Mechanical Ventilation
Welcome to this video tutorial on mechanical ventilation.
Ventilators can be quite overwhelming, so let’s go over some of the basics.
Mechanical ventilation is a means to mechanically assist or replace spontaneous respiration. It is indicated for patients who aren’t breathing (apneic) or breathing ineffectively (causing ventilation problems).
Mechanical ventilation is used to help the patient ventilate and oxygenate, with the end goal being to wean before complications arise. Mechanical ventilation can be provided noninvasively by face or nasal mask, or by an invasive airway (intubation).
Noninvasive Ventilation (NIV)
Noninvasive mechanical ventilation requires an alert, cooperative, hemodynamically stable patient. Ventilation is delivered by a CPAP or BiPAP device by way of a face or nasal mask.
CPAP
CPAP is Continuous Positive Airway Pressure. It provides a constant end-expiratory pressure that keeps the airway open, providing supplemental oxygen if needed. Whether breathing in or out, the continuous pressure keeps the alveoli open and the tongue forward, preventing sleep apnea.
BiPAP
BiPAP is Bi-level Positive Airway Pressure. This is similar to CPAP, but it delivers different levels of pressure during inspiration and expiration. If the patient is not a candidate for NIV or if NIV is not maintaining adequate ventilation, intubation is required for invasive mechanical ventilation.
Invasive Mechanical Ventilation
Intubation is achieved with an endotracheal tube (ETT) or tracheostomy (trach).
Endotracheal Tubes
ET tubes are inserted into the trachea through the mouth or nose. Oral intubation is preferred and more common (due to increased risk of pneumonia and sinus infections with nasal intubation). ET tubes can be left in place for several weeks, but after 10-14 days, tracheostomy is considered.
Tracheostomy
A tracheostomy is an artificial opening in the trachea, into which a tracheostomy tube is inserted.
Two basic types of ventilators are available: volume-cycled ventilators and pressure-cycled ventilators.
Volume-cycled vents are used most frequently. They deliver a constant volume of air with each breath.Pressure-cycled vents deliver a volume of gas to the airway using positive pressure during inspiration. Once the set pressure has been reached, the machine cycles off and exhalation occurs passively.
Our focus will be on volume-cycled ventilators, since they are used more frequently.
Ventilator Modes
Assist Control (AC) mode provides full support.
Synchronized Intermittent Mandatory Ventilation (SIMV) mode provides intermediate support.
Pressure Support Ventilation (PSV) mode is the stepping stone before extubation.
Assist Control (AC) Mode
This is the most common ventilation mode used in ICU. “Full support” mode means that every breath is a positive pressure ventilator breath, but it can be initiated by the machine or the patient. The patient receives a mandatory rate (RR) and a mandatory volume (TV).
Benefit of AC Mode
AC mode decreases the work of breathing. It can be used for patients who have some spontaneous breathing and those who don’t. It provides the set number of breaths every minute, but also allows the patient to initiate breaths on their own, decreasing anxiety.
Disadvantage of AC Mode:
Due to the low work of breathing, respiratory muscles weaken, which can cause breath-stacking, where air never fully exits the alveoli before another breath is taken. Every breath is the same size and if patient wants larger breaths than the set TV, it can cause anxiety, leading to tachypnea and hyperventilation.
Synchronized Intermittent Mandatory Ventilation
SIMV is another common mode used in ICU and often in surgical patients requiring ventilator support for a short time post-op. A set number of breaths are delivered each minute, but patient can breathe as often as he feels the need to; vent breaths are synchronized with the patient’s spontaneous breaths.
The ventilator breaths deliver the full TV, but patient-initiated breaths require the patient to inhale the TV independently. This helps work the respiratory muscles by providing periods of decreased support. The rate can be turned down and PT can start to be weaned.
Pressure Support
Used alone or with SIMV, this provides a small amount of pressure during inspiration to help the patient take in a spontaneous breath. The patient regulates his own respiratory rate and tidal volume.
PS provides a constant end-expiratory pressure that keeps the alveoli open, providing supplemental oxygen if needed. This mode is used as a stepping stone between a dependent ventilator mode and extubation, because it reduces the work of breathing for the patient.
Ventilator Settings
These are the four basic settings we will cover:
- Respiratory rate (RR)
- Tidal volume (TV)
- Fractional inspiration of oxygen (FiO2)
- Positive end-expiratory pressure (PEEP)
Respiratory Rate
This is the minimum amount of breaths the patient will take (between 12-18).
Tidal Volume
This is the amount of air going into the patient’s lungs with each breath (set between 400mL – 800mL, based on body weight).
Fractional Inspiration of Oxygen
This is the percentage of O2 concentration going into the lungs required to maintain adequate blood oxygen levels.
Positive End-Expiratory Pressure
This is the amount of pressure left in the circuit at the end of exhalation. It helps keep alveoli open, allowing for better oxygenation (set between 5-10 cm H2O).
Ventilator alarms
Anytime an alarm goes off, always look at your patient first! Do not immediately silence the alarm; we need to figure out why the vent is alarming. Alarm systems vary between machines, but there are 2 basic alarms:
High-pressure alarm: Increased resistance somewhere in the system, equipment obstruction, patient obstruction (coughing or secretions), or water/condensation in the tubing.Low-pressure alarm: A leak in the system or a disconnection somewhere; check all your connections.
Troubleshoot the alarm, starting with the patient and moving toward the machine.
Weaning off the ventilator
Prolonged mechanical ventilation is dangerous and expensive, with the occurrence of pneumonia increasing each day the patient is on the vent.
Certain criteria must be met for weaning to be initiated:
- Acceptable ABG’s and normal hematocrit
- Patient breathing on their own, TV > 10ml/kg
- FiO2 less than 0.5 (room air is 0.21 or 21% oxygen)
- Intact gag reflex, ability to cough and take in a deep breath
Review
2 Basic Noninvasive Ventilator Modes
CPAP – Continuous Positive Airway Pressure
BiPAP – Bi-level Positive Airway Pressure
3 Basic Invasive Ventilator Modes
AC – Assist Control – full support
SIMV – Synchronized Intermittent Mandatory Ventilation – intermediate mode
Pressure Support Ventilation (PSV) – last step before extubating
4 basic settings:
- Respiratory Rate (RR)
- Tidal Volume (TV)
- Fractional inspiration of oxygen (FiO2)
- Positive end-expiratory pressure (PEEP)
Thank you for watching this tutorial on medical ventilators!
Frequently Asked Questions
Q
What is mechanical ventilation?
A
Mechanical ventilation is a means to mechanically assist or replace spontaneous respiration in the context of medically- or physically-induced apnea or ineffective breathing.
Q
What is non-invasive vs. invasive mechanical ventilation?
A
Non-invasive ventilation provides ventilatory support via face mask or nasal mask whereas invasive ventilation provides ventilatory support via endotracheal tube (through the nose or mouth to the lungs) or tracheostomy (a surgical opening of the trachea through which a tube is placed). Both methods are then connected to a mechanical ventilator that controls the mode of delivery.
Q
What are the 2 methods of invasive mechanical ventilation?
A
Invasive mechanical ventilation can be volume-cycled or pressure-controlled (or both). Volume-cycled ventilation delivers a set volume of air (known as tidal volume) with each breath. Pressure-controlled ventilation delivers a set pressure of air using positive pressure with each breath.
Q
What are the 3 modes of invasive mechanical ventilation?
A
The 3 modes of invasive mechanical ventilation are assist control (AC) which can be either volume- or pressure-controlled, synchronized intermittent mandatory ventilation (SIMV) which can be either volume- or pressure-controlled, and pressure support ventilation (PSV).
Q
What is CPAP?
A
CPAP, or continuous positive airway pressure, is a method of non-invasive mechanical ventilation that provides constant end-expiratory pressure via face or nasal mask to keep airways open during inspiration and expiration.
Q
What is BiPAP?
A
BiPAP, or bi-level positive airway pressure, is a method of non-invasive mechanical ventilation that delivers differing levels of pressure during inspiration and expiration through face or nasal mask.
Q
What is PEEP in mechanical ventilation?
A
PEEP, or positive end-expiratory pressure, is a setting on the mechanical ventilator that reflects the pressure left in airways after expiration, that is meant to assist in keeping the alveoli open. The normal setting is 5-10 cm H2O. Excessive PEEP can damage the lungs and must be used judiciously.
Q
What is \(FiO_2\) in mechanical ventilation?
A
\(FiO_2\), or fractional inspiration of oxygen, reflects the concentration of oxygen that is in the gas administered via mechanical ventilation. 21% is equal to the amount of oxygen in room air.
Q
What is tidal volume in mechanical ventilation?
A
Tidal volume measures the total volume of air being breathed in upon inspiration. This can be controlled by the patient or by the mechanical ventilator. The normal range for the controlled delivery of tidal volume is 400–800 mL.
Q
What does a high pressure alarm mean in mechanical ventilation?
A
High pressure alarms on a mechanical ventilator indicate increased resistance in the system most commonly due to coughing, kinked tubing, or obstruction.
Q
What does a low pressure alarm mean in mechanical ventilation?
A
Low pressure alarms on a mechanical ventilator indicate a leak in the system, possibly due to disconnection of the tubing, or unintended or patient-induced extubation.
Q
What are weaning criteria for mechanical ventilation?
A
In order to consider weaning a patient off the ventilator, the patient must meet specific criteria that indicate readiness to wean. These include the improvement of the original indication for ventilation, acceptable ABGs (usually a pH > 7.25), hemodynamic stability, and the ability to breath spontaneously (with a TV > 10 mL/kg, RR 12-20, and \(FiO_2\) of < 21% [equivalent to room air]).
