The days of the iron lung (shown) have passed. The management of modern ventilators is a complex, multifactorial process with which all clinicians involved in critical care should be familiar. Understanding when and how to use different ventilator modes is essential. Review our slideshow to see if you are familiar with best practices.

Image courtesy of the Centers for Disease Control and Prevention (CDC).

The decision to intubate a patient and initiate mechanical ventilation is based on a risk-benefit assessment individually tailored for each patient. Mechanical ventilation is indicated when the patient's spontaneous ventilation is inadequate to sustain life. Emergent and elective indications are shown.

In volume-cycled ventilation, a set tidal volume (TV) is delivered that is followed by passive exhalation. Gas is delivered with a constant inspiratory flow pattern, resulting in peak pressures applied to the airways higher than that required for lung distention (plateau pressure). Due to the constant volume delivered, applied airway pressures vary with changing pulmonary compliance (plateau pressure) and airway resistance (peak pressure). Volume-cycled ventilation is the most common choice as an initial ventilatory mode in the emergency department.

Image courtesy of Medscape.

In pressure-cycled ventilation, a peak inspiratory pressure (PIP) is applied. Ventilation pauses when the peak pressure is attained and passive exhalation follows. The volume delivered with each respiration is dependent on the pulmonary and thoracic compliance. Pressure-cycled ventilation is particularly beneficial in the management of patients with acute respiratory distress syndrome (ARDS), for which there is generalized alveolar dysfunction and the lungs are most vulnerable to the effects of barotrauma and volutrauma. It is important to pay close attention to the minute ventilation due to the demands for varying tidal volumes.

Image courtesy of Medscape.

Assist-control ventilation employs a control breath, which is a breath initiated by the machine without any input from the patient, and an assist breath, in which the patient makes an effort to initiate a breath, which is identified by the machine and followed by the ventilator providing the breath. This is depicted above, with the graph demonstrating no negative deflection (negative pressure generated by the patient's respiratory effort) before the control breath, compared with the assist breath.

In patients with obstructive lung diseases like chronic obstructive pulmonary disease (COPD)/asthma who are breathing rapidly, assist-control ventilation may result in increased intra-alveolar pressures (auto-positive end-expiratory pressure [auto-PEEP]) due to decreased exhalation time. This can be prevented by using low tidal volumes.

Image courtesy of Medscape.

Synchronized intermittent mandatory ventilation (SIMV) can be understood as a combination of assist ventilation, in which the patient makes an initial effort to breathe and the ventilator delivers the breath, and spontaneous breathing, in which the patient initiates and takes a breath without any support from the machine. This mode of ventilation prevents overinflation/auto-PEEP, due to the time that the patient is given to breathe spontaneously.

However, SIMV can increase the work of breathing, as the patient has to breathe against the resistance of the ventilator's tubing. To overcome this pressure, support can be applied to the spontaneous breaths

Do not use this mode in patients with left ventricular dysfunction, as SIMV can decrease cardiac output.

Image courtesy of Medscape.

Pressure support ventilation is the most commonly used weaning mode. In this, the patient initiates and takes a breath with only pressure support from the ventilator. The inflation volume and the duration of the respiratory cycle are decided by the patient. The applied pressure increases the inspired volume and also helps to overcome the resistance of the ventilator circuit. Usual applied pressures range from 5-10 cm of H₂O.

Image courtesy of Jolliet P, Tassaux D. Clinical review: patient-ventilator interaction in chronic obstructive pulmonary disease. Crit Care. 2006;10(6):236.

High-frequency oscillatory support uses very high respiratory rates (180-900 breaths per minute), with very small tidal volumes and high airway pressures.^[1] It is a commonly accepted ventilatory setting for premature infants. The above radiograph demonstrates diffuse respiratory distress syndrome in a 27-week premature infant.

In a study, the use of high-frequency oscillatory ventilation had no significant effect versus usual ventilator care, on 30-day mortality in patients with ARDS.^[2]

Image courtesy of Medscape.

Ventilator settings must be titrated for each individual patient. Studies suggest that lung protective ventilation with low tidal volumes may also be beneficial in patients who do not have ARDS at the initiation of mechanical ventilation.^[3-5]

The approach to lung protective ventilation includes preventing volutrauma, preventing atelectasis, ensuring adequate ventilation and to titrate the fraction of inspired O₂ (FiO₂) to prevent hyperoxia.^[3] To avoid hypoxemia, the FiO₂ should be titrated to peripheral oxygen saturation (SpO₂) levels of 88 to 95%.^[3]

Image courtesy of Medscape.

This radiograph shows a patient with chronic obstructive pulmonary disease (COPD) who has classic findings of a depressed diaphragm, increased retrosternal air space, and hypovascularity of the lung parenchyma. Patients with asthma and COPD benefit from a reduced inspiratory-to-expiratory (I:E) ratio in order to avoid air trapping and auto-positive end-expiratory pressure (PEEP).

Inverse ratio ventilation is helpful in cases where prolonged inspiration helps keep the alveoli open as in ARDS and helps in oxygenation. Inverse ratio ventilation means that the inspiratory time is more than the expiratory time, i.e. 2:1 instead of the usual 1:2. The downside of this strategy is that shorter expiration can result in air-trapping and auto PEEP.

Image courtesy of Medscape.

Mechanical Ventilation in ARDS

This chest radiograph is from a patient who had been in respiratory failure for 1 week with the diagnosis of the inflammatory lung condition, ARDS. This image shows an endotracheal tube, a left subclavian central venous catheter in the superior vena cava, and bilateral patchy opacities in mostly the middle and lower lung zones.

The definition of ARDS was updated in 2012 (the Berlin Definition).^[6] In the Berlin Definition, ARDS is defined by timing (within 1 wk of clinical insult or onset of respiratory symptoms); radiographic changes (bilateral opacities not fully explained by effusions, consolidation, or atelectasis); origin of edema (not fully explained by cardiac failure or fluid overload); and severity based on the PaO₂/FIO₂ ratio on 5 cm of continuous positive airway pressure (CPAP). The 3 categories are mild (PaO₂/FIO₂ 200-300), moderate (PaO₂/FIO₂ 100-200), and severe (PaO₂/FIO₂ ≤100).^[6]

Image courtesy of Medscape.

Low tidal volume ventilation is the preferred method in patients with ARDS. The NIH-NHLBI ARDS Network protocol (shown) can be followed to achieve low tidal volume ventilation.^[7] The protocol recommends setting ventilator settings to achieve initial VT of 8 ml/kg predicted body weight (PBW) and then reducing VT by 1 ml/kg at intervals ≤ 2 hours until VT is 6ml/kg PBW. The pH goal is 7.30-7.45 and a minimum PEEP of 5cm H₂O should be used and incremental FiO2/PEEP combinations should be considered to achieve the oxygenation goal of PaO₂ 55-80mmHg or SpO₂ 88-95%.^[7]

Image courtesy of NIH-NHLBI ARDS Network.

Patients with congestive heart failure should be given an initial trial of noninvasive ventilation with either bilevel positive airway pressure or continuous positive airway pressure (shown) prior to initiating mechanical ventilation. Positive-pressure ventilation serves the dual role of opening alveoli and reducing preload to the right ventricle.

Image courtesy of Medscape.

Complications associated with mechanical ventilation include the following:

• Volutrauma: high inflation volumes causing overdistention of the alveoli and alveolar rupture causing lung injury. To prevent volutrauma, use low volume lung protective ventilation.
• Barotrauma: injury to the lung due to high peak inflation pressures (>40 cm H₂O) and may result in pulmonary interstitial emphysema (shown), pneumomediastinum, pneumoperitoneum, pneumothorax, and tension pneumothorax.
• Biotrauma: multiorgan damage due to release of cytokines from the lung secondary to volutrauma or barotrauma.
• High-inspired concentrations of oxygen (FiO₂ > 0.5) result in free-radical formation and secondary cellular damage.
• The increased intrathoracic pressure from ventilation can result in a decrease in cardiac output due to decreased venous return to the right heart, right ventricular dysfunction, and altered left ventricular distensibility.

Image courtesy Medscape.

Weaning From the Ventilator

Perform the first weaning test as soon as the patient meets the following criteria^[8]:

1. Resolution of the initial reason for intubation,
2. Cardiovascular stability with minimal or no need for vasopressors,
3. No continuous sedation, and
4. Adequate oxygenation defined as PaO₂/FIO₂ ≥ 150 mm Hg with PEEP up to 8 cm H₂O.

Daily screening followed by a weaning test and then by extubation if the test is successful can shorten the intubation time without increasing the risk of reintubation

Prophylactic Non Invasive Ventilation (shown) may help to prevent postextubation acute respiratory failure and decrease the need for reintubation.^[9]

Image courtesy of Medscape.