Measurement of Lung Volumes

More detailed assessment of lung function usually involves the measurement of the static lung volumes. These can be very helpful in sorting out restrictive and obstructive disorders. The most important measurements are total lung capacity (TLC), residual volume (RV) and functional residual capacity (FRC) which is the gas volume remaining in the lungs after the vital capacity (VC) has been exhaled. We are able to measure absolute lung volumes beyond the inspired and expired volumes measured by spirometry.

Several methods can accomplish this. Each method has its own advantages and disadvantages. The two dilutional lung volume methods, helium (He) dilution and nitrogen (N2) washout involve having the patient breathe gases or gas concentrations not normally present in the lungs whilst the third method uses the body plethysmograph to measure the volume of thoracic gas (VTG).

We are fortunate in our lab to have the “Gold-standard” measurement VMAX body plethysmograph which delivers precision engineering of the highest quality.

The body plethysmograph is a pulmonary function system consisting of a panorama glazed cabinet in which the patient sits during the test and a computer that controls testing and data output.

Our body box’s ease of operation, patient comfort and reliability will meet pulmonary function testing needs for years to come. The streamlined testing allows the scientist to focus on the patient.

FRC measured with the body plethysmograph (FRCpleth) refers to the volume of intrathorax gas measured when airflow occlusion occurs at FRC. The technique is based on Boyle’s law relating pressure to volume. Boyle’s law states that volume varies inversely with pressure if temperature is held constant. Measurement of FRCpleth is a complex procedure achieved by carefully instructing each patient in the required manoeuvres.

Plethysmography offers several advantages over other methods of measuring lung volumes.

1. VTG is not affected by the distribution of ventilation
2. Multiple measurements can be made quickly and averaged
3. and it provides a more accurate estimate of lung volumes in patients who have airway obstruction.

In general, the body box is regarded as the more acceptable technique, but is more demanding for the patient and more technically complex.

It is often useful to compare FRC values obtained by plethysmography with values obtained by gas dilution methods, particularly in patients with obstructive disease. The ratio of FRCpleth/FRCN2 or FRCpleth/FRCHE can be used as an index of gas trapping. This ratio is usually near 1.0 in patients with normal lungs, or even those with a restrictive lung disorder. Values greater than 1.0 indicate gas volumes detectable by the plethysmograph but hidden to the gas dilution techniques. This ratio has been used to evaluate candidates for lung volume reduction surgery.

Some evidence suggests that in severe airway obstruction, FRC may actually be overestimated when the plethysmographic technique is used. This occurs primarily because PMOUTH (measured when the shutter is closed) may not equal alveolar pressure if the airways are severely obstructed. Rapid panting rates aggravates this inaccuracy. Care should be taken that patients with spirometric evidence of obstruction pant at a rate of 0.5 – 1 HZ.

In the above photo of our plethysmograph in our Hamilton Lab you will also see four gas cylinders two are required for our gas transfer testing and the remaining two are gases used to calibrate the analyser and cross reference for calibration.

Also note the dual computer screen which is used with the cycle ergometer which you can just see the tip of at the right of the photo. I will showcase our cardio pulmonary exercise test next month.

Vanessa