Emergency intensivists are responsible for caring for a wide range of critically ill patients in acute, uncertain medical states. Many of these patients require invasive mechanical ventilation to support their breathing and can spend extended periods of time under the care of emergency intensivists.
To shoulder this responsibility, emergency intensivists must possess expertise in ventilator management. However, surveys indicate limited education and comfort with ventilator management. Clearly, there are opportunities for improvement.
Goals and General Principles of Mechanical Ventilation
Goal
The primary goal of mechanical ventilation is to provide physiologic support while minimizing harm. To this end, mechanical ventilation is used to maintain adequate gas exchange while minimizing harm caused by strain on lung pressures, volumes, and circulation. Like many critical interventions, it is supportive; it does not repair the underlying processes that warrant its use.
Respiratory Mechanics
Mechanical ventilation provides respiratory support by generating positive pressure airflow into the patient's lungs during inspiration and allowing passive expiration. In passive or paralyzed patients, inspiration is entirely controlled by the ventilator. In patients with active breathing, inspiratory airflow occurs as a result of both patient effort and ventilator effort. To deliver a breath, the ventilator pressurizes gas to overcome resistance to gas flow (from the ventilator tubing, endotracheal tube, and airway) as well as the elastic recoil of the lungs and surrounding structures. More simply, the pressure required to inflate the lungs is determined by the resistance and compliance of the respiratory system. When resistance increases, compliance deteriorates (the respiratory system becomes stiffer, manifested as a decrease in volume per unit change in pressure), or both, higher pressure is required.
Understanding where the problem lies—high resistance or poor compliance—helps determine the initial cause of respiratory failure or sudden ventilator decompensation and guide management accordingly. Common causes of high resistance and poor compliance are shown in the figure below.
Common causes of high airway resistance and poor compliance. Areas of the respiratory circuit that contribute to resistance include the ventilator tube, endotracheal tube, and the airway to the bronchioles. Areas of the respiratory circuit that contribute to compliance include the lung parenchyma (alveoli), pleural cavity, chest wall, abdomen, and any areas outside the chest wall that exert collapsing forces on the alveoli.
Exhalation is a passive process, triggered by a pressure gradient between the higher pressure in the alveoli and the lower pressure of the ventilator. Importantly, the ventilator can apply positive end-expiratory pressure (PEEP) to reduce this pressure gradient and prevent excessive lung collapse.
Defining Breathing
The amount and timing of airflow are determined by the ventilator's inputs. The vendor specifies when the ventilator delivers a breath, how it is delivered (e.g., the applied pressure or flow rate), and when it ends through triggers, control, and loop variables.
Trigger: The trigger variable determines when inspiration occurs. It is specified as time (from the last breath) or pressure or flow to detect when the patient is inspiratory. Pressure or flow as a trigger for a patient-initiated breath is rarely clinically meaningful.
Control (or Limiting): The control variable determines how the ventilator delivers the breath. It can be either flow or pressure. In the case of flow, a breath is delivered at a specific flow rate (e.g., 60 L/min). In the case of pressure, the ventilator maintains a specific pressure during inspiration, with the pressure difference between the ventilator and the patient's lungs causing the flow to occur.
The control variable must be either flow or pressure. It's impossible to specify both simultaneously.
Cycling: The cyclic variable determines when inspiration ends and expiration begins. It can be time, volume, or flow, where flow specifies the percentage of peak inspiratory flow at the end of inspiration.
The combination of triggering, control, and cyclic variables helps define a specific ventilator mode.
Common Modes of Mechanical Ventilation
A ventilator mode is a set of rules or algorithms used to deliver breaths throughout the respiratory cycle and is the first choice when mechanical ventilation is initiated.
The names of ventilator modes and the specific algorithms used to define them may vary between ventilator manufacturers, but generally speaking, they work in the same way. The most common ventilator modes are volume control (VC)—also known as volume assist control, pressure control (PC)—also known as pressure assist control, and forms of pressure support (PS). These modes are sufficient for most, if not all, clinical situations. Other modes rely on more complex algorithms, including pressure-controlled VC (PRVC), synchronized intermittent mandatory ventilation (SIMV), and airway pressure release ventilation (APRV).
Volume Control
In VC ventilation, the same preset tidal volume is delivered with each inspiratory cycle, regardless of whether the breath is time-triggered or patient-triggered.
Key settings include:
Flow rate
Tidal volume
Respiratory rate
Fraction of inspired oxygen (FiO2)
Depending on the ventilator, the user enters either flow rate and tidal volume or inspiratory time and tidal volume. In VC ventilation, pressure is not controlled. It is a dependent variable determined by airway resistance and lung compliance. As resistance increases or compliance deteriorates, flow remains constant and pressure rises.
Advantages of VC ventilation include guaranteed tidal volume, stable minute ventilation, and the ability to specify a flow rate, which can be beneficial in conditions of high airway resistance.
Disadvantages include the potential for harmful pressure buildup in conditions of deteriorating lung compliance or high resistance.
Pressure Control
In PC ventilation, a constant inspiratory pressure is applied throughout inspiration, regardless of whether the ventilator or the patient initiates a breath.
Key settings provided include:
Inspiratory Pressure
Inspiratory Time
Respiratory Rate
PEEP
FiO2
Depending on the ventilator, the user will input either the desired total inspiratory pressure or the inspiratory pressure applied above PEEP.
In this mode, flow and tidal volume produced are dependent variables and vary with changes in airway resistance and lung compliance.
One advantage of PC is that airway and lung pressure never exceed the selected inspiratory pressure. Thus, the risk of barotrauma is minimized. Another advantage is that the patient can control their inspiratory flow rate—air flow increases proportionally with their inspiratory effort, improving patient comfort and minimizing patient-ventilator asynchrony.
A disadvantage is that the delivered tidal volume can vary. As resistance decreases or compliance increases, the same pressure may result in an excessive tidal volume. Alternatively, if resistance increases or compliance deteriorates, the same pressure may produce a smaller volume and lead to poor ventilation, carbon dioxide retention, and ventilatory failure. Pressure-Regulated Volume Control (PRVC) is a mode of mechanical ventilation that automatically adjusts inspiratory pressure to achieve a specified tidal volume using an adaptive targeting scheme.
Key settings provided include:
Target tidal volume
Inspiratory time
Respiratory rate
PEEP
FiO2
The common name for this mode, PRVC, is misleading. Flow and volume are not controlled. Pressure is the controlled variable, and flow varies with changes in airway resistance, lung compliance, and patient effort.
To achieve the target tidal volume, PRVC monitors the tidal volume produced by applied inspiratory pressure. If the tidal volume is above the target, the applied pressure is reduced for the next breath. If it is below the target, the applied pressure is increased for the next breath. In this way, PRVC allows breath-by-breath pressure adjustment to achieve the desired volume despite changing resistance, compliance, and patient effort. PRVC theoretically offers the benefits of variable flow from PC and the guaranteed minute ventilation of VC, without requiring the user to adjust inspiratory pressure.
A disadvantage of PRVC is the potential for patient-ventilator asynchrony in patients with a high respiratory drive. If the patient's work of breathing increases and a large tidal volume is produced at a given inspiratory pressure, the adaptive target strategy will continue to reduce inspiratory pressure with each subsequent breath. This results in reduced ventilator support and increased work of breathing. Therefore, PRVC should be used in patients with stable respiratory drive.
Pressure Support
PS ventilation is a mode of mechanical ventilation that controls inspiratory pressure, but all breaths, flows, and inspiratory duration are determined by the patient.
Key delivery settings include:
Inspiratory pressure
Percentage of peak inspiratory flow at which inspiration is terminated
PEEP
FiO2
In PS, there is no set respiratory rate or inspiratory time. All breaths are triggered by the patient, and inspiration continues until inspiratory flow decays below a selected value (e.g., 30% of peak flow).
Because the patient controls inspiratory flow rate, inspiratory duration, and respiratory rate, PS is often used to wean patients from mechanical ventilation. Patients with low respiratory drive, high oxygen consumption, or elevated airway resistance are not suitable for PS and are not typically used in the emergency department.
Assessing Respiratory Mechanics
Assessing Respiratory Mechanics in Different Modes
Changes in resistance and compliance manifest differently in different mechanical ventilation modes.
Volume Control
In VC ventilation, increased resistance, worsening compliance, or both lead to increased pressure. These two conditions can be distinguished by comparing the maximum pressure during inspiration, or peak pressure, and the pressure required to maintain lung inflation after inspiratory flow ceases, or plateau pressure.
Peak pressure is simply the highest pressure observed in the pressure vs. time relationship displayed on the ventilator. Plateau pressure is measured by performing an inspiratory hold maneuver, in which the ventilator stops airflow at the end of inspiration and measures the pressure in the breathing circuit. Because the lungs are fully inflated and expiration has not yet occurred, this represents the total lung pressure at the specified volume, including positive end-expiratory pressure (PEEP).
When the limiting factor in gas delivery is airflow resistance (e.g., airway obstruction vs. normal lungs), a large difference between peak and plateau pressures will be observed. When the limiting factor is respiratory system compliance (e.g., widely patent airways, diffuse alveolar disease), a smaller difference will occur.
Pressure Control or Pressure Support
In PC and PS ventilation, inspiratory pressure is fixed to the value set by the operator. Therefore, increased resistance or decreased compliance will result in an observed decrease in tidal volume, and it is impossible to reliably distinguish between the two.
Pressure-Regulated Volume Control
As an adaptive mode, PRVC varies inspiratory pressure based on the mechanics of the respiratory system. Increased resistance or decreased compliance causes the ventilator to increase inspiratory pressure to achieve the target tidal volume. The difference between peak and plateau pressures can be measured by inspiratory hold, just as in VC.
Intrinsic Positive End-Expiratory Pressure
A specific cause of decreased respiratory compliance occurs when the patient does not fully exhale before the next breath begins. Air is trapped in the lungs, and the retained pressure exceeds the applied positive end-expiratory pressure (PEEP), or intrinsic positive end-expiratory pressure (PEEP).
With the development of intrinsic positive end-expiratory pressure (PEEP), increasingly higher inspiratory pressures are required to deliver the required tidal volume. If uncontrolled, this can lead to pneumothorax or impaired venous return and cardiovascular collapse.
Intrinsic PEEP is measured using the expiratory hold maneuver. This maneuver stops airflow at the end of expiration and assesses the pressure at that point. This pressure represents the sum of the reserve pressure (auto-PEEP) and the applied pressure (PEEP), and is referred to as total PEEP. Typically, total PEEP will equal the applied PEEP. When air trapping occurs, the total PEEP measurement will be higher than the applied PEEP due to intrinsic PEEP.
When the flow versus time display does not return to a zero baseline at the end of expiration (representing incomplete expiration), it can often provide clues to the presence of air trapping and intrinsic PEEP. However, relying on this display can miss significant intrinsic PEEP, so an expiratory hold maneuver should be performed.
Intrinsic PEEP may occur due to severe bronchospasm, inappropriate ventilator settings, or both. Strategies to reduce intrinsic PEEP include reducing respiratory rate and shortening inspiratory time to prolong expiration.
Driving Pressure
Driving pressure is defined as the difference between plateau pressure and total PEEP. Conceptually, it represents the pressure above PEEP required to maintain lung expansion to the selected tidal volume. Static lung compliance is equal to tidal volume divided by driving pressure.
Thus, driving pressure is inversely proportional to lung compliance. A less compliant, or "stiffer," lung will require a higher driving pressure to achieve the same tidal volume.
Driving pressure is strongly associated with mortality in patients with ARDS, and values less than 15 cm H2O are considered protective.
When to Measure Respiratory Mechanics
Inspiratory and expiratory hold maneuvers are key to understanding a patient's respiratory mechanics. However, they should only be measured when the patient is passive and compliant with the ventilator. Otherwise, the patient's negative inspiratory effort will lead to an underestimation of plateau pressure and an overestimation of lung compliance. Importantly, paralysis should not be administered solely for the purpose of obtaining accurate measurements. Initial Settings
Tidal Volume
For most patients, the initial tidal volume should be 6 to 8 cc/kg predicted body weight, adjusted as needed to ensure a plateau pressure of ≤30 cm H2O.
Patients without ARDS can tolerate higher tidal volumes of 10 mL/kg predicted body weight without adverse effects. However, ARDS is often underestimated, so a target of 6 to 8 cc/kg predicted body weight is recommended for most patients, although a target of less than 6 cc/kg predicted body weight is recommended for patients with ARDS.
If using a PC, inspiratory pressure should be set to achieve these targets, and patients should be reassessed continuously to avoid excessive tidal volumes.
Positive End-Expiratory Pressure
Positive end-expiratory pressure (PEEP) should be set for all patients to minimize traumatic opening and closing of alveoli, known as atelectasis.
In the setting of ARDS, a higher PEEP value (5 mmHg) should be selected to minimize atelectasis, intrapulmonary shunt, and pulmonary edema, thereby reducing venous return and afterload.
PEEP optimization is a complex topic, with no single, consensus approach. A straightforward approach is to set PEEP based on the PEEP/FiO2 table used by the ARDS Network, which demonstrated a mortality reduction with low tidal volume ventilation in ARDS. Another strategy is to maximize compliance by setting PEEP at the level that results in the lowest driving pressure.
Respiratory rate
The initial respiratory rate should provide adequate ventilation and comfort for the patient. For most patients, 14 to 18 breaths per minute is reasonable. However, for patients with metabolic acidosis (such as salicylate overdose), the respiratory rate should be increased to match or exceed their pre-intubation minute ventilation. Failure to do so may worsen acidosis and precipitate complications, such as cardiac arrest.
Fraction of Inspired Oxygen
In the setting of hypoxia, FiO2 should initially be set to 100% and then rapidly stopped to achieve a PaO2 of 60 to 100 mmHg or an SpO2 of 92% to 96%.
Common Troubleshooting Methods
Elevated Peak Airway Pressure
As previously mentioned, plateau pressure should be measured using an inspiratory breath-hold technique to distinguish between conditions of high resistance (large peak-to-plateau pressure gradient) and poor compliance (small pressure gradient).
Asynchrony
Patient-ventilator asynchrony occurs when mechanical ventilation mimics but does not match the patient's spontaneous respiratory mechanics. It is common and can increase the work of breathing, cause patient discomfort, and reduce the effectiveness of ventilatory support.
Asynchrony is important to identify and can be easily identified on the ventilator waveform. There are three main types of patient-ventilator asynchrony: flow, trigger, and cycling.
Flow asynchrony, or underflow, occurs in VC ventilation when the flow rate does not meet the patient's needs. On the pressure vs. time curve, the normally convex shape becomes concave, and the observed airway pressure decreases. Underflow can be addressed by increasing the flow rate or switching to PC ventilation.
Trigger asynchrony occurs when the patient triggers too many or too few breaths. The most common types of trigger asynchrony are ineffective triggering, auto-triggering, or double triggering, and can occur in any of the previously discussed modes.
Ineffective triggering occurs when the ventilator does not deliver a breath after the patient's inspiratory effort. The most common cause is improper flow or pressure triggering settings. Clinicians observe a negative deflection in the flow or pressure vs. time curve (indicating a patient inspiratory effort) instead of a breath that follows the ventilator. Conversely, auto-triggering occurs when the ventilator delivers a breath without the patient's inspiratory effort. This is often caused by condensation in the ventilator tubing, vigorous cardiac activity, or circuit leaks when the flow or pressure trigger sensitivity is too sensitive. Adjusting the trigger sensitivity can often resolve both ineffective and auto-triggering issues. Figure 12 illustrates these two scenarios.
Cycling asynchrony occurs when inspiratory flow stops prematurely or continues into the patient's spontaneous expiratory phase. Figure 13 shows an example of a patient's expiratory phase beginning before the end of the ventilator-delivered breath. In PC, VC, and PRVC, this can be addressed by shortening or lengthening the inspiratory time, respectively. In PS, this issue is addressed by reducing or increasing the percentage of peak flow when cycling from inspiration to expiration occurs.
Double triggering occurs when a second breath is triggered immediately after the first, often referred to as "stacked breaths." This most commonly occurs in VC, PC, or PRVC and is due to a form of cycling asynchrony called premature cycling, in which the patient's respiratory drive exceeds the volume or inspiratory time delivered by the ventilator. Increasing sedation, tidal volume or flow, inspiratory pressure, or inspiratory time can correct this form of double triggering.
Leak
If the measured expiratory volume does not equal the inspiratory volume, or if the volume vs. time curve does not return to baseline before the next breath, a leak should be suspected.
Pitfalls
Inadequate Sedation
Attempting to troubleshoot respiratory mechanics or dyssynchrony without adequate analgesia will result in inaccurate measurements and misleading waveforms. Therefore, ensure adequate sedation before measuring plateau pressures, checking intrinsic PEEP, making significant changes to settings, or switching modes.
Assuming the ventilator mode addresses poor respiratory mechanics
As mentioned previously, ventilator settings should be selected to target a plateau pressure below 30 cm H2O and a driving pressure below 15 cm H2O. However, when lung compliance is extremely poor, these targets may be unachievable. Switching modes will not change this situation and may be harmful. For example, switching from VC to PC to achieve a lower inspiratory pressure will result in smaller and potentially inadequate volumes and hypoventilation.
Failure to Reassess
As with any intervention, reassessment is key. Airway pressure, tidal volume, oxygenation, and synchrony should be monitored frequently, especially as the patient's condition changes.
Discussion
Mechanically ventilated patients are common in the emergency department. Unfortunately, these patients sometimes remain in the emergency department for prolonged periods, leading to prolonged mechanical ventilation, prolonged ICU stays, and increased mortality.
Early ventilator management presents an opportunity for improvement. In fact, in one observational study, fewer than half of ED patients with confirmed ARDS received low tidal volume ventilation. This is particularly concerning, as ventilator-induced lung injury can occur in as little as 20 minutes. Patients without ARDS may also be at risk, as high tidal volumes within the first 48 hours are associated with subsequent development of ARDS. Fortunately, best practice strategies can be successfully implemented in the ED to reduce mortality, duration of ventilation, and length of hospital stay.
Education presents another opportunity for improvement. Ventilated patients are often managed by physicians without specialist training, and there is evidence that mechanical ventilation education and management are inadequate in this group. Furthermore, a survey of attending EM physicians revealed that many had received three hours or less of ventilator training in the past year, and many had completed ventilator training. Respiratory therapists were identified as primarily responsible for ventilator management. Higher ventilator management scores were associated with prioritizing mechanical ventilation during physician residency training; however, a previous study of EM residents reported infrequent exposure to mechanical ventilation and minimal mechanical ventilation education.
All of this data highlights several important findings. It is increasingly important for EM providers to understand various modes of mechanical ventilation, initiate best-practice ventilator settings early, and recognize and treat ventilator complications when they occur. Understanding the causes of changes in resistance and compliance, how they are represented graphically, and their anatomical correlations is crucial for troubleshooting.
Clinical Care Essentials
Acute respiratory failure requiring invasive mechanical ventilation is a common presentation to the emergency department. EM providers can further improve the care of these patients by understanding common modes of mechanical ventilation, recognizing changes in respiratory mechanics, and adjusting ventilator settings and therapy accordingly.
Key Points
* Respiratory failure requiring mechanical ventilation is common; emergency medicine providers must possess expertise in ventilator management.
* The ventilator overcomes both respiratory system resistance and compliance to deliver airflow. The relative contribution of each factor is easily measured and can help guide management.
* No single mode of mechanical ventilation is perfect. Each has advantages and disadvantages and can be adapted to most clinical situations. The best mode is generally the one with which providers and clinicians are most familiar.
