Mechanical ventilation modification of settings

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Vishnu Vardhan Serla M.B.B.S. [2] Syed Hassan A. Kazmi BSc, MD [3]

In adults when 100% FiO₂ is used initially, it is easy to calculate the next FiO₂ to be used and easy to estimate the shunt fraction. When using 100% FiO₂, the degree of shunting is estimated by subtracting the measured PaO₂ (from an arterial blood gas) from 700 mmHg. For each difference of 100 mmHg, the shunt is 5%. A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly. Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously. Specific clinical, ventilator and oxygenation criteria should be met in order to do a trial of spontaneous breathing in mechanically ventilated patients.

• Modification of ventilator settings:^[1][2][3][4]
• In adults when 100% FiO₂ is used initially, it is easy to calculate the next FiO₂ to be used and easy to estimate the shunt fraction.
• The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation.
• In normal physiology, gas exchange (oxygen/carbon dioxide) occurs at the level of the alveoli in the lungs.
• The existence of a shunt refers to any process that hinders this gas exchange, leading to wasted oxygen inspired and the flow of unoxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood).
• When using 100% FiO₂, the degree of shunting is estimated by subtracting the measured PaO₂ (from an arterial blood gas) from 700 mmHg. For each difference of 100 mmHg, the shunt is 5%.
• A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly.
• If such complications are not present, other causes must be sought after, and PEEP should be used to treat this intrapulmonary shunt. Other such causes of a shunt include:

• Alveolar collapse from major atelectasis.
• Alveolar collection of material other than gas, such as pus from pneumonia, water and protein from acute respiratory distress syndrome, water from congestive heart failure, or blood from hemorrhage

• Withdrawal from mechanical ventilation (also known as weaning) should not be delayed unnecessarily, nor should it be done prematurely.
• Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously.

Clinical criteria

• Resolution of acute phase of disease
• Adequate cough reflex
• No excessive secretions
• Hemodynamic stability

Ventilator criteria

• PaCO2 < 50 mm Hg
• Vital capacity > 10 ml/Kg
• Spontaneous tidal volume > 5 ml/Kg
• Spontaneous respiratory rate (f) < 35/min
• f/Vt <100 breaths/ml/min
• Minute ventilation < 10 L with ABGs within normal limits

Oxygenation criteria

• PaO2 without PEEP > 60 mm Hg at FiO2 0.4
• PaO2 with PEEP (< 8 cm H2O) > 100 m Hg at FiO2 0.4
• SaO2 > 90% at FiO2 0.4
• PaO2/FiO2 greater than equal to 150
• P(A-a)O2 < 350 mm Hg at FiO2 1.0

• This refers to all modalities that assist ventilation without the use of an endotracheal tube.
• Non-invasive ventilation is primarily aimed at minimizing patient discomfort and the complications associated with invasive ventilation.
• It is often used in cardiac disease, exacerbations of chronic pulmonary disease, sleep apnea, and neuromuscular diseases.
• Non-invasive ventilation refers only to the patient interface and not the mode of ventilation used; modes may include spontaneous or control modes and may be either pressure or volume modes.

• Some commonly used modes of NIPPV include:
  • Continuous positive airway pressure (CPAP)
  • Bi-level Positive Airway Pressure (BIPAP) pressures alternate between Inspiratory Positive Airway Pressure (IPAP) and a lower Expiratory Positive Airway Pressure (EPAP), triggered by patient effort. On many such devices, backup rates may be set, which deliver IPAP pressures even if patients fail to initiate a breath.
  • Intermittent positive pressure ventilation (IPPV) via mouthpiece or mask

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