Lung function tests are commonly performed on patients in specialised pulmonary function laboratories or in cardiopulmonary units in hospitals. Various cardiopulmonary function tests are performed, usually on hospital outpatients, and these require the co-operation of the patient and interaction between the clinician and patient.
In contrast, patients in hospital intensive care units (ICUs) are dependent upon complex life support and monitoring equipment, such that they cannot be moved easily to the testing unit.
The commonest reason for admission to the ICU is the need for mechanical ventilatory support, known as life-support. These patients are not able to participate in any form of lung function testing involving voluntary action, nor can these patients be easily transferred or disconnected from their ventilatory support.
Critical illness, including severe sepsis and mechanical ventilation itself, can lead to severe lung abnormalities, decreased effective lung volume and decreased efficiency of gas exchange between the lung and the blood. Those patients who would benefit most from lung function and cardiopulmonary tests are therefore the most difficult to assess by conventional means.
These patients will also most likely be dependent upon high oxygen concentrations in the inspired gases and will not tolerate brief periods of breathing lower inspired oxygen concentrations, as is required in some lung function tests.
Ventilated patients will additionally have a variety of ventilatory modes and this has an important influence on the ability to perform cardiorespiratory and lung function tests.
For these reasons, it is seldom possible to perform lung function tests on the ventilated patient in the ICU, except in certain centres of excellence or for detailed research purposes.
It is also not easily possible to measure lung function of the anaesthetised patient in the operating theatre, since the unconscious patient is unable to participate in volitional respiratory manoeuvres.
These measurements need to be able to be made non-invasively, with relatively simple equipment and gases that are routinely available.
The research proposal is to develop apparatus using oxygen as the measurement gas to measure lung volume, and ultimately also blood flow through the lung, in a way that does not require patient co-operation. It should also not interfere with the patient’s breathing pattern, whether the patient is breathing spontaneously or is being mechanically ventilated.
A laptop computer will control this apparatus, and the signals from gas analysis and gas flow sensors will be fed into a computer model of the human lung to determine at the bed side the patient’s end-expired and dead-space lung volumes.
The apparatus will be tested as part of the project work first on a mechanical bench lung model and then on spontaneously breathing volunteers. The apparatus and technique will then be transferred to the hospital ICU for routine daily use.
The grant is valued at £216,682 and the project runs from April 2007 to March 2009.