Mechanical ventilation initial ventilator settings
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Initial ventilator settings should be modified and tailored according to the clinical condition of the patient and specific goals of management. Selection of ventilatory mode, sensitivity at flow trigger mode, tidal volume, rate, inspiratory flow, positive end expiratory pressure (PEEP), pressure limit, inspiratory time and fraction of inspired oxygen (FiO2) should be made according to the underlying etiology of hypoxemia/hypercapnia. Other factors for example, age of the patient, weight and height also play an important role in deciding the initial ventilatory settings. General rules that help physicians to choose the initial settings in a time-efficient manner include choosing a tidal volume of 12 mL per kg body weight delivered at a rate of 12 a minute (12-12 rule) in adults and adolescents. In infants and children without existing lung disease a tidal volume of 4-10 ml/kg may be delivered at a rate of 30-35 breaths per minute. With respiratory distress syndrome (RDS), a decreased tidal volume and increased respiratory rate sufficient to maintain pCO2 between 45-55. Allowing higher pCO2 (sometimes called permissive hypercapnia) may help prevent ventilator induced lung injury. Ventilator triggered breaths may be initiated via either a pressure-triggered or a flow-triggered mechanism. Pressure triggered breaths are initiated when the ventilator senses a negative pressure (indicating that the patient is trying to initiate a breath). During the flow-triggered mechanism, a continuous flow is delivered and a ventilator-delivered breath is initiated when the return flow is less than the delivered flow.
Initial Ventilator Settings
Tidal Volume, Rate, and Pressures
- Adult patients and older children
- Without existing lung disease -- a tidal volume of 12 mL per kg body weight is set to be delivered at a rate of 12 a minute (12-12 rule).
- With COPD -- a reduced tidal volume of 10 ml/kg is to be delivered 10 times a minute to prevent overinflation and hyperventilation (10-10 rule).
- With acute respiratory distress syndrome (ARDS) -- an even more reduced tidal volume of 6-8 mL/kg is used with a rate of 10-12/minute. This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) but this elevation does not need to be corrected (termed permissive hypercapnia)
- Infants and younger children
- Without existing lung disease -- a tidal volume of 4-10 ml/kg to be delivered at a rate of 30-35 breaths per minute
- With RDS -- decrease tidal volume and increase respiratory rate sufficient to maintain pCO2 between 45-55. Allowing higher pCO2 (sometimes called permissive hypercapnia) may help prevent ventilator induced lung injury
- As the amount of tidal volume increases, the pressure required to administer that volume is increased.
- This pressure is known as the peak airway pressure. If the peak airway pressure is persistently above 45 cmH2O for adults, the risk of barotrauma is increased and efforts should be made to try to reduce the peak airway pressure.
- In infants and children, it is unclear what level of peak pressure may cause damage. In general, keeping peak pressures below 30 is desirable.
- Monitoring for barotrauma can also involve measuring the plateau pressure, which is the pressure after the delivery of the tidal volume but before the patient is allowed to exhale.
- Normal breathing pattern involves inspiration, then expiration. The ventilator is programmed so that after delivery of the tidal volume (inspiration), the patient is not allowed to exhale for a half a second.
- Therefore, pressure must be maintained in order to prevent exhalation, and this pressure is the plateau pressure. Barotrauma is minimized when the plateau pressure is maintained < 30-35 cmH2O.
- An adult patient breathing spontaneously will usually sigh about 6-8 times/hr to prevent microatelectasis, and this has led some to propose that ventilators should deliver 1.5-2 times the amount of the preset tidal volume 6-8 times/hr to account for the sighs.
- However, such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma.
- Currently, accounting for sighs is not recommended if the patient is receiving 10-12 mL/kg or is on PEEP.
- If the tidal volume used is lower, the sigh adjustment can be used, as long as the peak and plateau pressures are acceptable.
- Sighs are not generally used with ventilation of infants and young children.
- Because the mechanical ventilator is responsible for assisting in a patient's breathing, it must then also be able to deliver an adequate amount of oxygen in each breath.
- The FiO2 stands for fraction of inspired oxygen, which means the percent of oxygen in each breath that is inspired. (Note that normal room air has ~21% oxygen content).
- In adult patients who can tolerate higher levels of oxygen for a period of time, the initial FiO2 may be set at 100% until arterial blood gases can document adequate oxygenation.
- An FiO2 of 100% for an extended period of time can be dangerous, but it can protect against hypoxemia from unexpected intubation problems. For infants, and especially in premature infants, avoiding high levels of FiO2 (>60%) is important.
Positive End-Expiratory Pressure (PEEP)
- PEEP is an adjuvant to the mode ventilation used in cases where the functional residual capacity (FRC) is reduced.
- At the end of expiration, the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure.
- The presence of PEEP opens up collapse or unstable alveoli and increases the FRC and surface area for gas exchange, thus reducing the size of the shunt.
- Thus, if a large shunt is found to exist based on the estimation from 100% FiO2, then PEEP can be considered and the FiO2 can be lowered (< 60%) to still maintain an adequate PaO2, thus reducing the risk of oxygen toxicity.
- In addition to treating a shunt, PEEP is also therapeutic in decreasing the work of breathing. In pulmonary physiology, compliance is a measure of the "stiffness" of the lung and chest wall.
- The mathematical formula for compliance (C) = change in volume/change in pressure. Therefore, a higher compliance means that only small increases in pressure can lead to large increases in volume, which means the work of breathing is reduced.
- As the FRC increases with PEEP, the compliance also increases, since the partially inflated lung takes less energy to inflate further.
- PEEP is a cardio-depressant and can cause severe hemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular outflow. As such, it should be judiciously used and is indicated in two circumstances.
- If used, PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2.
- As PEEP increase intrathoracic pressure, there can be a resulting decrease in venous return and decrease in cardiac output. A PEEP of less than 10 cmH2O is usually safe if intravascular volume depletion is absent.
- Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring.
- More recent literature has failed to find outcome benefits with routine PA catheterization when compared to simple central venous pressure monitoring.
- If cardiac output measurement is required, minimally invasive techniques, such as esophageal Doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives.
- PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 < 40%. When withdrawing, it is decreased through 1-2 cmH2O decrements while monitoring hemoglobin-oxygen saturation.
- Any unacceptable hemoglobin-oxygen saturation should prompt re-institution of the last PEEP level that maintained good saturation.
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia. It improves FRC, drainage of secretions, and ventilation-perfusion matching (efficiency of gas exchange). It may improve oxygenation in > 50% of patients, but no survival benefit has been documented.
Most patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress. Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning.
- To protect against ventilator-associated pneumonia, patient's bed is often elevated to about 30° and Histamine-2 receptor blocker or proton pump inhibitors may be used.
- Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults.
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