Extra Topic 3.6 -- Modes of Ventilation

Question 1

What ventilator settings would you order for this patient?

(You are transferring a critically ill patient with chronic obstructive pulmonary disease (COPD), who failed to meet extubation criteria, to the ICU for postoperative mechanical ventilation.)

Answer 1

Keeping in mind that this is a critically ill patient with COPD, I would employ ventilator settings that minimize the risk of cardiovascular compromise or additional lung injury.

First, recognizing that the delivery of supraphysiologic tidal volumes ( >/= 8 mL/kg) to this critically ill patient with COPD could potentially lead to cardiovascular compromise (reduced venous return with increasing intrathoracic pressures), barotrauma, and ventilator-associated lung injury (VALI), I would set his initial tidal volumes at 6 mL/kg (ideal body weight should be used rather than actual body weight, since lung volumes more closely correlate with height than weight).

Given the risk of auto-PEEP (air trapping) associated with COPD secondary to limited expiratory flow (auto-PEEP increases the risk of barotrauma, pneumothorax, cardiovascular compromise, and VALI), and recognizing the increased morbidity and mortality associated with the development of auto-PEEP in COPD patients,

I would provide adequate sedation and attempt to prolong the expiratory time by employing low tidal volumes, a low rate of ventilation (8-12 breaths/minute), and a reduced inspiratory time.

Question 2

Wouldn’t this strategy result in increased peak airway pressures and risk the development of respiratory acidosis?

Does this concern you?

(You are transferring a critically ill patient with chronic obstructive pulmonary disease (COPD), who failed to meet extubation criteria, to the ICU for postoperative mechanical ventilation.)

Answer 2

A reduced inspiratory time does necessarily result in increased inspiratory flow rate and, subsequently, increased peak airway pressures.

However, most of the peak pressure is dissipated as gas flows through the endotracheal tube and large airways, making this an acceptable trade for an increased expiratory time, which can significantly reduce ventilator-associated complications in COPD patients.

Moreover, the increased expiratory time leads to a reduction in end-expiratory, static or plateau, and mean airway pressures, despite the increased inspiratory flow rate.

Finally, while hypercapnia and respiratory acidosis are not desirable, the benefits of avoiding significant auto-PEEP most likely outweigh the potential detrimental effects of respiratory acidosis in this patient.

Question 3

How would you wean this patient from mechanical ventilation?

(You are transferring a critically ill patient with chronic obstructive pulmonary disease (COPD), who failed to meet extubation criteria, to the ICU for postoperative mechanical ventilation.)

Answer 3

There are several options for weaning a patient from mechanically supported ventilation such as:

1. a progressive reduction in the number of mandatory breaths/minute while employing synchronized intermittent mandatory ventilation,
2. the incremental reduction of pressure-support ventilation, and
3. trials of total separation of the patient from mechanical ventilation (“T-piece trials”).

However, the most important component of separating the patient from mechanical ventilation is the resolution of the underlying condition responsible for the needed respiratory support.

Therefore, I would treat the underlying condition and employ daily T-piece trials once the inciting condition improved, the patient was hemodynamically stable, and his oxygenation was adequate.

Question 4

When would you extubate him?

(You are transferring a critically ill patient with chronic obstructive pulmonary disease (COPD), who failed to meet extubation criteria, to the ICU for postoperative mechanical ventilation.)

Answer 4

I would consider extubation when

he was awake and alert,

demonstrating active laryngeal reflexes,

generating an effective cough and clearing secretions, and

when he was able to comfortably breathe without ventilatory support for 2 hours without experiencing a deterioration of his cardiac function, mental status, or arterial blood gases.

Moreover, I would take into consideration other proposed extubation criteria such as:

1. a Pao2 above 60 mmHg with a Fio2 < 50%,
2. a PaCo2 less than 50 mmHg,
3. an arterial pH > 7.30,
4. a vital capacity > 15 mL/kg, and
5. required < 5 cm H2O.

Clinical Notes:

• Modes of Ventilation: Mechanical ventilators are typically volume or pressure cycled, although some newer models combine features of both.
  
  • Volume-cycled ventilation includes assist-control (A/C), Continuous mandatory ventilation (CMV), and synchronized intermittent mandatory ventilation (SIMV). This mode of ventilation delivers a set tidal volume, with airway pressures varying according to lung compliance and the selected flow rate.
    
    • A/C - This mode is the simplest and most effective means of providing full mechanical ventilation. In this mode, each inspiratory effort beyond the set sensitivity threshold triggers delivery of a fixed tidal volume. In order to ensure a desired minimum respiratory rate, a mandatory respiratory rate is established.
    • CMV - In this mode, the ventilator provides mechanical breaths according to a preset rate and volume, ignoring any patient respiratory effort. This mode of ventilation is uncomfortable, usually requiring patient sedation.
    • SIMV - Like CMV, this mode of ventilation delivers breaths at a preset rate and volume. However, in this mode, the breaths are synchronized to the patient’s efforts. Moreover, in contrast to A/C ventilation, patient efforts above the set respiratory rate are unassisted.
  • Pressure-cycled ventilation includes continuous positive airway pressure (CPAP), pressure control ventilation (PCV), pressure support ventilation (PSV), airway pressure-release ventilation (APRV), and several noninvasive modalities. With all of these modes, the ventilator delivers a set inspiratory pressure, with tidal volume varying according to lung compliance. Unfortunately, changes in respiratory system mechanics can result in unrecognized changes in minute ventilation.
    
    • CPAP - provides a continuous level of elevated pressure to maintain adequate oxygenation, and decrease the work of breathing. No cycling of ventilator pressures occurs and the patient must initiate all breaths. CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs.
    • Pressure control ventilation - is similar to A/C ventilation, except that each inspiratory effort beyond the set sensitivity threshold delivers a set amount of pressure support rather than a set tidal volume. This mode of ventilation is set to maintain a minimum respiratory rate and the preset pressure is maintained for a fixed inspiratory time.
    • Pressure support ventilation - provides ventilation support only when triggered by the patient. Pressure is typically cut off when backpressure causes flow to drop below a certain point. Thus, a longer or deep inspiratory effort by the patient results in a larger tidal volume. This mode is commonly used to liberate patients from mechanical ventilation by letting them assume more of the work of breathing. However, there is not sufficient evidence to indicate that this approach is more successful.
    • APRV - This mode of ventilation cycles between two different levels of CPAP - an upper pressure level (inspiratory) and a lower pressure level (expiratory). The bi-level positive airway pressure allows gas movement in and out of the lung, while maintaining continuous positive pressure. It is important to understand that the baseline airway pressure is the upper CPAP level, and that this baseline pressure is intermittently “released” (decreased) to a lower level in order to eliminate waste gas.
  • Some of the newest modes integrate volume- and pressure-targeted concepts as in SIMV + PS, in which the mandatory (SIMV) breaths deliver a target volume during inspiration and the patient’s spontaneous breaths are supported with a target pressure.