Set Your Sights on Patient-Ventilator Synchrony

An array of ventilator modes is available today, although many are prone to patient-ventilator asynchrony. The inability to deliver assist with sufficient support in a timely fashion is a significant problem.

Studies show that between 25 percent and 50 percent of patients ventilated by conventional modes of mechanical ventilation have more than 10 percent of breaths delivered inadequately.^1-5 These numbers worsen when using noninvasive interfaces.⁶

Consequently, patients may fall into situations where both levels of assist and sedation are higher than necessary - factors now known to be detrimental and that may prolong duration of mechanical ventilation and hospital stay. The risks of mechanical ventilation increase with time. These include ventilator-associated pneumonia (VAP), ventilator induced diaphragm dysfunction (VIDD), and ventilator-induced lung injury (VILI).

 Ventilator-associated pneumonia

The most common nosocomial infection in the critical care unit is VAP, causing prolonged hospitalization and increased mortality. Most of the focus of VAP has been directed toward the role of the endotracheal tube, which negatively affects cough reflex, mucocilliary clearance, tracheal epithelial surface, and provides a pathway for bacteria.⁷

While many effective interventions have been suggested to limit VAP, clinicians also must optimize weaning practices. Ways to reduce the time of invasive ventilation include the appropriate choice and application of ventilator modes, use of noninvasive interfaces, and adequate sedation practices.

A longstanding belief has been that sedation reduces patient-ventilator asynchrony and increases comfort. In fact, deep sedation levels increase asynchrony. The patient's discomfort is induced by inappropriate delivery of ventilator assist that is blunted by the sedation.³ When breath delivery synchrony is achieved, sedation levels can be minimzed.

Ventilator-induced diaphragm dysfunction

Suggestions to limit major clinical implications of VIDD include limited use of controlled mode ventilation (CMV) in favor of partial (assisted) modes of ventilatory support that encourage spontaneous breathing whenever feasible.

Experimental application of CMV has been demonstrated to induce dysfunction of the diaphragm, evidenced by rapid decrease in diaphragmatic force-generating capacity in association with muscle atrophy, oxidative stress, structural injury, myofiber remodeling and mitochondrial dysfunction.^8,9 In support of experimental findings, researchers demonstrated that 18 to 69 hours of complete diaphragmatic inactivity and CMV in humans induced severe atrophy of diaphragm myofibers in association with increased diaphragmatic proteolysis during inactivity.¹⁰

Mechanically ventilated critical care patients frequently demonstrate extreme reductions in diaphragm force-generating capacity.¹¹ The development of diaphragm weakness is clearly more pronounced in weaning failure patients relative to post-operative patients who are successfully weaned after a few hours on mechanical ventilation.¹² These findings are also consistent with evidence that encouragement of spontaneous breathing shortens time on mechanical ventilation.¹³

Ventilator-induced lung injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are serious conditions demanding mechanical ventilation. The conditions are characterized by rapid shallow breathing due to markedly diminished compliance, increased work of breathing and elevated respiratory drive.¹⁴ If inadequately applied, positive pressure ventilation can exacerbate ALI/ARDS inducing ventilator induced lung injury (VILI). This could be due to application of too high transpulmonary pressures causing cyclic overdistension of the lungs as well as too low transpulmonary pressures causing cyclic collapse and reopening of lung tissue.

Recommendations to reduce the risk for VILI are limitation of tidal volumes and plateau pressures. The adequate level of positive end expiratory pressure is less clear.^15,16 Studies indicate that maintaining spontaneous breathing (using airway pressure release ventilation) reduces sedation and improves cardiopulmonary function, shortens duration of ventilatory support and ICU stay in ALI patients, and improves ventilation perfusion distributions in ARDS patients;  however, the impact of spontaneous breathing and partial ventilatory assist has not been evaluated in larger randomized controlled trials.^17,18