature is monitoring during trigger. Here is a diagram of a pressure wave showing all the parameter settings that can effect MAP. Mean Airway Pressure (MAP) is the average pressure applied to the airway during a respiratory cycle. It is calculated as the area under the pressure wave for one repiratory cycle. In premature babies, you need to use quite short inspiratroy time settings in order to achieve synchrony - typically a Ti of 0.2 - 0.3s. However as you can see in the diagram, this reduces the area under the pressure wave and may therefore reduce your MAP which effects oxygenation. What you can do to compensate for this loss of MAP due to shorter Ti (given that your ventilator has no problem with inadvertent PEEP), is increase the flow rate. Increasing the flow creates a quicker rise of pressure at the airway - more time at peak pressure and thus increases MAP.

Inspiratory Flow Pattern Beginning of expiration exhalation valve opens Peak inspiratory flow rate PIFR Inspiration Inspiratory Time TI Expiratory Time TE Flow (L/min) Total cycle time TCT Time (sec) Beginning of inspiration exhalation valve closes Expiration Peak Expiratory Flow Rate PEFR

Flow vs Time Inspiration Time (sec) Flow (L/min) Expiration

Flow Patterns SQUARE DECELERATING ACCELERATING SINE

Flow Patterns and Effects of Volume SQUARE DECELERATING ACCELERATING SINE

Mechanical vs Spontaneous Inspiration Expiration

Inspiratory Tidal Volume Volume vs. Time Inspiratory Tidal Volume Volume (ml) Inspiration Expiration TI Time (sec)

FRC and PV Loop Normal Compliance TLC FRC FRC VOLUME Negative Positive DISTENDING PRESSURE Normal Compliance FRC FRC

Components of Pressure-Volume Loop VT Expiration Volume (mL) Inspiration PIP Paw (cm H2O)

PEEP and P-V Loop VT PIP Volume (mL) PEEP Paw (cm H2O)

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