The goal of this chapter is to go through basic respiratory physiology, ventilator terminology, set up and diagnostics. Portable ventilators are becoming increasingly robust in capability, and their prehospital use is more widespread, whether for inter-facility transport or long distance emergency transports.
Respiratory physiology can be broken down into 2 separate processes – ventilation and oxygenation.
Ventilation is the process of air moving in and out of the lungs.
Oxygenation refers to the diffusion of oxygen from the alveolus into the blood. Ventilation is an active process, meaning the lungs cannot ventilate without the actions of the respiratory muscles (or mechanical ventilator with the intubated patient). Oxygenation is passive, meaning it relies solely on the diffusion gradient across the alveolar membrane. The removal of carbon dioxide requires both diffusion and ventilation but is more dependent on ventilation. Pulse oximetry is a dynamic measure of oxygenation, end-tidal capnography is a dynamic measure of ventilation.
Mechanical ventilation will need to be initiated whenever a patient has been intubated since the patient is no longer breathing spontaneously; however, the positive effects of mechanical ventilation may also be an indication to intubate in the first place. If the patient has a failure of ventilation (hypercapnia) or failure of oxygenation (hypoxia), mechanical ventilation allows the provider to correct these derangements by reducing work of breathing, controlling minute ventilation, increasing alveolar recruitment to improve gas exchange and reduce the ventilation/perfusion mismatch.
Basic Variables of Mechanical Breath
Before discussing the mechanics of the ventilator, it is important to understand the basic variables that determine the generation of a mechanical breath.
The volume of air delivered with each breath, measured in ml.
the number breaths delivered per minute
The volume of air exchanged over one minute, determined by the tidal volume multiplied by the respiratory rate, expressed in liters per minute (L/Min).
Peak Airway Pressure
The maximum pressure exerted on the airways during inspiration, measured in cm H2O.
The rate of inspiratory flow required to overcome the resistance of the circuit to deliver the inspiratory breath, measured in liters per minute (L/Min).
The ratio of inspiratory time compared to expiratory time.
Fraction of Inspired Oxygen (FiO2)
The percentage of oxygen in the inspired air.
Positive End Expiratory Pressure (PEEP)
The airway pressure applied at the end of exhalation, measured in cm H2O.
Modes of Ventilation
Mechanical ventilators have several modes of ventilation that deliver breaths based on 3 preset factors called the trigger, target, and termination.
How the breath is initiated, which could be ventilator-initiated based on a preset time or rate, or patient-initiated based on negative pressure generated by the patient
A preset inspiratory flow rate or pressure limit that the ventilator targets to generate the breath.
The end point of the inspiratory breath which could be a preset duration of inspiration, volume target, or rate of inspiratory flow.
Types of Breaths
There are 3 different types of breaths delivered during mechanical ventilation that vary based on the trigger of the breath and how the work is performed.
Mandatory breaths are triggered by the ventilator, and the ventilator does all the work of inspiration.
The patient triggers assisted breaths, but the ventilator does the work of inspiration.
The patient triggers spontaneous breaths, and the patient does all the work of the inspiration.
Categories of Mechanical Ventilation
There are 2 broad categories of mechanical ventilation that are determined by the breath strategy, or the target and termination of the inspiration.
Volume Control (Volume Limited)
Volume limited breaths target a preset flow rate, and inspiration is terminated when a preset volume is achieved. Airway pressures are determined by the intrinsic resistance of the circuit and airways, and lung compliance.
Pressure Control (Pressure Limited)
Pressure limited breaths target a preset inspiratory pressure, and inspiration is terminated when a set inspiratory time is achieved. Tidal volume and ultimately, minute ventilation, are variable and dependent on lung compliance and airway resistance.
Each ventilator mode will vary based on the trigger, the breath strategy, and the types of breaths delivered.
Controlled Mechanical Ventilation (CMV)
All breaths are mandatory and triggered by the ventilator based on the preset respiratory rate, with the target and termination depending on volume or pressure limited strategy. There are no assisted or spontaneous breaths.
Assist/Control Ventilation (AC)
Mandatory breaths are triggered by the ventilator at a preset minimum respiratory rate; however, this mode allows for patient triggered assisted breaths as well. The target and termination are determined by volume or pressure limited breath strategy. There are no spontaneous breaths.
Pressure Regulated Volume Control (PRVC)
A similar mode to assist/control; however the ventilator will adjust the inspiratory flow rate to regulate the amount of pressure delivered to the airways.
Intermittent Mechanical Ventilation (IMV)
Mandatory breaths are triggered by the ventilator at a preset rate. However, this mode allows for patient triggered breaths which can be either spontaneous or assisted depending on the settings. The ventilator can be set to provide a level of pressure support to spontaneous breaths to reduce work of breathing, or breaths can be fully spontaneous. The target and termination of mandatory breaths vary depending on breath strategy.
Pressure Support (PS)
Breaths are fully patient triggered, and the ventilator delivers a set driving pressure which each breath. Inspiration is determined by cessation of inspiratory force generated by the patient. Tidal volume varies depending on compliance and resistance.
Continuous Positive Airway Pressure (CPAP)
There is no cycling of the ventilator. The ventilator provides a fixed amount of airway pressure and breaths are entirely spontaneous.
General Guidelines for Ventilator Settings
Understanding the underlying physiology and the indication for mechanical ventilation is essential to establishing proper ventilator settings, and your initial set up should target correcting that derangement.
Initial Set Up
Choosing a mode of ventilation is somewhat arbitrary (initially, at least), and will most likely depend on provider experience, comfort level with various modes, and/or local policies/protocols. For the purpose of this review, this chapter strictly discusses volume assist/control, as transport ventilators vary widely in their settings, but AC is standard across all.
The ventilator is not a "set-and-forget" device. Just as a clinician continuously reassesses vital signs, they should do the same for ventilator settings. Without the ability to check blood gases in the field, rely on pulse oximetry and end-tidal capnometry to guide adjustments.
End Tidal Capnometry (EtCO2)
This is a rough approximation of the PaCO2 or the pressure of carbon dioxide in the blood as it passes through the alveoli. Goal EtCO2 for most patients is around 40 to 45 mm Hg. Keep in mind actual PaCO2 may be higher than EtCO2, but trending is as important as the absolute number. For example, if the patient was intubated for respiratory failure secondary to COPD exacerbation and the initial EtCO2 was 80 mm Hg, after mechanical ventilation, this number should trend down toward normal. If EtCO2 trends upward, this is a sign of inadequate ventilation, and the minute ventilation must be increased by increasing first the respiratory rate and then, tidal volume. Avoid tidal volumes beyond 8 ml/kg to prevent lung injury.
Pulse Oximetry (SpO2)
This is a dynamic monitor of oxygenation. Again, goal SpO2 should be 92% to 98%, using as little oxygen as possible. If SpO2 is 100%, back down on the FiO2. 21% FiO2 is considered room air. Adding PEEP is another method of increasing oxygenation, as this helps prevent collapse of the alveoli and increases the diffusion gradient for oxygen. When possible PEEP beyond 10 cm H2O should be avoided to reduce the incidence of lung injury.
Under normal conditions, the chest cavity is under negative pressure, and the negative inspiratory pressure generated by the expansion of the chest cavity not only pulls air into the lungs, but it also augments venous return to the heart. When the patient transitions to positive pressure ventilation, venous return to the heart (and thus preload) is reduced, and it may reduce blood pressure as a result. This is often associated with the concurrent drop in blood pressure caused by many of the sedative agents used. The clinician's response to this drop in blood pressure ultimately depends on the patient and is beyond the scope of the chapter; however, a common error is setting tidal volume too high, and reducing the tidal volume may lead to lower pressures and help mitigate some of this effect.
This is not an exhaustive list by any means, but there are several common scenarios and the initial moves to diagnose and correct them.
Sudden Loss of EtCO2 Waveform
Check the tube placement. Losing the end-tidal waveform may be a sign that something has dislodged the tube. Check the sensor; secretions may impair the function of the sensor.
Increase minute ventilation (increase respiratory rate and/or tidal volume).
Worsening Hypoxia or Sudden Desaturation
Again, confirm tube placement. Listen to breath sounds, check for tracheal deviation or subcutaneous emphysema that may suggest the development of a pneumothorax. Consider increasing PEEP and/or FiO2. You may need to remove the ventilator and manually ventilate with a bag-valve mask and 100% oxygen.
High Inspiratory Pressures
Check the circuit for any kinks or obstructions. Ensure adequate sedation and ventilator synchrony. Treat any underlying obstructive disease. Monitor for breath stacking.
Breath stacking occurs when the patient does not fully exhale, thus with each successive breath the volume of air in the lungs (and as a result airway pressure) rises. This is dangerous and puts this patient at risk for barotrauma. This can occur when patients have severe obstructive disease, particularly asthma. Some ventilators will either display the volume waveform or display inspired and exhaled volumes. The volume of exhaled air should equal the inspired volume, or the waveform should return to zero. If breath stacking is occurring, briefly disconnect the ventilator circuit, push on the patient’s chest to exhale all the excess volume, and reconnect the ventilator. Reduce the respiratory rate to allow more time between breaths for exhalation, and if the machine has the capability, decrease the inspiratory time of the breath.
Patients with severe metabolic acidosis such as diabetic ketoacidosis rely on respiratory compensation to mitigate the acidemia, and thus will require significant minute ventilation above normal. While you ought to avoid intubating these patients at all cost, if it is unavoidable, you will need to be extremely careful with setting the respiratory rate. Try to match what rate the patient was breathing before intubation. Note the initial end tidal immediately after intubation and try to maintain that number or lower. Failure to do so will cause worsening acidemia and further decompensation of the patient’s condition.
Acute Respiratory Distress Syndrome (ARDS)
ARDS is a complicated condition of severe lung injury and inflammation. Ventilation strategies for these patients should be aimed at minimizing lung injury. Tidal volumes in these patients should be reduced to 6 ml/kg or lower with higher PEEP and FiO2. PEEP should still not go higher than 10 cm H2O without expert consultation. In these patients, you may wish to tolerate a SpO2 of 88 % to 90%, and you may also be forced to allow for permissive hypercapnia due to such low achievable tidal volumes. You may not know in the acute setting the patient has ARDS as there are specific diagnostic criteria; however, if you note markedly high oxygen requirements, consider ARDS as an underlying diagnosis and adopt this lung protective strategy.
Patients intubated in the field will need some form of artificial ventilation started. BVM ventilation is unreliable and inconsistent and requires a dedicated provider to continue ventilation. Using mechanical ventilators in the prehospital environment allows for precise control of ventilation, particularly in areas with longer transport times. Understanding basic respiratory physiology and ventilator settings is essential to start mechanical ventilation safely and correcting the underlying respiratory derangement. Improper ventilator management can not only worsen the acute disease process but can also set off an inflammatory cascade causing worsening lung injury.
When possible, before initiating intubation and mechanical ventilation make every effort to obtain the patient's wishes. Medical Orders for Life-Sustaining Treatment (MOLST) and DNR orders should be considered. Mechanical ventilation is a complex process and requires teamwork and interprofessional coordination. When possible, alert the receiving facility to be prepared to continue ventilation and ensure a smooth transition of care.