Ventilation

Changes in the volume of the lungs associated with inspiration (inflation) and expiration (deflation).  

Gases like carbon dioxide and oxygen move between the air in the lungs and the blood in the blood vessels flowing through pulmonary capillaries.  Carbon dioxide is the exhaust gas of the body's metabolic engines.  Carbon dioxide leaves the body in the lungs and is blown off with exhalation (deflation)  

Dead space is the space occupied by gas in the lung where there is no gas exchange.  

A shunt is when blood flows through the lungs without exchanging gas (losing CO2, gaining O2 are physiologically most important). 

Ventilation perfusion mismatch 

A pathologic state in which the volume of air available to contain gases in the lungs over time is greater than or less than the amount of blood carrying carbon dioxide waste gas to the lungs.  Too much gas exchange capacity results in elimination of carbon dioxide faster than it is produced.  Hypocarbia (too little carbon dioxide) disturbs the balance of acid and electrolytes.   Too little gas exchange results in accumulation of acid. 

The condition most commonly associated with too much ventilation for the amount of perfusing blood is pulmonary embolism in which clots block blood headed from the lungs.  The lungs continue to inflate and deflate, but there is no blood with which to exchange carbon dioxide and oxygen.  The problem with inadequate oxygen exchange is that the blood arriving to tissues may not contain enough oxygen to sustain metabolism resulting in tissue death.  

The most common symptom of ventilation perfusion mismatch with too little ventilation and gas exchange is shortness of breath.   Too little oxygen in the blood (hypoxia) can result in confusion, agitation, sleepiness, or even unconsciousness.

V/Q scan

Mechanical ventilation

Mechanical ventilation requires placing a tube down the patient's throat into the upper part of the trachea (windpipe). A balloon can be blown up that occludes the space between the endotracheal tube and the inner wall of the trachea to prevent air pumped into the lungs under pressure from escaping.

Patients with neurological diseases are placed on mechanical ventilation for one or both of two reasons:

1. unconsciousness with inability to cough out secretions and objects that can potentially occlude the trachea preventing flow of air necessary for ventilation. Patients who are unconscious do not "clear" their windpipes normally for at least two reasons:

a) depressed or absent signals from ventilatory centers in the brainstem (uncommon)

b) depressed or absent reflex normally activated by stimulation (irritation, pressure, etc.) of the inner surface of the trachea (windpipe), back of throat (oropharynx), and mouth. In unconscious patients there is decreased reflex (automatic) control of the tongue which can flop back into the throat (when the patient is supine [on his or her back] ) blocking the upper part of the passageway between the lungs and the mouth/nose.

2. conscious but unable to inflate the lungs due to problems with 1. the nerves that control contraction of the diaphragm and/or other ventilatory muscles or 2. with the muscles of ventilation themselves.

Conscious patients on mechanical ventilation are fully aware of the discomfort of the tube in their throat as well as of the regular intermittent pressurized air pulses through the endotracheal tube -- these are somewhat less after tracheostomy.

Assisted

Mandatory

Neurointensive Care Specialty Center

figure: ventilator

figure: suctioning a patient on a ventilator

table: mechanical ventilation indications

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