Ventilation refers to the movement of air between the earth's atmosphere and the lungs. Breathing, or pulmonary ventilation as it is also known, is the process of ventilation which consists of two phases: inspiration (or inhalation), and expiration (or exhalation).
Inspiration is the process of air entering the lungs. The thoracic cavity increases in volume due to contraction of the diaphragm muscle located below the lungs. This increase in volume causes the pressure inside the lungs to decrease, which causes air to flow from the atmosphere into the lungs.
Expiration is the process of air exiting the lungs. The diaphragm muscle relaxes, which causes the lungs to decrease in volume. This decrease in volume causes the pressure inside the lungs to increase, which causes air to flow from the lungs out into the atmosphere.
Natural ventilation refers to ventilation caused by the body's own musculoskeletal system.
The volume inside the lungs increases when the diaphragm muscle contracts. The lungs return to a decreased volume when the diaphragm muscle relaxes. These changes in volume cause inversely proportional changes in pressure within the lungs; an increase in volume causes a decrease in pressure, and vice versa. The volume multiplied by the pressure is equal to a constant.
Weakness of the diaphragm muscle prevents the lungs from increasing significantly in volume, which correspondingly precludes the decrease in pressure required to cause significant flow of air into the lungs. Mild or moderate decreases in flow volume are tolerable, however, more serious decreases will require ventilation support.
A lack of airflow into the lungs during inspiration will also cause a lack of airflow out of the lungs during expiration. Together, these two phenomena cause blood oxygen levels to decrease, and blood carbon dioxide levels to increase. Therefore, oxygen therapy would not benefit an individual in this condition, as the oxygen therapy does not in any way cause the excess carbon dioxide in the blood to be removed.
Mechanical ventilation refers to ventilation that is supported by a mechanical device. There are two types of ventilators: non-invasive, and invasive.
Mechanical ventilators are typically used to assist with natural ventilation, by relieving the diaphragm muscle of at least some of the load. At the extreme, they are able to entirely take over the task of providing adequate ventilation.
Mechanical ventilators cause inspiration by using pressures higher than the surrounding atmospheric pressure to force air into the lungs. Expiration is caused by reducing the pressure somewhat; the lungs return to a smaller volume, which causes air to flow back out of them.
Non-invasive ventilation refers to mechanical ventilators which do not enter the body. Removable masks and nasal fittings are used to deliver air into the airway.
Bi-Level ventilators, also known by the trade name BiPAP®, are non-invasive ventilators that alternate between a high and low pressure to facilitate breathing.
Continuous positive airway pressure (CPAP) machines are not considered to be mechanical ventilators as they do not regulate the flow of air into and out of the lungs. As the name suggests, a CPAP machine creates and maintains a continuously positive pressure in the airway, which is used to help keep the airway open. CPAP machines are thus useful for treatment of obstructive sleep apnea, where the airway becomes blocked during sleep.
CPAP machines are not associated with managing the symptoms of ALS; some individuals with ALS may use them if they suffer from obstructive sleep apnea, and do not yet require the use of a true mechanical ventilator.
Invasive ventilation refers to mechanical ventilators that enter the body. Two different methods are used to enter the airway: intubation, which involves placement of a removable tube down the throat, and tracheostomy, a surgical procedure where a hole is placed at the base of the neck and the tube is inserted at that point.
| Table: pressures at various locations during natural ventilation | ||
|---|---|---|
| Location | Pressure | Lung Volume |
| Outer space | 0 psi | - |
| Earth's atmosphere at sea level | Approximately 14.7 psi | |
| Lungs, after expiration and before inspiration | Equal to atmospheric | Small |
| Lungs, during inspiration | Below atmospheric but significantly higher than outer space | Increasing |
| Lungs, after inspiration and before expiration | Equal to atmospheric | Large |
| Lungs, during expiration | Above atmospheric | Decreasing |
| Table: pressures at various locations during mechanical ventilation | ||
|---|---|---|
| Location | Pressure | Lung Volume |
| Outer space | 0 psi | - |
| Earth's atmosphere at sea level | Approximately 14.7 psi | |
| Ventilator outlet | Always above atmospheric, alternating between high and low thresholds to facilitate inspiration and expiration, respectively | |
| Lungs, after expiration and before inspiration | Equal to ventilator's low-pressure threshold | Small |
| Lungs, during inspiration | Increasing toward the high-pressure threshold according to ventilator's inspiration profile | Increasing |
| Lungs, after inspiration and before expiration | Equal to ventilator's high-pressure threshold | Large |
| Lungs, during expiration | Decreasing toward the low pressure threshold according to ventilator's expiration profile | Decreasing |