Lung volumes

VT tidal volume during normal, quiet breathing ERV expiratory reserve volume FRC functional residual capacity EVC expiratory vital capacity IVC inspiratory vital capacity TLC total lung capacity.


Mechanical ventilation/ventilators, which often referred to as mechanical respiration/respirators are used to inflate and deflate the lungs of patients that are unable to breathe on their own. Their are two types of ventilator designs: negative pressure and positive pressure ventilation.

Positive Pressure[]

The principle of positive pressure is opposite from negative pressure. Positive air flow pressure is a closed system. Air flow during inhale creates pressure in the tubes connected to the patient circuit and exhale controls close a valve at the outlet of the tubing to the atmosphere. Consequently if there is any leak from the positively pressured system it will egress into the surrounding environment. Hospitals may have positive pressure rooms for patients with compromised immune systems. Air will flow out of the room instead of in, so that any airborne microorganisms (e.g., bacteria) that may infect the patient are kept away.

Negative Pressure[]

The principle of negative pressure almost uses the same natural respiration techniques we use to breathe. The cause of air flow into the enclosed area within the thoracic walls (chest) has lower pressure than the patients lungs around it. This allows the patients chest to rise while deflating the pressure inside the lungs. Air flow during exhale is caused by the natural spring-like motion from the chest walls returning back to normal. [1] [2]



Assist Control (AC) mode provides the ventilator a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath. Traditional assist-control used only a pre-set tidal volume—when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV. However, the initiation timing is the same—both provide a ventilator breath with every patient effort. In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic. Although a maximum rate is not usually set, an alarm can be set if the ventilator cycles too frequently. This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt).


Functional Residual Capacity (FRC) is the volume of air present in the lungs, specifically the parenchyma tissues, at the end of passive expiration. At FRC, the elastic recoil forces of the lungs and chest wall are equal but opposite and there is no exertion by the diaphragm or other respiratory muscles.

The FRC formula is...

FRC measures approximately 2400 ml in a 70 kg, average-sized male. It can not be estimated through spirometry, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body body plethysmography. [3]


Positive end-expiratory pressure (PEEP) is a term used in mechanical ventilation to denote an airway pressure that is kept above atmospheric pressure at the end of the expiratory cycle. The purpose is sometimes used to keep alveoli in the lungs from collapsing during exhale.


Residual volume (RV) is the amount (volume) of gas remaining in the lungs after maximum exhale. In children and adolescents residual volume grows slightly faster than the total lung capacity. In healthy adults RV increases with age since a maximal expiration is increasingly impeded by airway closure, preventing dependent alveoli from emptying. In contrast, the total lung capacity does not change with age in adults. As a result the VC decreases with age in healthy patients.[4]


Synchronized Intermittent Mandatory Ventilation (SIMV). In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 seconds - thus a respiratory rate of 12 results in a 5 second cycle time). Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath. However, SIMV may be combined with pressure support (see below). SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath.


Tidal volume (Vt) is the amount (volume) of air from the lungs inhaled or exhaled during normal breathing. Tital volume is calculated in milliliters per kilogram. Traditionally 10 ml/kg was used but has been shown to cause barotrauma, or injury to the lung by over-extension, so 6 to 8 ml/kg is now common practice in ICU. Hence a patient weighing 70 kg would get a Vt of 420–480 ml. In adults a rate of 12 strokes per minute is generally used.


Vital capacity is the amount (volume) of gas exchange of the lung between a full inhale and a maximum exhale. It is equal to the inspiratory reserve volume plus the tidal volume plus the expiratory reserve volume. A person's vital capacity can be measured by a spirometer which can be a wet or regular spirometer. In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease. The unit that is used to determine this vital capacity is the milliliter (ml).

The VC formula is...

A normal adult has a vital capacity between 3 and 5 liters. After the age of 20 the vital capacity decreases approximately 250 cc per ten years.


  1. [1]Smith, S.E., "What is Negative Pressure", Wise Geek dot com, 08 September 2010
  2. David, Y., von Maltzahn, W., Neuman, M. and Bronzino, J., Clinical Engineering, CRC Press LLC, Boca Roton, Fl., 2003
  3. [2] Mechanical ventilation, Wikipedia,14 December 2010
  4. Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Official Statement of the European Respiratory Society. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629