Continuing Education Activity

Patients intubated in the field will need some form of artificial ventilation started. Bag Valve Mask (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. Recognition of basic respiratory physiology and ventilator settings is essential to start mechanical ventilation safely and correct 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. This activity reviews the different ventilator modes used in intubated patients and their pros and cons.

Objectives:

• Review the indications for ventilation.
• Summarize the initial settings on the ventilator of a patient who has just been intubated.
• Describe the various classifications of ventilators.
• Explain the importance of improving care coordination among the interprofessional team to enhance the management of a patient needing intubation and ventilation.

Introduction

Prehospital mechanical ventilation is a mainstay treatment for respiratory failure and airway management by emergency medical services (EMS).[1] Despite the wide spread of mechanical ventilation use in hospital care after intubation, EMS still practice manual ventilation in prehospital care. However, manual ventilation can lead to variable tidal volumes and potential barotrauma.[1] According to the US National Emergency Medical Services Information System (NEMSIS) public research dataset (2011–2014), EMS activations (911 calls) with reported ventilator use is approximately 1 % among over 28 million EMS activations.[2] 

Therefore there is a need to provide access and educate healthcare responders on using portable mechanical ventilation in prehospital settings based on best practices established by organizations such as the National Association of EMS Physicians (NAEMSP).[3] This chapter aims to outline basic respiratory physiology, ventilator terminology, setup, 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.

Anatomy and Physiology

Respiratory physiology can be divided into two separate processes – ventilation and oxygenation.

Ventilation is the process of carbon dioxide clearance via the lungs (i,e., alveolar ventilation). Removing carbon dioxide requires diffusion and ventilation but is more dependent on ventilation. Pulse oximetry is a dynamic measure of oxygenation, and end-tidal capnography is an active measure of ventilation. 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 ventilators in intubated patients). Oxygenation is passive, meaning it relies solely on the diffusion gradient across the alveolar membrane. Respiratory failure occurs if one or both of these respiratory process is impaired (ventilation and oxygenation).[4] 

In type 1 respiratory failure, the partial pressure of oxygen (PaO2) is less than 60 mmHg with a normal or decreased partial pressure of carbon dioxide (PaCO2). While in type 2 respiratory failure, the partial pressure of carbon dioxide (PaCO2) > 45 mmHg due to the inability to ventilate from respiratory pump failure with a pH < 7.35 and normal or reduced PaO2.

Indications

Mechanical ventilation will need to be initiated whenever a patient has been intubated since the patient is no longer breathing spontaneously. In addition, it is recommended by the NAEMSP to use mechanical ventilation once a hypoxic and/or hypercapnic respiratory failure is identified and for airway protection.[1] 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 reducing the ventilation/perfusion mismatch.[5][6] 

In one study, respiratory problems were the most common complaints for EMS calls that required ventilator use (63.9%).[3] In addition, studies have reported that prehospital intubation was associated with a decrease in mortality among patients with severe head injuries.[7]

Equipment

Mechanical ventilation techniques and parameters for prehospital care should be disease-specific and similar to the in-hospital standard of care.[1]

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.[8]

• Tidal Volume: The volume of air delivered with each breath, measured in ml.
• Respiratory Rate: the number of breaths delivered per minute
• Minute Ventilation: 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.
• Flow Rate: 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).
• I: E Ratio: 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 three preset factors: trigger, target, and termination.

• Trigger: 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
• Target: A preset inspiratory flow rate or pressure limit that the ventilator targets to generate the breath.
• Termination: The endpoint of the inspiratory breath, which could be a preset duration of inspiration, volume target, or rate of inspiratory flow.

Types of Breaths

Three different types of breaths delivered during mechanical ventilation vary based on the trigger of the breath and how the work is performed.

• Mandatory: Mandatory breaths are triggered by the ventilator, and the ventilator does all the work of inspiration.
• Assisted: The patient triggers assisted breaths, but the ventilator does the work of inspiration.
• Spontaneous: The patient triggers spontaneous breaths and does all the work of inspiration.

Categories of Mechanical Ventilation

Two broad categories of mechanical ventilation 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.

Ventilator Mode

Each ventilator mode will vary based on the trigger, the breath strategy, and the types of breaths delivered.[9]

• 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 the work of breathing, or breaths can be fully spontaneous. The target and termination of mandatory breaths vary depending on the 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 the cessation of inspiratory force generated by the patient. Tidal volume varies depending on compliance and resistance.
• Continuous Positive Airway Pressure (CPAP): There is no ventilator cycling. The ventilator provides a fixed amount of airway pressure, and breaths are entirely spontaneous.

Preparation

General Guidelines for Ventilator Settings

Before initiating mechanical ventilation in prehospital settings, patients must receive appropriate sedation and analgesia.[1] The underlying physiology and the indication for mechanical ventilation are essential to establishing proper ventilator settings, and your initial setup should target correcting that derangement.

• If the patient was intubated for airway protection only, whether for trauma, obstruction, burns, or other reasons, but there was no failure of oxygenation or ventilation, the ventilator should be set to mimic normal physiology as closely as possible.
• If the patient was intubated for hypoxic respiratory failure, the strategy should be to maximize oxygenation while keeping normal ventilation. Acute pulmonary edema or pneumonia are examples where oxygenation is the primary derangement.
• If the patient was hypercapnic but requires minimal oxygen supplementation, the strategy is to augment ventilation to return CO2 to normal and keep excess oxygenation to a minimum. The exacerbation of COPD is an example of this patient.
• In general, the clinician needs to meet the minute ventilation demanded by the patient before intubation. This is especially important to a patient with severe metabolic acidosis that requires markedly high minute ventilation to maintain appropriate respiratory compensation. An example of this would be a patient in DKA with tachypneic, hyperpneic respirations requiring both high tidal volume and a high rate to meet the minute ventilation demand.

Initial Setup

There are no prehospital validated data that provide definitive outcomes. Most parameters recommended for prehospital care by EMS are extrapolated from hospital settings. Nevertheless, lung injury from using large tidal volumes can develop quickly in only 20 minutes and is associated with increased mortality.[10]

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.[11]

1. Determine tidal volume. The initial setup parameters include tidal volumes of 6 mL/kg of ideal body weight (IBW), a plateau pressure (Pplat) <30 cmH2O, and a driving pressure (Pplat - PEEP) <15 cmH2O. IBW is a predicted weight based on the patient’s sex and height, and the calculation is below. IBW and not actual weight is used because lung volume does not change with body mass. In other words, a 65” tall male weighing 200 kg has the same size lungs as the same height male who weighs 70 kg. Therefore it is important to use actual height using a measuring tape or to ask the family for accurate height. 
2. Set respiratory rate. Fourteen to 16 breaths per minute is an average starting point for patients with respiratory failure, however, consider higher rates for patients with metabolic acidosis, as discussed above, who will require much higher minute ventilation. Note your initial EtCO2 after intubation and use this dynamic measure as a guide to monitor the adequacy of ventilation.
3. Start with PEEP of 5 cmH2O. This is usually just enough to overcome the intrinsic resistance of the ventilator circuit and maintain a physiologic amount of PEEP.
4. Titrate FiO2. Most will default to 100% FiO2 initially, but hyperoxemia can be detrimental to the patient long term, so quickly titrate FiO2, the goal being to use as little oxygen as necessary to maintain SpO2 of 92% to 98%.[12]
5. Ensure adequate sedation. Ventilator synchrony is essential to maximizing the effectiveness of ventilation, increasing patient comfort, and preventing downstream lung injury. The choice of sedative will vary depending on the local protocol.

• IBW (Men) = 50 kg + (2.3 kg x (height (in) - 60))
• IBW (Women) = 45.5 kg + (2.3 kg x (height (in) - 60))

Technique or Treatment

Monitoring

After initiating mechanical ventilation, close and continuous monitoring is required for the patient, and settings are adjusted accordingly. 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 downward 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, the goal SpO2 should be 92% to 98%, using as little oxygen as possible. Adding PEEP is another method of increasing oxygenation, as this helps prevent the collapse of the alveoli and increases the diffusion gradient for oxygen. 

Complications

Hemodynamic Considerations

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, which may lower blood pressure. This is often associated with the concurrent drop in blood pressure caused by many sedative agents. 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 the tidal volume too high, and reducing the tidal volume may lead to lower pressures and help mitigate some of this effect.

Diagnostics

This is not an exhaustive list, 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 indicate that something has dislodged the tube. Check the sensor; secretions may impair the function of the sensor.

Rising EtCO2

Increase minute ventilation (increase respiratory rate and/or tidal volume).

Worsening Hypoxia or Sudden Desaturation

In cases of sudden worsening of hypoxia, it is essential to confirm tube placement and assess breath sounds. It is also important to evaluate for any tracheal deviation or subcutaneous emphysema that may suggest the development of a pneumothorax. If hypoxia is confirmed, it is prudent to increase PEEP and/or FiO2. In addition, it is important to disconnect the patient from the ventilator and manually ventilate them with a bag valve mask and 100% oxygen.

High Inspiratory Pressures

When there is an alarm for high inspiratory pressures, it is advisable to check the circuit for any obstructions and ensure adequate sedation and ventilator synchrony. Moreover, if the high peak pressure alarms are triggered with normal plateau pressure, then underlying obstructive ventilatory defects should be suspected and treated using bronchodilators. 

Special Considerations

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 the inspired and exhaled volumes. The volume of exhaled air should equal the inspired volume, or the waveform should return to zero. If breath stacking occurs, 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.

Metabolic Acidosis

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 should avoid intubating these patients at all costs, if it is unavoidable, you must 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 of ideal body weight or lower with higher PEEP and FiO2. PEEP usually should be increased to higher than 10 cm H2O in prehospital settings without expert consultation. In these patients, lower SpO2 values of 88 % to 90% can be tolerated with permissive hypercapnia due to low tidal volumes. ARDS is not easily identifiable in prehospital settings as there are specific diagnostic criteria; however, it can be suspected when there are markedly high oxygen requirements.[13]

Clinical Significance

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 correct the underlying respiratory derangement. Improper ventilator management can worsen the acute disease process and set off an inflammatory cascade causing worsening lung injury.[14]

Enhancing Healthcare Team Outcomes

When possible, before initiating intubation and mechanical ventilation, the caregivers should 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. EMTs should coordinate their activities with the rest of the interprofessional healthcare team and consult with clinicians if necessary. Interprofessional care coordination and open communication will help drive improved patient outcomes. [Level 5]