Mechanical Ventilation

Published on 14/03/2015 by admin

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Last modified 14/03/2015

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3 Mechanical Ventilation

Jairo I. Santanilla

Key Points

• Noninvasive positive pressure ventilation reduces the need for endotracheal intubation.

• Rapid titration and adjustment of the noninvasive ventilator to reduce the work of breathing are essential for the success of noninvasive positive pressure ventilation.

• Monitoring of airway pressure and use of a low–tidal volume strategy minimize the risk for ventilator-induced lung injury.

• Appropriate selection of ventilator mode, tidal volume, positive end-expiratory pressure, fraction of inspired oxygen, and inspiratory flow rate is essential to minimize the work of breathing and enhance correction of hypoxemia.

• Correction of hypoxemia is critical, but correction of hypercapnia is not.

Epidemiology

Patients with severe respiratory complaints account for about 12% of emergency department (ED) visits.¹ Almost 800,000 hospitalizations per year involve mechanical ventilation, which costs nearly $27 billion and represents 12% of all hospital costs. Although the overall number of patients requiring mechanical ventilation is small (2.8%), the relative mortality is as high as 34%.² Twenty-six percent of asthmatic patients who required intubation reported the ED as their primary source of health care.³ Thorough knowledge of noninvasive and invasive mechanical ventilation, lung-protective ventilation strategies, and methods to enhance patient-ventilator synchrony is essential in the practice of emergency medicine.

Pathophysiology

A wide variety of conditions can lead to respiratory failure. Generally, respiratory failure is characterized as hypoxic (inability to adequately oxygenate), hypercapnic (inability to adequately ventilate), or both hypoxic and hypercapnic. In addition, respiratory failure can be caused by an inability to protect the airway.

Alterations in the normal physiology and anatomy of the respiratory system can lead to respiratory failure requiring mechanical ventilation. Anatomic alterations causing airway obstruction, such as tumors, edema, direct or indirect trauma, burns, or other such pathology, may result in respiratory failure. Central nervous system alterations caused by traumatic brain injury, intoxicants, and hemorrhagic or ischemic stroke can cause overt respiratory failure or loss of protective reflexes. Diseases of the peripheral nervous system may result in hypoventilation. Primary pulmonary diseases such as pneumonia can be manifested as ventilation-perfusion mismatch. Asthma and chronic obstructive pulmonary disease (COPD) can lead to hypercapnic respiratory failure. Cardiovascular disease may be accompanied by respiratory failure secondary to acute cardiogenic pulmonary edema, cardiac arrest, myocardial infarction, acute valvular insufficiency, cardiomyopathy, or arrhythmias. Finally, global states such as shock from any cause can lead to respiratory failure.

Presenting Signs and Symptoms

Patients requiring mechanical ventilation will typically be seen in extremis. Vital signs are paramount in the initial management. A rapid history and physical examination are also important. Patients may have a wide range of heart rate, respiratory rate (RR), and blood pressure. Pulse oximetry may be difficult to perform. Patients may complain of dyspnea, chest pain, anxiety, or generalized malaise. They may have altered mental status, tachypnea, hypopnea or apnea, diaphoresis, tachycardia, or bradycardia and occasionally arrive in full cardiac arrest. The history and physical examination should be focused on determining the need for mechanical ventilation and the cause of the respiratory failure.

Differential Diagnosis and Medical Decision Making

The decision to place someone on mechanical ventilation should be a clinical one performed at the bedside. Five basic questions can assist in determining the need for mechanical ventilation. First, is the patient failing to maintain an adequate airway or protect the airway? Second, is adequate oxygenation being maintained? Third, is adequate ventilation being maintained? Fourth, is the patient’s expected clinical course such that intubation is indicated? “Yes” answers to any of these questions should prompt consideration for intubation. Finally, is the patient a candidate for noninvasive positive pressure ventilation (NPPV)? Selected patients may be given a trial of NPPV instead of intubation.

Facts and Formulas

Ideal body weight (IBW, kg)

• Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet

• Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet

Treatment

Prehospital Setting

Mechanical ventilation in the prehospital setting is typically limited to continuous positive airway pressure (CPAP) or NPPV and critical care transport ventilators. The presence of CPAP/NPPV machines is becoming more commonplace in emergency medical system (EMS) vehicles. Patients who require intubation typically need bag-valve ventilation during transport. Critical care transport ambulances are fewer in number but carry transport ventilators, which have become increasingly smaller and more complex. An important step in transporting a patient on a transport ventilator is to test the patient on the transport ventilator before leaving the facility, preferably before moving the patient to the stretcher. This allows paramedics and EMS nurses to coordinate ventilator settings with respiratory therapists at the transferring facility. Critically, it allows early determination of intolerance to the transport ventilator.

Hospital Setting

The typical setting for initiation of mechanical ventilation will be the prehospital or hospital setting. Occasionally, patients with chronic respiratory failure managed with home ventilators will arrive at the ED in acute respiratory failure; such patients should be placed on critical care ventilators while the work-up is in progress, and their home ventilator should accompany them to the ED. This functions in two ways: first, it addresses any possible complications caused by the home ventilator, and second, it allows ease of familiarity with the ventilator.

Consultation

Patients suffering from respiratory failure requiring mechanical ventilation will need to be admitted to a critical care unit or be transferred to a facility able to take care of mechanically ventilated patients. Consultation with a critical care medicine provider is preferable, though not always available.

Techniques and Methods of Mechanical Ventilation

Mechanical ventilatory support may be provided through a noninvasive or invasive approach. Furthermore, each technique may be applied with a variety of ventilator modes. The key differences in ventilatory support are determined by the trigger, the limit, and the cycle. The trigger is the event that starts inspiration: either patient-initiated or machine-initiated respiratory effort. Limit refers to the airflow parameter that is regulated during inspiration: either airflow rate or airway pressure. The cycle terminates inspiration: either a set volume is delivered (volume-cycled ventilation [VCV]), a pressure is delivered for a set period (pressure-cycled ventilation [PCV]), or the patient ceases inspiratory effort (pressure support ventilation [PSV]).

The plethora of terms associated with mechanical ventilation can cause confusion and misunderstanding, especially because some terms are used interchangeably. Knowing a few simple terms can improve understanding and aid management. The ventilator can be set to reach either a target volume or a target pressure. Other terms used for this target are cycle and limit. Volume cycled, volume limited, and volume targeted all refer to the same thing. Similarly, pressure cycled, pressure limited, and pressure targeted also refer to the same mode. “Control” breaths are ventilator-initiated breaths. “Assist” breaths are patient-initiated breaths. Therefore, a ventilator that is set on volume-targeted (cycled, limited) assist/control (AC) mode has breaths that are initiated by the patient (assist breaths) and the ventilator (control breaths) and reaches a set volume target (cycle, limit).

Modes of Invasive Mechanical Ventilation

Control Mode

Control mode ventilation (CMV) is used almost exclusively in anesthesia, but knowledge of this mode’s limitations aids in comprehension of other modes’ features (Fig. 3.1). In CMV, all breaths are triggered, limited, and cycled by the ventilator. In volume-targeted mode, the physician selects a tidal volume (VT), RR, inspiratory flow rate (IFR), fraction of inspired oxygen (FIO₂), and positive end-expiratory pressure (PEEP). The machine then delivers positive pressure and applies as much pressure as required to deliver the set VT at the set IFR. (In pressure-targeted mode the physician sets the pressure high, RR, FIO₂ and pressure low or PEEP.) Note that patients can set their own flow rate in pressure-targeted modes. The machine then delivers positive pressure and applies as much pressure as required to reach the set pressure high. The VT values generated are a function of respiratory system compliance. The patient is not able to initiate or terminate a breath. If inspiratory effort is initiated before the machine is triggered to deliver a breath, airflow would not occur regardless of the patient’s inspiratory effort. If exhalation is incomplete and the time for the machine to deliver a breath has occurred, the ventilator would provide as much pressure as necessary to cause inhalation. Imagine forcibly exhaling, or coughing, when the ventilator begins to deliver a breath. This lack of synchrony would cause distress and risk structural lung or airway injury. For these reasons, CMV is never used except for apneic, paralyzed, or anesthetized patients.

Fig. 3.1 Control mode.

Tidal volume, respiratory rate, inspiratory flow rate, FIO₂, and positive end-expiratory pressure are controlled. In this mode there is no synchronization with the patient’s respiratory effort.

Assist/Control Mode

AC mode usually provides the greatest level of ventilatory assistance (Fig. 3.2). In volume-targeted ventilation, the physician sets VT, RR, IFR, FIO₂, and PEEP. (In pressure-targeted mode, the physician sets the pressure high, RR, FIO₂ and pressure low or PEEP.) In contrast to all other modes, the trigger that initiates inspiration can be either an elapsed time interval (determined by the set RR) or the patient’s spontaneous inspiratory effort. When either occurs, the machine delivers the set VT (in volume-targeted mode) or pressure high (in pressure-targeted mode). The machine follows a time algorithm that synchronizes the machine with patient-initiated breaths. If the patient is breathing at or above the set RR, all breaths are initiated by the patient. If the patient breathes below the set RR, machine-initiated breaths are interspersed among the patient’s breaths. Work of breathing (WOB) is primarily the effort that the patient exerts to cause airway pressure to drop to the threshold that triggers onset of the ventilator. (Manipulating the sensitivity of the ventilator sets this threshold.) Furthermore, WOB may be performed to a variable degree during inspiration, depending on how much the respiratory muscles are activated. WOB with the volume-targeted AC mode may be extreme in two situations: when the VT drawn by the patient is greater than the set VT and when the patient inspires at a rate that exceeds the set IFR (see later).

Fig. 3.2 Volume-targeted, assist/control mode.

Tidal volume is controlled. A minimum mandatory respiratory rate is set and synchronized with patient effort. If the patient breathes at a rate higher than the set rate, all breaths are assisted. The dashed line represents esophageal/intrathoracic pressure dynamics. The first two breaths represent machine-initiated (M) breaths; the second two breaths represent patient-initiated (P) breaths.

In the majority of situations, AC mode is used as described earlier and is termed volume-targeted or volume-cycled ventilation. As an alternative, some ventilators allow pressure-targeted (cycled) ventilation (PCV, not to be confused with PSV, described later) (Fig. 3.3). Instead of IFR, the limit during PCV is a set airway pressure. Instead of VT, the cycle during PCV is a set inspiratory time (TI). On some ventilator models, RR and the inspiratory-to-expiratory (I : E) ratio are set, and TI is calculated from these settings. On other models, TI is available as a setting. Because VT is not set, the VT delivered varies slightly from breath to breath, depending on lung compliance, airway resistance, and patient effort. PCV may offer a slight advantage over VCV in clinical scenarios that require control of the I : E ratio, but a body of literature investigating this concept does not exist. Historically, PCV was commonly used in neonates and infants, although modern ventilators that precisely measure small VT are currently favored. PCV may be the only mode available on some portable and transport ventilators.

Fig. 3.3 Pressure-targeted, assist/control mode.

Inspiratory pressure support (PS) and inspiratory time (TI) are controlled. Tidal volume may vary from breath to breath. The minimum mandatory respiratory rate is set and synchronizes with patient effort. The dashed line represents esophageal/intrathoracic pressure dynamics. The first two breaths represent patient-initiated breaths; the third breath is a machine-initiated (mandatory) breath. PEEP, Positive end-expiratory pressure.

Synchronized Intermittent Mandatory Ventilation and Pressure Support

Synchronized intermittent mandatory ventilation (SIMV) is probably the most commonly misunderstood mode of mechanical ventilation (Fig. 3.4). The physician sets VT, RR, IFR, FIO₂, and PEEP, as in AC mode. In contrast to AC mode, however, the trigger that initiates inspiration depends on the patient’s RR relative to the set RR. When the patient breathes at or below the set RR, the trigger can be either elapsed time or the patient’s respiratory effort. In this case, WOB is equivalent to AC. When the patient breathes above the set RR, the ventilator is not triggered to assist in making spontaneous breaths in excess of the set RR. The work associated with such breaths may be quite high because the patient must generate enough negative force to pull air through the ventilator and overcome the resistance to airflow caused by the ventilator circuit tubing and the endotracheal tube (ETT), in addition to the WOB required as a result of the underlying disease process.

Fig. 3.4 Volume-targeted, synchronized intermittent mandatory ventilation (SIMV).

Tidal volume is controlled only during machine-assisted breaths. Tidal volume may vary during nonassisted (patient-initiated) breaths. The minimum mandatory respiratory rate is set. If the patient breathes more slowly than the set rate, the machine synchronizes with patient effort (third breath). If the patient breathes faster than the set rate, breaths in excess of the set rate are not assisted (second and fourth breaths). M, Machine-initiated breath; P, patient-initiated breath. The dashed line represents esophageal/intrathoracic pressure dynamics.

This limitation of SIMV can be diminished by the addition of PSV (Fig. 3.5). PSV causes inspiratory positive pressure to be applied during patient-initiated breaths that exceed the set RR. The patient initiates and terminates inspiration, thereby determining VT. Once the patient triggers pressure support, it is maintained until the machine detects cessation of patient effort, as indicated by a fall in inspiratory airflow. VT, IFR, and TI are not controlled but instead are determined by patient effort. The WOB performed during PSV involves triggering the ventilator to deliver the pressure and maintaining inspiratory effort throughout inhalation. Contrast this with machine-assisted ventilation in AC or SIMV, where WOB involves triggering the ventilator but lung inflation continues regardless of the patient’s inspiratory effort. WOB during PSV also depends on the set level of pressure support. Insufficient pressure support is associated with high WOB, which leads to a small VT and a high RR. Adequate pressure support reduces WOB and improves VT and RR. Many experts view RR as the best index of the adequacy of the level of pressure support. It should be adjusted to maintain an acceptable RR of less than 30 but preferably less than 24 breaths per minute.