Measuring D L co: What Could Possibly Go Worng? Brian Graham, PhD Division of Respirology, Critical Care and Sleep Medicine University of Saskatchewan Saskatoon, Canada

Disclosures: In the past 2 years, funding/support has been received from: AstraZeneca Boehringer-Ingelheim COSMED GSK Merck Frosst Novartis Nycomed Pfizer Prairie Oxygen Roche Trudell Vitalaire zu.com

Objectives: Describe limitations and sources of error in D L co measurements Describe steps to ameliorate their effects Describe how current technology can be better applied to D L co measurements

It’s not just diffusion It’s not a capacity It’s more precisely called Transfer Factor Diffusing Capacity of the lung for carbon monoxide: D L co [single breath]

gas P alv P cap DMDM flow = pressure x conductance for CO, assume Pcap = 0 V A ·ΔF A co/Δt = P A co · D L co Krogh equation: D L CO = V A ·ln[F A CO (t 2 )/F A CO (t 1 )] P B ·(t 2 -t 1 ) only valid during breath hold Krogh M. The diffusion of gases through the lungs of man. J Physiol (London) 1914

Roughton & Forster, J Apple Physiol 1957  co Vc 1. D L co varies with blood volume 2. D L co varies with oxygen tension D L co D M co  co·Vc =+  co  as Pa O 2 

gas concen- tration 0 0.3% time volume RV TLC 0 Ogilvie CM et al. J Clin Invest 1957 breath hold time D L CO = V A ·ln[F A CO /F I CO ·F I Tr /F A Tr ] P B ·t BH alveolar gas sample Ogilvie

gas concen- tration 0 0.3% time volume RV TLC 0 breath hold time alveolar gas sample (85ml) Jones RS, Meade F. Q J Exp Physiol 1961 D L CO = V A ·ln[F A CO /F I CO ·F I Tr /F A Tr ] P B ·t BH Jones & Meade

gas concen- tration 0 0.3% time volume RV TLC 0 Graham et al, IEEE Trans Biomed Eng 1980 V MAX ·dF A CO (t)/dt = -DL CO ·P B ·F A CO (t) 3 equation V(t)·dF A CO (t)/dt = -DL CO ·P B ·F A CO (t)+(F I CO (t)-F A CO (t))dV(t)/dt alveolar gas V(t)·dF A CO (t)/dt = -DL CO ·P B ·F A CO (t)

All methods yield similar D L co values in young, healthy, trained subjects 3 equation Ideal J&M Ogilvie ATS* Graham et al, J Appl Physiol 1981

fast 10s BH slow 10s BH slow 5s BH 150% 125% 100% 0% normalised D L co % fast 3 equation Ideal J&M Ogilvie ATS* Effect of lower flow and shorter breath hold Graham et al, J Appl Physiol 1981

gas concen- tration 0 0.3% time volume RV TLC litre 1.0 litre ATS/ERS dead space washout – 1 litre litres if VC < 2 litres ATS/ERS DLco Standardisation. Eur Resp J 2005 Problems with dead space washout

time volume RV TLC % ATS/ERS dead space washout allow 0.75 to 1 litre 1.0 litre 0.75 litre 1 litre sample collection

With current technology we can measure the point of dead space washout so that dead space gas is not included in the alveolar sample Graham et al. Can Resp J 1996 TLC Measure dead space washout

Effect of delaying alveolar sample collection Graham et al, J Appl Physiol 1981 x x x x x x x o ○ Jones & Meade in 10 healthy subjects x - 3 equation method  - J&M lung model fast manoeuvre

Effect of delaying alveolar sample collection Graham et al, J Appl Physiol 1981 x x x x x x x o x - 3 equation method  - J&M lung model slow manoeuvre ○ Jones & Meade in 10 healthy subjects

0 gas concen- tration 0.3% Calculation of alveolar volume ATS/ERS DLco Standardisation. Eur Resp J 2005 V I ·F ITr = V A ·F ATr + V D ·F ITr V A = (V I – V D ) · (F ITr /F ATr )

Measure all of the tracer gas that went onto the lung and subtract all of the tracer gas that came out to get the amount of tracer gas left in the lung at end expiration. Use the concentration measured at end expiration to estimate the concentration of tracer gas in the lung: Lung volume = amount of tracer gas - dead space concentration of tracer gas Graham et al. Can Resp J 1996 Why not use all of the available information for a more accurate measurement of V A ??? V A = F Tr (t)dV(t) / F Tr (t f ) - V D  t0t0 tftf At end exhalation:

0 0.3% gas concen- tration time volume RV TLC 0 Calculate absolute lung volume at end exhalation CH 4 concentration at end exhalation

TLC litres 2 sec 5 sec 10 secbody box breath hold time Comparison of lung volume measurements 15 normal, healthy trained subjects ATS 3-equation Graham et al, J Appl Physiol 1985

COPD Patient CH 4 CO pressure volume flow

COPD Patient CH 4 CO

TLC litres 2 sec 5 sec 10 secbody box breath hold time Comparison of lung volume measurements 10 COPD patients * * * Graham et al, J Appl Physiol 1985 ATS 3-equation

TLC litres 2 sec10 sec body box breath hold time Comparison of lung volume measurements 11 Asthma patients Graham et al, J Appl Physiol 1985 ATS 3-equation

D L co = V A · Kco The Krogh coefficient: Kco = ln(Δ[CO])/Δt/P B Kco units are min -1 · mmHg -1 Using ml · min -1 · mmHg -1 · litre is misleading Kco should never be labelled D L co/V A The relationship between D L co and lung volume is not linear, so D L co/V A and D L co/TLC do NOT provide an appropriate way to normalise D L co for lung volume Do we really need to measure V A? ATS/ERS DLco Standardisation. Eur Resp J 2005 YES!

ATS/ERS 2005 Guidelines state: V D can be estimated from various formulae or simply use 150 ml as a fixed value Effect of V D error on D L co depends on V A, ranging from 1% to 3% BUT equipment dead space (including filter – up to 350 ml) must also be subtracted or the error can be up to 10% ATS/ERS DLco Standardisation. Eur Resp J 2005 Calculation of V A : dead space

Graham et al. Can Resp J 1996 TLC Why not measure V D ???

D L co predicted adjustment for Hb Hbadjustment malesfemales 20% high % 10% high % standard % 10% low % 20% low % ATS/ERS DLco Standardisation. Eur Resp J 2005 D L co adjustment: 1.7·Hb /(102.2+Hb) for males D L co adjustment: 1.7·Hb /(93.8+Hb) for females & children< 15 yrs

CO in the lung prior to the D L co test has both an “anemia effect” and a CO back pressure effect Predicted D L co is reduced 1% for every 2% increase in COHb for COHb levels > 2% Baseline COHb in smokers can be as high as 15% COHb rises with each DLco manoeuvre (almost 1% per manoeuvre in normal healthy subjects) ATS/ERS DLco Standardisation. Eur Resp J 2005 D L co predicted adjustment for COHb and CO back pressure

Graham et al. Am J Resp Crit Care Med 2002 fractional concentration of alveolar CO (ppm) COHb (%)

D L co predicted adjustment for COHb and CO back pressure Graham et al. Am J Resp Crit Care Med 2002 D L co ml/min/mmHg no correction F A co correction both corrections control COHb = 1.2%  CO COHb = 11.2% **

Effect of alveolar oxygen on D L co D L co increases ~ 0.35% for each 1 mmHg decrease in P A O 2 The alveolar oxygen tension can be calculated from the simplified alveolar gas equation: P A O 2 = F I O 2 (P B -47) - Pa CO 2 (F I O 2 + [1- F I O 2 ] / R) As P B decreases or Pa CO 2 increases, P A O 2 decreases and D L co increases Correction for P I O 2 = 1/( (P I O )) where P I O 2 = F I O 2 (P B -47) ATS/ERS DLco Standardisation. Eur Resp J 2005

Toronto Winnipeg Saskatoon Edmonton Calgary Vancouver Halifax Montreal 8% 6% 4% 2% 0% -2% Increase in D L CO due to decreasing P I O 2 with altitude

Pay attention to the O 2 in test gas Typically test gas has 21% O 2 in North America In Europe, many test gas was often % Crapo’s DLco reference values were measured with a test gas concentration of 25% to correct for altitude. A single breath of test gas at a different O 2 might mix sufficiently well in normals but probably not in lung disease patients. Kendrick Thorax 1993; Crapo Am Rev Resp Dis 1981

Other factors affecting D L co Pressure during breath hold: Valsalva  Muller  Volume history – previous deep breath  D L co Menstrual cycle variations (independent of Hb) ATS/ERS DLco Standardisation. Eur Resp J 2005

Calibration Daily - volume - 3 litre syringe Weekly – leak check – use 3 litre syringe as test subject – “biologic control” Quarterly – gas analyser linearity check ATS/ERS DLco Standardisation. Eur Resp J 2005

Summary The standardised D L co test was not designed to measure gas exchange in patients with airflow obstruction The standardised D L co test remains constrained by limitations to the original instrumentation and manual computation Using existing equipment, more accurate and precise D L co measurements can be made from a patient-friendly, more physiologic manoeuvre

Summary In spite of the many limitations of the standardised test, D L co is a valuable measure of pulmonary function, but it has the potential to be a much more useful test

If the application of technology to communication had proceeded in the same manner as the application of technology to diffusing capacity, this is what a smart phone would like today

Thanks to the Saskatchewan D L co Team David J Cotton Joseph T Mink