To pant or not to pant - measuring specific airway conductance.
The respiratory tree consists of tubes having many different diameters, from the smallest respiratory bronchial to the much larger trachea and larynx. The Raw depends on the size of the tube's lumen and it also depends upon the lung volume at which the measurement is being taken. Raw is less at large lung volumes, as encountered at the end of forced exhalation, and greater when measured at small lung volumes or at the beginning of the exhalation. This is partly due to the fact that at larger lung volumes, the flow through the smaller airways is laminar resulting in decreased Raw, unless the patient has a degree of severe airway obstruction in which the smaller airways might begin to close with increased trans-pulmonary pressure.
The body plethysmograph directly measures airway resistance by measuring the pressures needed to produce resultant flows and thus the units for Raw are cm H2O/liter/ second. Airway conductance (Gaw) is the reciprocal of Raw. It is, in essence, a way of looking at the same phenomena, but from the opposite viewpoint, so to speak. So instead of looking at resistance to flow, we are looking at the conductance or ease of flow. This reciprocal relationship is demonstrated in the following equation:
[G.sub.aw] = [1/[R.sub.aw]]
It is the measure of flow that is generated from the available drive pressure and its units are L/second/cmH2O.
Since the accurate measurement of Raw is dependent on the lung volume, the standard technique is to make the measurements at functional residual capacity (FRC) or at end of normal exhalation. It is evident that patients with large lungs will have higher Gaw than patients with smaller lungs and so to make a fair comparison with patients possessing different sized lungs, the Gaw is divided by the thoracic gas volume (VTG), which is comparable to the patient's FRC, to produce specific airway conductance or SGaw which is simply Gaw corrected for lung volume at FRC. So specific airway conductance (sGaw) is the ratio of airway conductance (Gaw) to thoracic gas volume (Vtg).
We saw that since airway resistance (Raw) is the opposition or "resistance" to flow in the human airways as the result of the forces of molecular friction, it is defined as the ratio of driving pressure or force to the rate of air flow. Resistance to flow in the airways depends on whether the flow is laminar or turbulent, on the dimensions of the airway, and on the viscosity of the gas.
R = [[DELTA]P/V]
For laminar flow the resistance is very low and it is evident that a relatively small driving pressure is able to produce a significant resultant flow rate. The calculation of resistance with laminar flow can be calculated with a rearrangement of Poiseuille's Law.
R = 8nl/n r4
R = resistance n = viscosity I = length r = radius
As the result of the fourth power in the denominator, resistance increases rapidly as diameter decreases. The most critical variable here is the radius of the airway, which, by virtue of its elevation to the fourth power, has a tremendous impact on the resistance. Thus, if the diameter of a tube is doubled, resistance will drop by a factor of sixteen. Turbulent flow results in a relatively larger airway resistance when compared with laminar flow and a much larger driving pressure is required to produce the same flow rate. Because the pressure to flow relationship ceases to be linear during turbulent flow, no equation or graph is possible to calculate its resistance.
It is true that a single small airway results in much more resistance than a single large airway. But total resistance to air flow in the smaller airways depends on the number of parallel pathways present. Because of this, the large and particularly the medium-sized airways actually provide greater resistance to flow than do the more numerous small airways. So airway resistance decreases as lung volume increases because the percentage of are flowing from the small airways increases and results in an overall decrease in resistance, even though the resistance of individual larger airways is much less. It can easily documented in any laboratory that when a person is breathing at an increased rate, as is the case with any form of panting above two cycles per second, the airway resistance is significantly affected throughout the lung but especially in the larger airways where there will be a relative gradient increase in airway resistance due to the increase in turbulent flow. The increased resistance will be seen down into the smaller airways until the inherent laminar flow effect from the multitude of parallel airways, takes effect. At that point, deep in the bronchial tree, any increase in breathing rate will have no effect on resistance. But when panting, the overall resistance throughout the lung will be increased, mainly due to the increased turbulent flow from the larger airways. On the hand when breathing at a normal rate during the measurement of the resistance loop, the turbulent flow is still present at the smaller lung volumes but is markedly decreased. This difference in turbulence found in the airways between breathing normally and panting is more marked in children.
The American Thoracic Society's (ATS) recommendation for breathing patterns during body plethysmography measurements is to pant at a rate of 0.5 to 1.5 Hz or cycles per second. Ranting faster than 1.5 Hz may result in increased resistance as described above and panting 0.5 Hz or less may cause problems with the controlled leak of the body plethysmograph cabinet. I would recommend that the term "panting" not be used since it leads patients to breath at much faster rate. A more effective instruction is to ask the patient to breath "along with me as I say in--out, in--out." In this way the patient's breathing rate can be more easily controlled and word "panting" does not become a temptation to breath faster and faster. The measurement of resistance is affected by the flow rate in the larger airways and for that reason the breathing rate during SGaw measurements should not exceed 1.5 Hz or cycles per second. It is recommended that the term panting itself should not be used and, most often, the best instructions are to ask the patient to breath normally during the both the Vtg and Raw measurements.
Jim Harvey MS, RPFT, RCP works in the Pulmonary Function Laboratory at Stanford Hospital and Clinics in Palo Alto, and teaches Pulmonary Function at Skyline College in San Bruno, California.
by Jim Harvey MS, RPFT, RCP
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||PULMONARY FUNCTION TESTING|
|Publication:||FOCUS: Journal for Respiratory Care & Sleep Medicine|
|Date:||Nov 1, 2010|
|Previous Article:||Physiologic integration, good timing, and short loop feedback.|
|Next Article:||Fifteen years of Focus on education.|