Introduction to Mechanical Ventilation
Spontaneous Breathing
Positive Pressure Breath
Goals of Mechanical Ventilation Maintain ABG’s Optimize V/Q Decrease Myocardial Workload
Indications for Mechanical Ventilation Apnea Acute Ventilatory Failure Ph 7.30 or <, with PaCO2 50 or > Clinical Signs Impending Ventilatory Failure Acute Respiratory Failure
Two Ways to Achieve Continuous Mechanical Ventilation, ie CMV Negative pressure Positive pressure
Positive Pressure Flow Pattern Considerations Flow = Pressure divided by resistance
Positive Pressure Flow Patterns Constant flow or Square Wave Flow stays constant as resistance varies Thus pressure and resistance vary directly
Positive Pressure Flow Patterns Accelerating/decelerating or sine wave Peak flow occurs at mid-inspiration Mimics spontaneous breathing
Positive Pressure Flow Patterns Constant Pressure or tapered flow Flow (and hence tidal volume) vary with resistance
Flow Patterns Summary Constant flow or square wave Sine Wave Constant Pressure or tapered wave
Compare & Contrast
Cycling Cycling refers to how the ventilator ends the inspiratory phase of the breath
Cycling Mechanisms Volume cycling – inspiration ends when a preset tidal volume is delivered Pressure cycling – inspiration ends when a preset pressure is reached on the airway Time cycling – inspiration ends when a preset inspiratory time has elapsed Flow cycling – inspiration ends when a preset flow has been reached
The mechanism that starts the inspiratory phase Triggering The mechanism that starts the inspiratory phase
Trigger Mechanisms Pressure triggered – a drop in airway pressure triggers the ventilator Flow triggered – a constant (bias) flow of gas passes through the ventilator circuit. When the patient starts to inhale the ventilator detects the drop in bias flow and triggers Types of triggered breaths: patient = assisted; ventilator = controlled, operator = manual
Hazards – Positive Pressure CMV Increased mean intrathoracic pressure Decreased venous return Increased intracranial pressure Pulmonary Volu/Barotrauma Fluid retention Gastric Ulcers Muscle Atrophy & Patient Dependence Mechanical Failure Mismanagement Contamination/Infection
Preventing Hazards Maintain good I:E ratio Make sure flow meets patient’s demand Attention to patient and ventilator FREQUENT HANDWASHING!
Ventilator “Modes”
Control Mode
Assist Mode
Assist/Control
IMV – Intermittent Mandatory Ventilation
PEEP
CPAP
Other Modes High Frequency Ventilation (HFV) Pressure Control ( time cycling) Pressure Support (flow cycling) Airway Pressure Release Ventilation (APRV)
Some Practical Applications
Peak Pressure Pressure on manometer immediately at end of inspiratory phase Represents pressure needed to overcome both elastic and airway resistance Used to calculate dynamic compliance Cdyn = VT/Peak pressure PEAK PRESSURE WILL CHANGE WHEN EITHER ELASTIC OR AIRWAY RESISTANCE CHANGES!
Plateau Pressure Pressure on manometer after inspiration has ended but before expiration has started Represents pressure needed to overcome elastic resistance only Used to calculate static compliance Cstat = VT/plateau pressure PLATEAU PRESSURE CHANGES ONLY WHEN ELASTIC RESISTANCE CHANGES
Clinical Analysis By Comparing Peak and Plateau Pressure Changes Remember – a change in elastic resistance will affect both peak and plateau pressure Remember – a change in airway resistance only affects the peak pressure Compare the change in plateau pressures first, then compare the changes in peak pressure
Resistance and Pressure Vary Directly Resistance and Pressure Vary Inversely With Compliance
Initial Values 2 Hours later Peak = 28 cmH2O Plateau = 23 cmH2O
Initial Values 2 Hours Later Peak = 31 cmH2O Plateau = 25 cmH2O
Initial Values 2 Hours Later Peak = 49 cmH20 Plateau = 30 cmH2O Peak = 49 cmH2O Plateau = 26 cmH2O
Initial Values 2 Hours Later Peak = 36 cmH2O Plateau = 29 cmH2O
Initial Values 2 Hours Later Peak = 29 cmH2O Plateau = 22 cmH2O
Initial Values 2 Hours Later Peak = 33 cmH2O Plateau = 21 cmH2O
Now lets have some Fun with more math!