7.2 DLCO Calculations, Hemoglobin/COHb/Altitude Corrections, and VA

7.2 DLCO Calculations, Hemoglobin/COHb/Altitude Corrections, and VA

The interpretation of single-breath Carbon Monoxide Diffusing Capacity ($DL_{CO}$) requires an in-depth understanding of the mathematical models governing gas transfer, the simultaneous measurement of Alveolar Volume ($V_A$), and the physiological variables that modify carbon monoxide binding kinetics. Because $DL_{CO}$ reflects both the anatomical membrane thickness and the functional mass of circulating hemoglobin in the pulmonary capillaries, raw measured values must be adjusted for hemoglobin concentration ($Hb$), carboxyhemoglobin ($COHb$), and barometric pressure / altitude ($P_B$). This section details the equations, correction factors, and diagnostic algorithms required for the NBRC CPFT examination.

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The Krogh Equation and Mathematical Derivation of DLCO

Single-breath $DL_{CO}$ is calculated using the modified Krogh equation, which expresses gas diffusion as a first-order logarithmic rate of CO disappearance from alveolar gas during the breath-hold period:

$DL_{CO} = \frac{V_A \times 60}{t \times (P_B - P_{H2O})} \times \ln \left( \frac{F_{A,CO,0}}{F_{A,CO,t}} \right)$

Where:

• $DL_{CO}$: Diffusing capacity in $\text{mL CO (STPD)/min/mmHg}$ (or SI units of $\text{mmol/min/kPa}$).
• $V_A$: Total alveolar gas volume present during the breath-hold, expressed in Liters STPD (Standard Temperature and Pressure, Dry) for diffusion calculations, derived from $V_A$ in Liters BTPS.
• $t$: Breath-hold time in seconds (measured by the Jones-Meade method), multiplied by 60 to convert to minutes.
• $P_B - P_{H2O}$: Dry barometric pressure ($P_B - 47\text{ mmHg}$ at $37^\circ\text{C}$).
• $F_{A,CO,0}$: Initial fractional concentration of CO in the alveoli at the start of the breath-hold (calculated from tracer gas dilution).
• $F_{A,CO,t}$: Final fractional concentration of CO in the exhaled alveolar gas sample after breath-hold time $t$.

Determination of Initial Alveolar CO Concentration ($F_{A,CO,0}$)

Because inspired test gas dilutes upon entering the Residual Volume of the lungs, the initial alveolar CO concentration cannot be measured directly. Instead, it is calculated from the dilution ratio of the inert, non-absorbable tracer gas (e.g., Helium or Methane):

$\frac{F_{A,CO,0}}{F_{I,CO}} = \frac{F_{A,\text{tracer}}}{F_{I,\text{tracer}}} \implies F_{A,CO,0} = F_{I,CO} \times \left( \frac{F_{A,\text{tracer}}}{F_{I,\text{tracer}}} \right)$

Where $F_{I,CO}$ and $F_{I,\text{tracer}}$ represent the inspired concentrations of CO ($0.3%$) and tracer gas ($10% \text{ He}$), and $F_{A,\text{tracer}}$ is the concentration of tracer gas measured in the exhaled alveolar sample.

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Alveolar Volume ($V_A$) Determination and Single-Breath vs. Plethysmographic TLC

Simultaneously during the single-breath $DL_{CO}$ maneuver, Alveolar Volume ($V_A$) is measured using the single-breath tracer gas dilution principle:

$V_A = \left[ V_I \times \left( \frac{F_{I,\text{tracer}}}{F_{A,\text{tracer}}} \right) - V_D \right] \times \text{BTPS Factor}$

Where $V_I$ is the inspired volume and $V_D$ is the dead-space volume of the mouthpiece and apparatus.

Physiological Significance of $V_A$ Discrepancies

In healthy subjects with uniform ventilation, $V_A$ measured by single-breath dilution closely equals the Total Lung Capacity ($TLC$) measured via body plethysmography or multi-breath helium dilution ($V_A \approx TLC$).

However, in patients with severe obstructive lung disease (e.g., emphysema, chronic bronchitis, severe asthma, or bullous lung disease):

• Gas mixing is extremely maldistributed, and poorly ventilated or trapped bullous gas spaces do not equilibrate with inspired tracer gas during a brief 10-second breath-hold.
• Consequently, single-breath $V_A$ significantly underestimates true TLC ($V_A < TLC_{\text{pleth}}$).
• Technologists must recognize that an artificially reduced $V_A$ due to airflow obstruction will falsely lower the absolute calculated $DL_{CO}$ value, even if the alveolar-capillary membrane itself is normal.

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The Krogh Factor ($DL_{CO}/V_A$ Ratio or $K_{CO}$)

To differentiate whether a low $DL_{CO}$ is caused by loss of intrinsic alveolar-capillary tissue or simply by a reduction in lung volume, clinicians evaluate the $DL_{CO}/V_A$ ratio (also termed the Krogh factor or $K_{CO}$):

$\text{Krogh Factor } (K_{CO}) = \frac{DL_{CO}}{V_A}$

Diagnostic Interpretation Algorithms

1. Low $DL_{CO}$ with Low $DL_{CO}/V_A$ Ratio ($K_{CO}$):
   • Indicates true loss or destruction of the alveolar-capillary membrane unit, thickening of the interstitial membrane, or loss of pulmonary capillary blood flow.
   • Clinical Differential Diagnoses:
     • Parenchymal Interstitial Lung Disease (ILD): Idiopathic Pulmonary Fibrosis (IPF), Sarcoidosis, Asbestosis, Systemic Sclerosis.
     • Emphysema: Destruction of alveolar walls and capillary beds (differentiates emphysema from chronic bronchitis/asthma, which typically have normal $DL_{CO}$).
     • Pulmonary Vascular Disease: Primary Pulmonary Arterial Hypertension (PAH), Chronic Thromboembolic Pulmonary Hypertension (CTEPH), Pulmonary Embolism.
2. Low $DL_{CO}$ with Normal or Elevated $DL_{CO}/V_A$ Ratio ($K_{CO}$):
   • Indicates that the intrinsic alveolar-capillary membrane structure per unit of lung volume is healthy, but total accessible lung volume is restricted.
   • Clinical Differential Diagnoses:
     • Extrapulmonary Restriction: Kyphoscoliosis, chest wall deformities, diaphragm paralysis, neuromuscular disease (ALS, Myasthenia Gravis, Guillain-Barré).
     • Surgical Resection: Post-pneumonectomy or lobectomy (remaining lung tissue exhibits normal diffusion density and hyperperfusion).
     • Submaximal Inspiration: Patient failed to inspire to true TLC during testing ($V_I < 85% VC$).
3. Elevated $DL_{CO}$ ($> 120% \text{ to } 140%$ of Predicted):
   • Reflects increased pulmonary capillary blood volume ($V_c$) or enhanced hemoglobin binding.
   • Clinical Differential Diagnoses:
     • Bronchial Asthma: Increased negative intrathoracic pressures and hyperperfusion recruit apical capillaries.
     • Acute Pulmonary Hemorrhage: Goodpasture syndrome, Granulomatosis with Polyangiitis (Wegener's)—free RBCs in alveolar spaces absorb CO rapidly.
     • Polycythemia: Elevated circulating hemoglobin concentration.
     • Left-to-Right Cardiac Shunts: Increased pulmonary blood flow (e.g., ASD, VSD).
     • Supine Posture or Exercise: Increased venous return and capillary recruitment.

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Hemoglobin (Hb) Corrections

Carbon monoxide uptake is directly dependent on the concentration of functional hemoglobin in pulmonary capillary blood. Anemia reduces the available binding sites, lowering measured $DL_{CO}$, whereas polycythemia increases binding sites and elevates $DL_{CO}$.

Standard ATS/ERS Hemoglobin Adjustment Formulas

To compare a patient's measured $DL_{CO}$ against population predicted norms, the measured $DL_{CO}$ must be adjusted to a standardized reference hemoglobin concentration:

• Reference Hb for Adolescent/Adult Males ($\ge 15\text{ years}$): $14.6\text{ g/dL}$
• Reference Hb for Adult Females and Children ($< 15\text{ years}$): $13.4\text{ g/dL}$

$\text{Adult / Adolescent Males } (\ge 15\text{ yrs}): \quad DL_{CO,\text{adjusted}} = DL_{CO,\text{measured}} \times \left[ \frac{10.22 + Hb}{1.7 \times Hb} \right]$

$\text{Adult Females and Children } (< 15\text{ yrs}): \quad DL_{CO,\text{adjusted}} = DL_{CO,\text{measured}} \times \left[ \frac{9.38 + Hb}{1.7 \times Hb} \right]$

• Clinical Impact: In an anemic patient ($Hb = 8.0\text{ g/dL}$), the measured $DL_{CO}$ will be artificially low; applying the correction formula elevates the $DL_{CO}$ to reflect what the diffusing capacity would be if hemoglobin were normal.

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Carboxyhemoglobin (COHb) and Altitude / Barometric Pressure Corrections

Carboxyhemoglobin ($COHb$) Correction

Cigarette smoking or environmental exposure elevates blood $COHb$. Elevated $COHb$ impacts $DL_{CO}$ via two distinct mechanisms:

1. Anemia Effect: $COHb$ ties up hemoglobin binding sites, reducing functional $Hb$ capacity.
2. Back-Pressure Effect: Dissolved CO in capillary blood reduces the partial pressure gradient between alveolar gas and capillary plasma.

• Rule of Thumb: Every $1%$ increase in blood $COHb$ reduces measured $DL_{CO}$ by approximately $1%$.
• ATS/ERS Back-Pressure Correction Formula: $DL_{CO,\text{COHb adjusted}} = \frac{DL_{CO,\text{measured}}}{1 - \left( \frac{\%COHb}{100} \right)}$

Altitude and Barometric Pressure ($P_B$) Correction

At high altitude, atmospheric barometric pressure ($P_B$) is significantly reduced compared to sea level ($760\text{ mmHg}$).

• Lower $P_B$ reduces the alveolar partial pressure of oxygen ($P_{A,O2}$).
• Lower $P_{A,O2}$ decreases competitive $O_2$ occupancy of hemoglobin binding sites, causing the rate of CO binding ($\theta_{CO}$) to increase.
• As a result, unadjusted $DL_{CO}$ measured at high altitude is artificially elevated.
• ATS Barometric Pressure Correction Formula: $DL_{CO,\text{sea-level equivalent}} = DL_{CO,\text{measured}} \times \left[ 1 + 0.0058 \times (P_B - 760) \right]$ (Note: Since $P_B < 760$ at altitude, $(P_B - 760)$ is negative, which appropriately lowers the measured $DL_{CO}$ to its sea-level equivalent).