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Mechanical Ventilation in ARDS

Mechanical ventilation is often used in the management of Acute Respiratory Distress Syndrome (ARDS). ARDS occurs due to acute inflammation of the alveoli capillaries, resulting in severe oxygen exchange disruption. Many cases of ARDS in the Intensive Care Unit (ICU), with mortality rates between 27-45%.

Mechanical Ventilation in ARDS
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Many factors cause ARDS, but the most frequent etiology is pneumonia. Mechanical ventilation is needed to ensure optimal gas exchange. However, the use of mechanical ventilation must be careful because it can cause lung injury and multiple organ failure.

Tidal Volume and Breath Frequency

The 2017 American Thoracic Society guidelines recommend that tidal volumes should be lower in the range of 4-8 mg/kg weight, and inspiratory pressure (plateau pressure <30cm H2O), by administering tidal volume at the beginning of ventilation by 6 mg/kg weight. The purpose of these actions is to reduce the risk of barotrauma.

In their study, Sklar et al. reported that the use of lower tidal volumes could reduce lung lesions and reduce systemic inflammation, thereby accelerating organ healing and increasing survival rates in patients with Acute Respiratory Distress Syndrome (ARDS).

However, to reduce the dangerous increase in PaCO2, the use of mechanical ventilation with low tidal volume can be offset by increasing the breathing frequency to 25-30 cycles/minute by using a heated humidifier to reduce dead space. After achieving the PaCO2 target (<50 mmHg), adjust the breathing frequency to prevent pulmonary edema.

In this management, it is necessary to consider the risks that occur due to the administration of low tidal volumes, such as synchronization and double triggering due to differences in inspiration time between the ventilator machine with the body's compensation reaction. Thereby, observation of inspiration duration on ventilation machines needs to be performed periodically.

Conversely, the provision of high tidal volume makes plateau pressure and driving pressure increase. If the increase in driving pressure is too high, the risk of an Induced Lung Injury (VILI) Ventilator also increases. Giving an end-inspiratory pause of 0.2-0.3 seconds to the ventilator can prevent this increase. 

Positive End-Expiratory Pressure

Positive End-Expiratory Pressure (PEEP) is one component of mechanical ventilation in the management of patients with Acute Respiratory Distress Syndrome (ARDS). Giving PEEP in higher pressure is one of the strategies to prevent Ventilator Induced Lung Injury (VILI), but PEEP titration is still being debated and requires further study.

Papazian et al. Recommended the use of an initial PEEP of 5 cmH2O in all patients with ARDS. However, in severe ARDS, PEEP can be increased to 14.6-19.5 cmH2O. The increased PEEP can prevent the risk of collapsing alveoli and small airways, so reduce the risk of atelectrauma or volutrauma.

PEEP can reduce intrapulmonary shunts and increase oxygenation by opening the collapsed alveoli. Therefore, alveoli can function again as organs for gas exchange. The use of PEEP with higher pressure has no association with an increase in the incidence of barotrauma or multiple organ failure after ventilator installation.

However, it should be noted that the installation of high-pressure PEEP can increase the risk of many dead spaces, alveolar overdistention, and can increase pulmonary vascular resistance. Also, avoid breaking the circuit to maintain PEEP. Examples of activities that can break a circuit and reduce PEEP are the provision of manual ventilation and open suctioning.

Recruitment Maneuvers

Recruitment maneuvers (RM) can be performed under certain conditions such as the presence of endotracheal aspiration, accidental loss of the ventilator circuit, or re-intubation to increase oxygenation. According to American Thoracic Society guidelines, RM can be performed in various ways, namely providing continuous positive ventilation of 30-40 cmH2O for 30-40 seconds and increasing PEEP gradually.

Although RM can reduce intrapulmonary shunt, RM can increase the risk of barotrauma and hemodynamic complications. Therefore, its use needs to examine each patient's clinical and benefit-risk ratio. 

Airway Suctioning

The endotracheal suctioning is one component of the management of patients with Acute Respiratory Distress Syndrome (ARDS) using mechanical ventilation. The procedure aims at clearing lung secretions. There are two suctioning systems, namely open and closed suctioning. In open suctioning, do suction by breaking the ventilator circuit. Whereas in closed suctioning, do suction with a catheter or bronchoscopy.

Until now, the efficacy of the two suctioning systems is still being debated, because both systems can exert negative pressure on the lungs. The closed suctioning systems should be minimized and consider the benefit to risk ratio. Meanwhile, open suctioning should be avoided, especially in patients who need high PEEP.

I: E Ratio

Inspiration: Expiration Ratio (I: E ratio) is a proportion of the breath cycle that illustrates the comparison of the duration of inspiration and expiration. Prolonging the duration of inspiration is thought to increase oxygenation into the body because it increases intrinsic PEEP and the quality of ventilation in the lungs. However, animal studies have reported otherwise, increasing the duration of inspiration does not provide brain oxygenation in Acute Respiratory Distress Syndrome (ARDS). Therefore, recommendations using the I: E ratio at the beginning of ARDS management at 1: 2.

Humidifier

The heating and moisturizing oxygen process on the ventilator can be performed using a heated humidifier or heat and moisture exchanger. The use of heat and moisture exchanger is the best option because of the relatively low cost, easy maintenance, and condensation to the patient's expiratory air. However, its use increases airway resistance and dead space, so the risk of hypercapnia increases.

In patients with Acute Respiratory Distress Syndrome (ARDS), the use of heated humidifiers can reduce PaCO2 without having to change ventilator settings. Also, Moran et al. reported that tidal volume could decrease with the use of a heated humidifier in ARDS patients.

Patient Position

In severe Acute Respiratory Distress Syndrome (ARDS) case (P / F ratio ≤200 mmHg), Fan et al. recommended treatment with a pronation position at least 12 hours a day. The pronation position is thought to improve ventilation through increased lung perfusion, increased end-lung expiratory volume, and tidal volume distribution more evenly in all parts of the lung. However, the pronation position can also increase the risk of endotracheal tube displacement.

Endotracheal Tube

In treating patients with Acute Respiratory Distress Syndrome (ARDS) using mechanical ventilation, support for airway patency using the endotracheal tube is essential. Using the endotracheal tube with a too-small diameter increases the airflow resistance, thereby increasing the patient's breathing effort.

In adult male patients, generally, the endotracheal tube used has a diameter of 8.0 mm and in adult female patients using a diameter of 7.5 mm. Varshney et al. Reported that the average length of the endotracheal tube installation was 23 cm for men and 21 cm for female patients to reach the carina.

In preventing gastric fluid aspiration, use an endotracheal tube with a cuff that can be filled with air with a 10-20 ml syringe to develop it. Ideally, the pressure on the cuff is maintained ≤20 cm H2O considering that applying too much pressure can block blood flow.

Conclusion
Until now, mechanical ventilation is still an integral component in the management of Acute Respiratory Distress Syndrome (ARDS). There are several aspects of the use of mechanical ventilation that must be considered, such as the provision of a tidal volume of 6 ml/kg weight, increasing breath frequency at the beginning installation, and initial PEEP pressure of 5 cmH2O. Initial PEEP can be increased periodically according to the patient's clinical conditions.

The use of recruitment maneuvers routinely by providing positive pressure ventilation and suctioning is not recommended. The recommendation that the use I: E ratio in ARDS as physiologically as possible, which is 1: 2.

Pronation position is only for severe ARDS cases (P / F ratio of ≤200 mmHg). The given air should be warmed with a heated humidifier to reduce the tidal volume. Also, to ensure proper ventilation, airway patency with adequate endotracheal tube size and insertion depth needs to be done.


References
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