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- Mechanical ventilation expands the lungs and chest wall by pressurizing the airway during inflation. The stretched lungs and chest wall develop recoil tension that drives expiration.
- Positive pressure developed in the pleural space may have adverse effects on venous return, cardiac output and dead space creation.
- Stretching the lung refreshes the alveolar gas, but excessive stretch subjects the tissue to tensile stresses which may exceed the structural tolerance limits of this delicate membrane.
- Disrupted alveolar membranes allow gas to seep into the interstitial compartment, where it collects, and migrates toward regions with lower tissue pressures.
- Interstitial, mediastinal, and subcutaneous emphysema are frequently the consequences. Less commonly, pneumoperitoneum, pneumothorax, and tension cysts may form.
- Rarely, a communication between the high pressure gas pocket and the pulmonary veins generates systemic gas emboli.
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The routine management of mechanical ventilation in the ICU includes monitoring of peak airway pressures, plateau pressures and determining airway resistance.
When volume or pressure is pushed through an airway, a peak pressure is generated. This peak pressure is the sum of the amount of pressure necessary to get through the airways, inflate the alveoli and displace the chest wall and diaphragm. An inspiratory hold is performed on the ventilator to measure how much this pressure (plateau pressure) is actually being sensed in the alveoli once the lungs are inflated. By subtracting the plateau pressure from the peak pressure, we can calculate the resistance from the airways.
In managing mechanical ventilation, we routinely look at the plateau pressure to determine the limits to which we can increase our ventilating volumes. For the majority of patients, the chest wall and diaphragm are relatively compliant so are not a major factor in ability to ventilate patients. In cases of stiff chest wall or distended abdomens, the plateau pressure may be misleading as the pressure sensed within the alveoli is in part due to the pressures from the stiff chest wall or diaphragm.
Recently, esophageal catheters have been used to help optimize ventilation of patients with concerns re. stiff chest walls or diaphragms (distended abdomens). A catheter inserted in the esophagus is in close proximity to the pleural space. Esophageal pressures can be used as a surrogate to pleural pressures. Use of esophageal pressure monitoring can then help to differentiate between:
o pressure in the pleural space, attributable to chest wall and diaphragm and
o pressure distending the lungs (transpulmonary pressure) which might result in barotraumaPtpt (transpulmonary) = Paw (plateau) – Pes (esophageal)
New Ventilation Modes
•Dual Control
Within-a-breath switches from PC to VC during the breath
• VAPS and pressure augmentation
Breath-to-Breath
– Pressure-Limited, Flow-Cycled Ventilation
• Volume support ventilation VSV Siemens 300
• Variable-pressure-support Cardiopulmonary
corporation Venturi,
– Pressure-Limited, Time-Cycled Ventilation
• Pressure-regulated volume-control PRVC Siemens 300
• Adaptive pressure ventilation APV Hamilton Galileo
• Auto-flow Draeger Evita 4
• Volume-control Puritan Bennett 840
• Variable pressure control Cardiopulmonary
corporation Venturi
•Proportional-Assist Ventilation
•Adaptive Support Ventilation
•Automatic Tube Compensation
•Airway Pressure-Release Ventilation
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The University Hospital of Alberta in Edmonton is a leading Canadian clinical, research and teaching hospital. The institution was one of the first to implement and utilize NAVA – Neurally Adjusted Ventilatory Assist. Two specialty critical care units have been gaining experience in specific patient categories: the Cardiovascular ICU and the Neuro ICU, and the technology is in line to be implemented at the pediatric and neonatal ICU at Stollery Children’s Hospital within the institution.
