Mechanical Ventilation PrimerFiras Rabi, MD

Indications Airway Compromise

Airway patency is in doubt Loss of gag or cough reflex

Respiratory Failure Hypoxemic – generally, PaO2 ≤ 60 mmHg

Needs help oxygenating Hypercarbic – generally, PaCO2 ≥ 50 mmHg

Needs help ventilating

Origins of Ventilators Negative pressure

ventilators “Iron” lung First used in Boston

in 1928 Used extensively for

polio

Mechanical Ventilators Depend on place of employment All have similar ventilator patterns, but

may call them different names ALL MODES ARE HARMFUL TO THE LUNG

Goal is always to extubate ASAP.

Alveoli (cartoon)

Alveoli (actual)

Servo 300

Servo i

High Frequency Oscillating Ventilator (HFOV)

LTV 1000

Ventilator Terminology A/C – Assist/Control IMV – Intermittent mechanical ventilation SIMV – Synchronized IMV PRVC – Pressure regulated, volume

control PEEP – Positive end-expiratory pressure CPAP NIPPV

Volume vs. Pressure Volume control – tidal volume is constant,

pressure will vary Pressure control – pressure is constant,

tidal volume will change based on lung compliance

Pulmonary Compliance Compliance = Volume/Pressure This equation is worth memorizing since it

provides the basis for understanding pulmonary and ventilator interactions

Modes of Ventilation

Control Mode Each breath has a pre-set volume, time,

and flow rate Patient cannot generate spontaneous

breaths

Assist/Control Mode Each breath has a pre-set volume, time,

and flow rate Each patient generated respiratory effort

over and above the set rate are delivered at the set volume and flow rate

Pressure Control If the pressure is set at PC 16 above PEEP

of 4, then the ventilator will deliver a top pressure (peak pressure) of 20 (PC level of 16 plus PEEP) with an end pressure of 4.

This keeps the airways slightly open, making it easier to inflate them, and helps prevent collapse and consolidation

SIMV Each breath has a pre-set volume, time,

and flow rate Allows patient to generate own breaths

with own volumes and flow rates If patient initiates a breath, machine will

not initiate a ventilator breath no breath stacking

PRVC Each breath is volume controlled (you set

the tidal volume) The machine decides how much pressure

to use to deliver that tidal volume based on the required pressures for previous 2-3 breaths

PRVC You get to limit the amount of pressure

used to deliver the volume. If this pressure is reached, the ventilator will alarm “regulation pressure limited”, switch to expiration, and will not be able to deliver the preset tidal volume

Automode (only in servo i) Automatically controls the transition

between controlled (vent triggered) and support (patient triggered) mode in accordance with patient’s effort

PC PS VC VS PRVC VS

Pressure Support Each patient-initiated breath is supported

by the machine

Combo modes SIMV VC with PS SIMV PC with PS SIMV/PRVC with PS

PEEP Not a specific mode but rather an

adjunct to any of the other modes PEEP is the amount of pressure remaining

in the lung at the END of the expiratory phase.

Utilized to keep otherwise collapsing lung units open while hopefully also improving oxygenation

Improving oxygenation FiO2

Simplest maneuver Free radical damage with prolonged

exposure to > 60% PEEP

Reverses pulmonary shunting Atelectasis, pneumonia, ARDS, CHF,

pulmonary hemorrhage

PEEP Side-effects Barotrauma Diminished cardiac output Regional hypoperfusion

CPAP This IS a mode and simply means that a

pre-set pressure is present in the circuit and lungs throughout both the inspiratory and expiratory phases of the breath

CPAP serves to keep alveoli from collapsing, resulting in better oxygenation and less WOB

CPAP The CPAP mode is very commonly used as

a mode to evaluate the patients readiness for extubation

CPAP vs. BiPAP CPAP is essentially PEEP BiPAP is PEEP plus pressure support

Summary of ModesControlled Modes

Supported Modes

Combined Modes

Spontaneous breaths

VC VS Automode: VC + VS

CPAP

PC PS Automode:PC + PS

BiPAP

PRVC Automode:PRVC + PS

SIMV: VC/PS

SIMV: PC/VS

SIMV: PRVC/PS

Initial Settings Decide on mode Rate based on age

Infants 30 Children 20 Adolescents 10

Most can start with tidal volume of 6mL/kg PEEP of 5

Inspiratory Trigger Normally set automatically Two modes

Airway pressure Flow triggering

I:E Ratio Normally 1:2 Asthma 1:3 or 1:4 Severe hypoxia 1:1 or 2:1 (inverse ratio)

FiO2 Start 100% Target lowest possible for PaO2 >

60mmHg or sat > 90%

High Frequency Ventilation

Oxygenation in HFOV Oxygenation is primarily controlled by the

Mean Airway Pressure (Paw) and the FiO2 Mean Airway Pressure is a constant

pressure used to inflate the lung and hold the alveoli open.

Since the Paw is constant, it reduces the injury that results from cycling the lung open for each breath

Ventilation in HFOV Generally controlled by frequency

Increased frequency reduces amount of time for exhalation, so leads to decreased ventilation↑ Hz ↑ pCO2

Increased amplitude/power will increase ventilation

Not usually the primary reason for using HFOV

Troubleshooting Sudden deterioration

Disconnection Obstruction Pneumothorax Equipment failure

Anxious patient Appropriate mode Trigger set appropriately

Troubleshooting Call the respiratory therapist

Thank You Questions