Mechanical Ventilation

CHAPTER 207 Mechanical Ventilation



Joe Esherick


Modern mechanical ventilation was developed out of necessity during the polio epidemic of the 1930s. The original machines were negative-pressure ventilators, known as “iron lungs.” These devices soon became obsolete with the development of positive-pressure ventilators during the 1950s. The advent of positive-pressure ventilation ushered in the era of modern-day surgery, anesthesia, and critical care medicine.


Mechanical ventilators assist in the oxygenation and ventilation of patients. Ventilators improve pulmonary gas exchange and aim to reverse hypoxemia and acute respiratory acidosis. Mechanical ventilation also unloads the respiratory muscles and therefore significantly decreases the body’s oxygen consumption in both shock and respiratory failure. Although positive-pressure ventilation can aid in pulmonary mechanics, it can also lead to ventilator-induced lung injury if improperly applied.



Classification of Mechanical Ventilation


All modern ventilators use positive-pressure ventilation. Positive-pressure ventilation can be administered noninvasively (e.g., bilevel positive airway pressure [BiPAP] or continuous positive airway pressure [CPAP] machines); however, this chapter focuses on positive-pressure ventilation delivered by an endotracheal or tracheostomy tube.



Indications



• Inability to protect one’s airway

• Hypoxic respiratory failure

• Hypercapnic respiratory failure

• Cardiac arrest


Contraindications


Advanced directives specifying no intubation or resuscitation are the contraindications for mechanical ventilation.



Equipment and Supplies



• Ventilator

• Ballard suction catheter

• Suction canister and suction tubing

• Yankauer suction tip

• Bite block or oral airway

• Heat and moisture exchanger

• Ventilator circuit tubing

• Metered-dose inhaler adapter

• Bag-valve-mask device


Precautions



• Using inadequate sedation may lead to patient–ventilator dyssynchrony.

• High plateau pressures (>30 cm H2O) lead to increased risk of barotrauma (e.g., pneumothorax, pneumomediastinum, pneumopericardium).

• Inadequate head elevation predisposes to ventilator-associated pneumonia.

• High positive end-expiratory pressure (PEEP) levels can decrease cardiac output, leading to hypotension, and can increase intracranial pressure.

• High tidal volume (VT > 10 mL/kg predicted body weight) ventilation may cause ventilator-induced lung injury and acute renal failure.

• Prolonged mechanical ventilation predisposes to
• Stress gastric ulcers

• Subglottic stenosis and tracheomalacia

• Sinusitis

• Decubitus pressure ulcers

• Intensive care unit psychosis


Technique



Modes of Ventilation


There are four main modes of ventilation: controlled mode, assist-control (AC) mode, synchronized intermittent mandatory ventilation (SIMV) mode, and support mode (Table 207-1). Each of these modes is subclassified into volume-cycled or pressure-cycled methods of ventilation.



TABLE 207-1 Modes of Ventilation

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Controlled Mode


Controlled ventilation (or intermittent mandatory ventilation) is restricted to use in heavily sedated or paralyzed patients. It is the principal mode of ventilation used for general anesthesia in the operating room. This mode will deliver a preset VT at a specified rate independent of patient effort. The advantage to its use is that the clinician controls with absolute certainty the patient’s minute ventilation. However, the disadvantage is that heavy sedation is required to prevent the development of patient–ventilator dyssynchrony.



Assist-Control Mode


Assist-control ventilation is capable of both assisted ventilation and controlled ventilation. If patients are spontaneously breathing, the ventilator will synchronize with a patient-initiated breath and thereby assist ventilation. If the patient fails to initiate a breath within a given time (as determined by the preset ventilator rate), the machine will provide a controlled breath. AC mode can be volume-cycled (volume control), pressure-cycled, (pressure control), or a hybrid of the two (pressure-regulated volume control).


With volume control (VC) ventilation, each AC breath delivers a preset VT. In other words, both assisted breaths and controlled breaths receive a preset VT regardless of the pressure required. The machine ensures that the patient will receive a minimum number of breaths per minute, based on the set ventilator rate, even in the absence of patient effort. An advantage is that this mode requires less patient sedation. However, AC cannot limit the respiratory rate of patients who have a high spontaneous respiratory rate. Patients with obstructive lung disease (e.g., chronic obstructive pulmonary disease [COPD], status asthmaticus) and a rapid respiratory rate may develop air trapping. Air trapping can cause auto-PEEP and potentially barotrauma.


With pressure control (PC) ventilation, the ventilator delivers gas at a flow rate necessary to achieve a preset peak pressure. As in VC, the ventilator synchronizes with patient effort, when present, and ensures a minimum ventilatory rate. In PC ventilation the peak inspiratory pressure (PIP) remains constant, but its major disadvantage is that the VT varies from breath to breath depending on the dynamic lung compliance; therefore, the VT can fall to very low levels if the lungs are stiff, which can compromise the minute ventilation.


Pressure-regulated volume control (PRVC) is a hybrid between VC and PC. PRVC is essentially a volume-cycled mode of ventilation with gas flow characteristics similar to those of PC ventilation. The ventilator will deliver a preset tidal volume with each breath using a decelerating flow curve. PRVC allows the delivery of a preset VT at lower peak and mean airway pressures compared with VC.


The primary disadvantage for all modes of AC ventilation is the potential for developing auto-PEEP. At the same minute ventilation, auto-PEEP occurs with fairly equal frequency in VC, PC, and PRVC. See the discussion of auto-PEEP, later, for more details.



Synchronized Intermittent Mandatory Ventilation Mode


Synchronized intermittent mandatory ventilation is a mode of ventilation that ensures a preset minimum number of “machine breaths” while allowing spontaneous patient breaths in between. The ventilator waits for a preset time, allowing the patient to breathe spontaneously, and then delivers a “machine breath” synchronized with patient inspiratory effort. The main difference between the AC and SIMV modes is that every breath in the AC mode is ventilator assisted, whereas only a minimum number of breaths per minute are ventilator assisted in SIMV. SIMV can be administered in VC, PC, or PRVC mode; therefore, the ventilator mode can be set as SIMV/VC, SIMV/PC, or SIMV/PRVC. If necessary, pressure support (PS) or PEEP can be added to assist spontaneous breathing in the SIMV mode.



Pressure Support Mode


The PS mode is used solely for spontaneously breathing patients. All breaths in this mode are patient-initiated breaths and can be augmented by varying degrees of PS. The least amount of PS that should be added is 6 to 10 cm H2O to overcome the resistance of the endotracheal or tracheostomy tube. Higher PS levels may be applied to aid respiratory mechanics and achieve an adequate VT.



Ventilator Settings and Terminology



• Inspiratory time: The inspiratory time (IT) can be adjusted to change the inspiratory–expiratory (I:E) time ratio. The normal I:E ratio is 1 : 2. A decreased IT is often helpful for conditions requiring a prolonged expiratory time, such as severe bronchospasm (e.g., COPD, asthma exacerbations). An increased IT may be indicated for severe hypoxia refractory to high PEEP levels. A significantly prolonged IT is usually very uncomfortable, typically requires heavy sedation, and may lead to auto-PEEP.

• Triggering sensitivity: The triggering sensitivity is the amount of negative pressure/flow needed to trigger a ventilator-assisted breath. This is set in all ventilator modes while watching patient effort. The aim is to achieve optimal patient comfort.

• Positive end-expiratory pressure: PEEP is applied by regulating the pressure in the expiratory limb of the ventilator circuit. The goal of PEEP is to keep the alveoli open after expiration to increase the surface area available for gas exchange. In addition, PEEP can recruit lung volume by opening closed alveoli; it also raises intrathoracic pressure, which can decrease cardiac preload. High levels of PEEP (>10 cm H2O) can improve oxygenation so that lower levels of inspired oxygen (FIO2) can be administered. Furthermore, a high level of PEEP is often needed during lung-protective ventilation in patients ventilated either for acute respiratory distress syndrome (ARDS) or for acute lung injury. PEEP must be used with extreme caution in shock states or if there is any evidence of increased intracranial pressure because high levels of PEEP can worsen both of these conditions.

• Auto-PEEP: Auto-PEEP occurs when there is inadequate time for expiration. It causes an increase in the functional residual capacity and raises intrathoracic pressure, increasing the risk of barotrauma. Volume-cycled AC modes have a higher risk of auto-PEEP compared with the SIMV modes of ventilation. Additional risk factors include severe bronchospasm, high respiratory rates, and a high I:E time ratio.

• Tidal volume: A preset VT is used for all volume-cycled modes of ventilation (VC or PRVC). The ventilator displays both an inspiratory VT and an expiratory VT, which should be the same unless the circuit is occluded or has a leak. VT should be monitored closely when ventilating in PS mode.

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May 14, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Mechanical Ventilation

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