Basics of Mechanical Ventilation

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Indications and Rationale for Initiating IPPV
Unprotected and unstable airways (e.g,, coma) 
Intubation and IPPV allows to
-    Secure the airways
-    Reduce the risk of aspiration
-    Maintain adequate alveolar ventilation 

Hypercapnic respiratory acidosis
IPPV and NIPPV 
-    Reduce the work of breathing and thus prevents respiratory muscle fatigue or speeds recovery when fatigue is already present
-    Maintain adequate alveolar ventilation (prevent or limit  respiratory acidosis as needed)

Hypoxic respiratory failure
IPPV and NIPPV help correct hypoxemia as it allows to
-    Deliver a high FiO2   (100% if needed during IPPV)
-    Reduce shunt by maintaining flooded or collapsed alveoli open

Others
Intubation to facilitate procedure (bronchoscopy), bronchial suctioning

Important Pitfalls and Problems Associated with PPV
Potential detrimental effects associated with PPV
Heart and circulation
-    Reduced venous return and afterload
-    Hypotension and reduced cardiac output
Lungs
-    Barotrauma
-    Ventilator-induced lung injury
-    Air trapping
Gas exchange
-    May increase dead space (compression of capillaries)
-    Shunt (e.g., unilateral lung disease - the increase in vascular resistance in the normal lung associated with PPV  tends to redirect blood flow in the abnormal lung)

Important Effects of PPV on Hemodynamics
Decreased preload
Positive alveolar pressure  ↑ lung volume  compression of the heart by the inflated lungs  the intramural pressure of the heart cavities rises (e.g., ↑ RAP)  venous return decreases  preload is reduced  stroke volume decreases  cardiac output and blood pressure may drop. This can be minimized with i.v. fluid, which helps restore adequate venous return and preload.

Patients who are very sensitive to change in preload conditions (e.g., presence of hypovolemia, tamponade,  PE, severe air trapping) are particularly prone to hypotension when PPV is initiated.

Reduced afterload
Lung expansion increases extramural pressure (which helps pump blood out of the thorax) and thereby reduces LV afterload.

When the cardiac performance is mainly determined by changes in afterload than in preload conditions (e.g., hypervolemic patient with systolic heart failure), PPV may be associated with an improved stroke volume. PPV is very helpful in patients with cardiogenic pulmonary edema, as it helps to reduce preload (lung congestion) and afterload. As a result stroke volume tends to increase... 

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