Notes

Outline

Understanding Mechanical Ventilators

Outline Self Study Guide Basic Definitions Equation of Motion Breath Types, Patterns, and Phase Variables What is a “Mode”? What does “Control” Mean? Open-Loop and Closed-Loop Control Proportional Assist Dual Control Mechanism Adaptive Support Automatic Tube Compensation Final Thoughts

Self Study Guide Learn the language: Basic definitions Use the language Translate marketing lingo Learn to recognize basic waveforms  Pressure, volume, and flow Use basic waveforms Understand how “modes” work

Basic Definitions WARNING! There is little standardization of nomenclature Some things that are slightly different are given the same names Some things that are the same are given different names Never rely on manufacturers for a general understanding of ventilators

Mechanical Ventilator Ventilator: A machine used to assist or replace the work generated by the ventilatory muscles. Mechanical Ventilation: Use of a ventilator to move gas into and out of the pulmonary system.

Mechanical Ventilator Machine: a system of related elements  designed to direct applied energy to do useful work. Electrical: Energy = volts x amps x time Compressed Gas: Energy = pressure x volume

Key Ideas Equation of Motion Ventilator/patient interaction Breath Types Mandatory/spontaneous Breath Patterns Combinations of mandatory/spont. Phase variables

Equation of Motion

Uses for the Equation of Motion Classify ventilators and modes Ventilator controls only one at a time pressure, volume, or flow Monitor lung mechanics Resistance & compliance, time constant Basis of newest modes Proportional Assist Adaptive Support

Waveforms

Breath Types Spontaneous Breath Inspiration is both initiated and terminated by the patient. Mandatory Breath Inspiration is either initiated or terminated by the ventilator.

Breath Patterns Continuous Mandatory Ventilation CMV All breaths mandatory Intermittent Mandatory Ventilation IMV or SIMV Mandatory and spontaneous breaths Continuous Spontaneous Ventilation All breaths spontaneous

Phase Variables TRIGGER starts inspiration Example: pressure drop when patient sucks in LIMIT preset inspiratory value Example: preset maximum inspiratory flow CYCLE stops inspiration Example: preset inspiratory time BASELINE preset expiratory pressure

What Is a “Mode”? Particular control variable pressure, volume, or flow Particular pattern of breaths CMV, IMV, CSV Particular set of phase variables trigger, limit, cycle Particular control logic for changing phase variables automatically

What Does “Control” Mean? 1. Open loop control 2. Closed loop control 3. Double loop “dual” control

Open Loop Control Mechanism 1. Preset control circuit to desired on/off periods Imagine a furnace and on/off timer Furnace turns on for an arbitrary 5 minutes/hour

Open Loop Control Mechanism Advantages Simple, inexpensive Disadvantages Room temperature not well controlled because outside air temperature (ie, weather) changes 5 minutes may be too long or too short

Open Loop Control of Ventilator Example Mechanical pressure release on older infant ventilators and some transport ventilators Advantage Easy to understand and use Disadvantage Leaks or flow changes affect pressure

Closed Loop Control Mechanism (Feedback/Servo) 1. Preset control circuit to desired output 2. Measure actual output 3. Change controller to get desired output if target not met Imagine a thermostat and furnace Furnace turns off when room temperature preset value

Closed Loop Control Mechanism Advantages Maintains constant room temperature regardless of outside air temperature changes Disadvantages More complex and expensive

Closed Loop Control of Ventilator

Closed Loop Control of Ventilator Example Pressure controlled ventilation with sensors and microprocessor Advantage Maintains consistent pressure waveform Disadvantage Changes in lung mechanics cause unstable blood gases

Advanced Closed Loop Control Proportional Assist Example: Draeger Evita 4 “proportional pressure support” Operator input “volume assist” level (elastance) “flow assist” level (resistance) FiO2 PEEP

Proportional Assist Advanced single loop pressure control Ventilator automatically adjusts pressure

Proportional Assist Phase Variables Trigger patient Limit resistive pressure (flow assist level) elastic pressure (volume assist level) Cycle flow

Proportional Assist

Proportional Assist Potential Advantages support matched to need only abnormal load is supported better machine-patient synchrony theoretically the best mode Potential Disadvantages leaks defeat ventilator algorithm no ventilation if patient stops breathing

Double Loop (Dual) Control Mechanism 1. Preset control circuit to desired output 2. Measure actual output 3. Change controller to desired output 4. Automatically change desired output as overall conditions change Imagine timer changing thermostat setting for day versus night room temperatures Furnace automatically turns off at one temperature during day, another at night

Dual Control of Ventilator Dual Control Between Breaths All breaths pressure controlled to preset pressure limit Automatic change in pressure limit to maintain target tidal volume Dual Control Within Breaths Switch from pressure control to volume control within breath to maintain target tidal volume

Dual Control of Ventilator Advantage Stabilizes delivered volume and blood gas values Improves synchrony Disadvantage Automatic changes may be inappropriate

Dual Control Between Breaths

Dual Control Between Breaths Potential Advantages Better synchrony like PCV Stable tidal volume like VCV Automatic weaning as patient improves Potential Disadvantages May result in autoPEEP May inappropriately decrease support patient increases drive due to agitation

Dual Control Within Breaths

Dual Control Within Breaths Potential Advantages better matching of flow to patient need like PCV stable tidal volume like VCV Potential Disadvantages difficult to understand and set properly may be uncomfortable for patient to switch between pressure and volume control

Advanced Dual Control Adaptive Support Ventilation Example: Hamilton Galileo Operator input ideal body weight FiO2 % of minute ventilation to support PEEP

Adaptive Support Ventilation Advanced dual control (between breaths) Ventilator monitors minute ventilation lung mechanics (expiratory time constant) Automatically adjusts minute ventilation rate pressure limit inspiratory time Sets pattern to minimize WOB as if patient was breathing spontaneously

Adaptive Support Ventilation Phase Variables Trigger patient or machine Limit inspiratory pressure Cycle time or flow

Adaptive Support Ventilation Potential Advantages matches ventilation to lung condition quicker, automatic weaning decreased risk of lung damage Potential Disadvantages leaks may defeat algorithm operator must select appropriate % of minute ventilation to support deadspace may cause problems

Adaptive Support Ventilation

Automatic Tube Compensation Example: Draeger Evita 4 Operator input endotracheal tube size % compensation Ventilator automatically sets flow assist level pressure control for resistive pressure eliminates resistive WOB

Automatic Tube Compensation Potential Advantages simulates breathing without tube decreases patient work of breathing Potential Disadvantages actual tube resistance may change secretions, kinking may not simulate actual conditions swelling after extubation

Unanswered Questions How do newer modes affect outcome? Which patients - which modes Incidence of adverse effects Duration of ventilation Length of hospital stay Cost per episode of care How to train users?

Final Thought