Discuss various methods of mechanical ventilation.
APRV as a treatment for Hypoxemia
The diagnosis and management of refractory hypoxemia involves various methods of mechanical ventilation. It is clear that even while the various methods of mechanical ventilation are potentially life-saving in many instances, they can themselves be responsible for lung damage. Injured lungs of patients with refractory hypoxemia have often been attributed to ventilator induced injuries. For patients with refractory hypoxemia then, new ventilator approaches are essential for their safe treatment. This is where the open-lung strategies come in. Airway Pressure Release Ventilation (APRV) and High Frequency Oscillatory Ventilation (HFOV) have been suggested as refractory hypoxemia treatment mechanisms with minimal lung injury (Zhou et al. 2017). APRV however tends to have more advantages that include allowing for spontaneous breathing, reduced sedation and shorter duration of mechanical ventilation. In these aspects then, APRV is considered more advantageous. This paper assesses these advantages.
One of the biggest benefits of APRV as a mechanical ventilation method is its capacity to offer spontaneous breathing to the victims of refractory hypoxemia in a short period of time. With spontaneous breathing during APRV, there is improved ventilation-perfusion matching, reduced intrapulmonary shunt, and even smaller dead space. With spontaneous breathing, the diaphragmatic muscle conditions are maintained (Zhou et al. 2017). For the patient with refractory hypoxemia, the discomfort that is caused by the losses of aerated lung tissue to the respiratory process always call for improvement. This discomfort demands that the ventilation method chosen be one that can offer relief in the short and long terms. APRV has the advantage of a better patient-ventilator synchrony (Facchin & Fan, 2015). It is this synchrony that eventually gives way to spontaneity in breathing. Once this spontaneous breathing is attained, then the comfort that the patient dearly needs is achieved.
The spontaneity in breathing is also seen through the manner in which the gas is distributed during the ventilation process to the nondependent regions of the lungs. The even distribution of gas to these regions of the lungs allows the lungs to operate in the normal capacity thus obtaining a regular breathing pattern (Facchin & Fan, 2015). APRV has a role to play in improving the ventilation and perfusion matching through the promotion of a more physiological gas distribution. It is important to notice in terms of spontaneity that spontaneous breathing relates differently with the ventilator strategies depending on which type of synchronization. APRV, being a non-synchronized mode, offers better protection to the lungs through reduction of lung strain and needless stress.
The use of sedatives during mechanical ventilation is a pretty standard practice. While the use of sedatives has demonstrable benefits, it is also true that is is laden with a number of disadvantages. Most notably is that fact that sedation reduces the manner in which the patient interacts with his or her environment and also increases the chances of pulmonary complications (Facchin & Fan, 2015). When the patient cannot interact with their environment consistently, it is highly unlikely that they would develop a synchronized breathing pattern which is essential for their recovery.
Research has shown that with reduces sedation, there is increased chances of lung-protective benefits to the patient. This lung-protection quality relies on the setting selection to make sure that there is ventilation on the advantageous portion of the pressure-volume curve (Daoud et al. 2012). The protection of the lungs also come in form of the long inflation time which results into less frequent inflation and deflation thereby avoiding shear stress to the alveoli. It has been proved that the use of sedation leads to the depression of cough reflex and the much fatal increased risk of aspiration of the pharyngeal secretions (Fredericks et al. 2020). Overall then, the decreased sedation increases the chances that spontaneous breathing will be possible and also opens up the possibility for the patient to interact more with their environment thus enhancing their chances of speedy recovery. Neuromuscular blocking agents, which also almost do the work that sedatives do of providing the temporary paralysis in critically ill patients still remain a controversial method of offering the sedation and attract mixed reactions on their effectiveness.
Shorter duration of mechanical ventilation
In line with the desired spontaneous ventilation for the patient with refractory hypoxemia, one of the chief concerns would be how quick the patient would be able to get off the mechanical ventilation and resume breathing on their own without any visible strain, stress or difficulty. When the patient has an unrestricted spontaneous breathing, they can cough during mandatory breaths. This contributes to the secretion clearance and possibly even the reduction in the VAP risk (Daoud et al. 2012). This can eventually lead to reduced duration of ventilation. This reduced time is however only possible because APRV, unlike HFOV, is applicable in less serious cases of the disease. Patients who are still in the more acute stages would need more time for ventilation and certainly more sedation.
Nowhere is the influence of APRV ventilation seen more than in the morbidly obese patients with already compromised lung functions (Facchin & Fan, 2015). For these patients, it is essential that the oxygenation capacity for their lungs is improved quickly following reduced lung volumes, compliance and the inefficiency of the muscles. For these patients, the positive pressure that comes about as a result of the APRV keeps lung recruitment alive while also preventing overdistention (Fredericks et al. 2020). When the ventilation time is reduced greatly, then the inflation time also lengthens, increasing the capability of the lungs to perform normally. Because hypoxemia is mainly caused by shunting following alveolar collapse and the reduction of the functional residual capacity, the lung units with limited compliance have short time constants while thse with normal compliance have longer time constants. Reduction in ventilation time has a direct impact on the ventilation time and how it eventually affects the breathing synchrony.
The limited duration of mechanical ventilation is also associated with increased cardiac output. When the pleural pressure decreases and abdominal pressure increases, then the increased cardiac output is realized (Daoud et al. 2012). When spontaneous breathing is suppressed in any form during mechanical ventilation, the cardiovascular function is compromised due to a decreased venous return and therefore the cardiac output. Patients on APRV have an increased cardiac index and oxygen delivery.
In conclusion, it is true that mechanical ventilation has a huge role to play in restoring the health of those who suffer from refractory hypoxemia. Treating this condition may involve the open-lung strategies of APRV and HFOV. These two strategies besides offering relief to the patient, also reduce the risk of ventilator-induced lung injury (VILI) to these patients. Of the two, APRV proves to be more advantageous in terms of how in improves spontaneous ventilation, reduces the chances of sedation and shortens the period of mechanical ventilation. In the end, APRV and HFOV ought to be used complementarily, with the latter used when the patient is acute and the former I situations where the patient has improved and has more spontaneous breathing.
Daoud, E.G., Farag, H.L., & Chatburn, R.L. (2012). Airway Pressure release ventilation: What do we know? Respiratory Care, 57(2), 282-292.
Facchin, F. & Fan, E. (2015). Airway Pressure release ventilation and high-frequency oscillatory ventilation: Potential strategies to treat severe hypoxemia and prevent ventilator induced injury, Respiratory Care, 60(10). 1509-1521.
Fredericks, A.S., Bunker, M.P., Gliga, L.A., Ebeling, C.G., Ringqvist, J.R., Heravi, H., Manley, J., Valladares, J., & Romito, B.J. (2020). Airway pressure release ventilation: A review of the evidence, theoretical benefits and alternative titration strategies, Clinical Medical Insights, 14: 1-9.
Zhou, Y., Jin, X., Lv, Y., Wang, P., Liang, G., Wang, B., & Kang, Y. (2017). Early application of airway pressure release ventilation may reduce the duration of mechanical ventilation in acute respiratory distress syndrome, Intensive Care Med, 43:1648-1659.