Transfer factor for carbon monoxide : a glance behind the scene

The carbon monoxide transfer factor (TLCO) is widely used in lung function laboratories where it complements the measurement of lung volumes and of forced inspiratory and expiratory flows. The TLCO is alternatively named the diffusing capacity (DLCO), but the former term is more appropriate for two reasons. First, this index is not uniquely determined by the diffusive characteristics of the lung, and second, it does not represent a maximal capacity in resting conditions because it easily increases with metabolic rate [1]. By measuring the surface area of the lung available for gas exchange, this test represents a unique window on the pulmonary microcirculation. Thus, the TLCO is a key measurement in conditions like interstitial lung diseases and in the evaluation before surgery for lung cancer or for lung volume reduction in emphysema. Furthermore, in the presence of normal lung volumes and spirometry, the TLCO may be the sole abnormal test hinting to a pulmonary vascular disorder like chronic thromboembolic disease or other causes of pulmonary vascular obliteration. Although the usefulness of the TLCO is not disputed, the complexity of this test is not always fully appreciated. For instance, although introduced long ago by Marie Krogh in 1915 [2], this test was still fuelling a lively scientific controversy at the dawn of the XXI century! [3–5]. The aim of this article is first to recall some neglected principles of the TLCO, and second to propose a practical scheme of interpretation which is largely inspired by the thorough work of Hughes and Pride [4, 6]. The transfer factor for carbon monoxide (TLCO) is widely used in pulmonary function laboratories because it represents a unique non-invasive window on pulmonary microcirculation. The TLCO is the product of two primary measurements, the alveolar volume (VA) and the CO transfer coefficient (KCO). This test is most informative when VA and KCO are examined, together with their product TLCO. In a normal lung, a low VA due to incomplete expansion is associated with an elevated KCO, resulting in a mildly reduced TLCO. Thus, in case of low VA, a seemingly “normal KCO” must be interpreted as an abnormal gas transfer. The most common clinical conditions associated with an abnormal TLCO are characterised by a limited number of patterns for VA and KCO: incomplete lung expansion, discrete loss of alveolar units, diffuse loss of alveolar units, emphysema, pulmonary vascular disorders, high pulmonary blood volume, alveolar haemorrhage.

The carbon monoxide transfer factor (T L CO) is widely used in lung function laboratories where it complements the measurement of lung volumes and of forced inspiratory and expiratory flows.The T L CO is alternatively named the diffusing capacity (D L CO), but the former term is more appropriate for two reasons.First, this index is not uniquely determined by the diffusive characteristics of the lung, and second, it does not represent a maximal capacity in resting conditions because it easily increases with metabolic rate [1].By measuring the surface area of the lung available for gas exchange, this test represents a unique window on the pulmonary microcirculation.Thus, the T LCO is a key measurement in conditions like interstitial lung diseases and in the evaluation before surgery for lung cancer or for lung volume reduction in emphysema.Furthermore, in the presence of normal lung volumes and spirometry, the TLCO may be the sole abnormal test hinting to a pulmonary vascular disorder like chronic thromboembolic disease or other causes of pulmonary vascular obliteration.
Although the usefulness of the T L CO is not disputed, the complexity of this test is not always fully appreciated.For instance, although introduced long ago by Marie Krogh in 1915 [2], this test was still fuelling a lively scientific controversy at the dawn of the XXI century![3][4][5].The aim of this article is first to recall some neglected principles of the T LCO, and second to propose a practical scheme of interpretation which is largely inspired by the thorough work of Hughes and Pride [4,6].
The transfer factor for carbon monoxide (TLCO) is widely used in pulmonary function laboratories because it represents a unique non-invasive window on pulmonary microcirculation.The T L CO is the product of two primary measurements, the alveolar volume (V A ) and the CO transfer coefficient (KCO).This test is most informative when V A and KCO are examined, together with their product TLCO.In a normal lung, a low VA due to incomplete expansion is associated with an elevated KCO, resulting in a mildly reduced TLCO.Thus, in case of low VA, a seemingly "nor-mal KCO" must be interpreted as an abnormal gas transfer.The most common clinical conditions associated with an abnormal TLCO are characterised by a limited number of patterns for V A and KCO: incomplete lung expansion, discrete loss of alveolar units, diffuse loss of alveolar units, emphysema, pulmonary vascular disorders, high pulmonary blood volume, alveolar haemorrhage.The TLCO measures the rate of transfer of CO from the alveoli to the blood.Inhaled carbon monoxide is used because of its very high affinity for haemoglobin: as a result, the plasma CO partial pressure remains close to zero, the gradient of partial pressure remains operational between the alveoli and the capillary blood, and the amount of CO transferred is limited by diffusion only.The single-breath T L CO measurement (T L CO SB ) was introduced by Ogilvie et al. [7] and will be described here because it is the most widely used method.

Glossary
The subject first fully exhales, then takes a rapid and full inspiratory vital capacity of a gas mixture composed of air, a tiny fraction of CO (0.003) and a fraction of an inert gas such as helium or methane.The breath is held for 10 seconds at complete inspiration before a rapid and full exhalation is made.After the first portion of exhaled gas has been discarded, gas is collected during mid-expiration as an alveolar sample for analysis of CO and of the inert gas.It is very important to control the quality of the manoeuvre by checking several points: that the inspiration is rapid enough (<2.5 s, or <4 s in case of airflow limitation), that the inspired volume is large enough (>90% vital capacity), and that the breath-holding time is correct (10 ± 1 s).From this, two primary measurements are made:

Rate constant for alveolar-capillary CO transfer (= permeability factor, kCO):
The initial alveolar fraction of CO is calculated as follows: F A CO 0 = F I CO • (F A He / F I He) where: F I CO = inspired fraction of CO F A He = alveolar fraction of helium F I He = inspired fraction of helium During breath-holding, the alveolar fraction of CO decreases exponentially: The permeability factor is calculated as follows: log e (F A CO 0 / F A CO t ) kCO = t where the unit for kCO is: min -1   Alveolar volume (V A ): F A He where: V I = inspired volume the unit for V A is: ml STPD The T L CO is then calculated as the product of the permeability factor and alveolar volume, divided by the effective barometric pressure: kCO • V A T L CO = P B -P H2O where: PB = barometric pressure PH2O = water vapour pressure The TLCO is expressed in units of conductance: mmol CO • min -1 • kPa -1 (or ml CO • min -1 • mm Hg -1 in traditional units).
It is important to understand that the TLCO is not the measurement of an actual physical variable.Rather, it is a calculation of what would be the flux of CO from the alveoli to the blood in the hypothetical condition of the subject's lungs being filled with 100% CO (which, if true, would expose to medico-legal consequences).Hence the introduction of alveolar volume which represents the volume of distribution of CO, and of barometric pressure which corresponds to the driving pressure for diffusion [6].

Components of TLCO
The TLCO is made of two conductances in series: the membrane conductance (D M) which represents the diffusion component, and the reactive conductance (q • Qc) where q is the rate of reaction of CO with haemoglobin and Qc is the blood volume in the pulmonary capillaries.The D M factor is reduced when the alveolar-capillary membrane surface is reduced or when its thickness is increased.The q factor varies with the concentration of haemoglobin: the low value of q explains the low values of T L CO measured in anaemia [8].

Confounding factors
Because of the effect of haemoglobin concentration [Hb], TLCO has to be adjusted to the normal [Hb] in particular when anaemia is present or can be suspected.The adjusted value (TLCO adj.or TLCO corr.) is considered for assessing gas exchange.
Carboxyhaemoglobin (HbCO) reduces T L CO by two mechanisms: first, it decreases the CO pressure gradient by increasing venous CO back pressure, and second, it decreases the mass of haemoglobin available for CO binding.The effect is a 1% fall of T L CO for each 1% increment of [HbCO].
To avoid this problem it is recommended that the subject refrains from smoking 24 hours before the test.
The T LCO is influenced by altitude because oxygen and CO are in competition for Hb: q increases when alveolar PO2 falls.Thus, TLCO increases by 0.31% per mm Hg decrease in inspired PO 2. This point has to be considered when the test is performed in a pulmonary function laboratory located at high altitude.
Exercise increases T L CO by increasing pulmonary capillary blood volume (Qc).Thus, T LCO increases by 20% for each increment of 5 L • min -1 of cardiac output.It is recommended that the subject refrains from strenuous exercise and remains seated for at least 5 min before testing [9].

The problem of V A in airflow limitation
Patients with chronic obstructive pulmonary disease (COPD), in particular those with emphysema, tend to have a higher than normal total lung capacity (TLC).Yet, the alveolar volume (V A ) measured during the T L CO SB manoeuvre is low.This discrepancy between V A and TLC is due to uneven ventilation distribution during the short breath-holding time leading to an incomplete mixing between the inspired gas and the residual volume gas.Indeed, the more severe is airflow limitation, the higher is the underestimation of TLC by VA.As TLCO is the product of kCO and VA divided by effective barometric pressure, this results in an underestimation of the potential true T LCO of the patient.However, the degree of underestimation is unknown, because poorly ventilated units are likely to be more severely affected by the disease.This is an inherent limitation of the T L CO SB in COPD.To circumvent this problem it has been proposed to use the V A measured by plethysmography [1].However, this would lead to an overestimation of true T L CO because it would include poorly ventilated units and assign them a kCO equal to that of well ventilated units [4].Moreover, this dual method is impractical and is consequently not applied.

What is KCO (or TLCO/VA)?
The carbon monoxide transfer coefficient (KCO) is often written as T LCO divided by alveolar volume (T LCO/VA).It is important to grasp what KCO actually represents.If we take the equation for TLCO: then, dividing by V A : the units for KCO are: mmol CO • min -1 • kPa -1 • L -1 , or ml CO • min -1 • mm Hg -1 • L -1 in traditional units.In the latter case, note that V A is expressed in ml STPD in the numerator because it represents a volume of CO, and in L BTPS in the denominator because it represents a volume of air.
Thus, the transfer coefficient KCO is simply another way to express the permeability factor kCO, one of the two primary measurements allowing to derive T LCO [4,6].
The expression TLCO/VA is misleading because it implies a "correction of T LCO for alveolar volume".According to this view, a low TLCO with a low V A and a normal KCO (or TLCO/VA) would be interpreted as a lung of reduced volume but with normal transfer of CO.This is definitely wrong because in a normal lung, KCO increases exponentially when alveolar volume is reduced, as during a voluntary incomplete expansion, or during a reduced expansion in a patient with a neuromuscular disorder (Figure 1).This is due to an increase in the surface to volume ratio for diffusion per alveolus as the alveoli become smaller.Thus, when V A is low, a seemingly "normal KCO" actually reflects an abnormal gas transfer.Practical considerations for interpretation of T LCO From the preceding considerations, it appears that the interpretation of T LCO is unfortunately not straightforward.Nevertheless, this test is highly informative and useful if the following steps are taken [6]: First, the quality of the manoeuvres must be scrupulously checked.This is now made easy by the display of error codes.A common failure is an insufficient inspired volume, whereas it should exceed 90% of vital capacity.
Second, KCO and V A should be considered as the primary measurements and should be analysed along with T L CO.The relationship between these variables can be seen as follows:

KCO • VA = TLCO efficiency per number of gas exchange lung unit contributing units capacity
For practical purposes, the most common disorders affecting T L CO can be grouped in a limited number of pathophysiological or clinical entities.

1) Incomplete lung expansion
The lung is normal but incompletely inflated, like in neuromuscular disorders, obesity, kyphoscoliosis, pleural effusion, or in case of a poorly performed test.The low VA is associated with an elevated KCO.As a result, T LCO is only mildly reduced, falling by 3% per 10% fall in VA.

2) Discrete loss of alveolar units
There is a discrete loss of lung, for instance a whole lung, a lobe, or several units in several lobes, but the lung remaining is normal.Examples are pneumonectomy, lobectomy, lobar collapse, local destruction (post-TB, bronchiectasis), localized alveolar infiltrate (sarcoidosis).The loss of alveolar units is reflected by a low V A. Because blood flow of lost units is diverted to remaining units, KCO increases slightly.As a result, TLCO falls relatively less than V A .

3) Diffuse loss of alveolar units
The alveolar units most severely affected by the disease are lost, but the remaining lung is affected as well by the disease.Examples are diffuse fibrosis (idiopathic pulmonary fibrosis, connective tissue diseases, pneumoconiosis), alveolar infiltrates (inflammatory infiltrate, hypersensitivity pneumonitis, pneumocystis carinii pneumonia), cardiovascular disorders (pulmonary oedema, chronic heart failure).The V A is low, KCO is low to "normal", and T L CO is markedly reduced.
Case 4: 79-year-old man, pulmonary asbestosis: Comment: the interstitial lung disease and the diaphragm weakness both contribute to a low VA.In case of diaphragm weakness with a normal lung, the KCO would be elevated, whereas the seemingly "normal" value observed here reflects the effect of interstitial disease.

4) Obstructive lung disease
The VA is low because of incomplete mixing between the inspired gas and the residual volume gas during the short breath-holding time.The KCO differs according to the underlying disease.In emphysema, KCO is low because of the loss of alveolar-capillary surface.As a result, TLCO is se-verely reduced.In contrast, KCO may be increased in asthma where the pulmonary microcirculation is preserved and cardiac output may be increased. Case

5) Pulmonary vascular disorders
Examples are pulmonary arterial hypertension, chronic thromboembolic pulmonary hypertension, vasculitis, sickle-cell disease, hepatopulmonary syndrome.The V A is normal or near normal, KCO is reduced due to the vascular disorder, and T L CO is reduced approximately to the same degree.

6) Increased pulmonary blood volume
Both KCO and T L CO are mildly to moderately increased when pulmonary capillary blood volume is increased, as in case of high cardiac output or of left-to-right shunt.

7) Alveolar haemorrhage
Intermittent alveolar haemorrhage occurs in anti-GBM disease, pulmonary vasculitis, systemic lupus erythematosus, and idiopathic haemosiderosis.The VA is mildly reduced by alveolar filling with blood, and KCO is markedly increased because inhaled CO reacts with extravascular haemoglobin.The KCO often increases to more than 150% of predicted, and a 30% increase in KCO over baseline values is suggestive of alveolar haemorrhage [10].After haemorrhage ceases, the halftime of the return of KCO to baseline is 24 hours.It is essential to adjust KCO and T L CO to a normal haemoglobin concentration because of fluctuating anaemia in these patients [6].
Thus, by analysing V A , KCO and T L CO together one is able to discriminate between the most common abnormal patterns (Table 1).Although not straightforward, this analysis of T L CO is worth the effort because major therapeutic decisions may depend on this test, like performing surgery or initiating immunosuppressive or cytostatic therapy.

Correspondence
Figure 1 Variation of carbon monoxide transfer coefficient (KCO) and transfer factor (TLCO) with lung expansion in normal subjects.The KCO, TLCO and alveolar volume (VA) are expressed in percent of values measured at full inspiration.The KCO increases exponentially with decreasing VA.As a result, TLCO decreases only mildly with decreasing VA.
Med Wochenschr (1871-2000) Swiss Med Wkly (continues Schweiz Med Wochenschr from 2001) Editores Medicorum Helveticorum 8: 76-year-old man, pulmonary emphysema: Single-breath method.VA: alveolar volume; KCO: carbon monoxide transfer coefficient; TLCO: carbon monoxide transfer factor.Official journal of the Swiss Society of Infectious disease the Swiss Society of Internal Medicine the Swiss Respiratory Society