BCRT: British Columbia Respiratory Therapy

DLCO: The diffusing capacity of the lung for carbon monoxide, also known as

TLCO : Transfer factor of the lung for carbon monoxide.

Used to determine transfer of gas from the distal airspaces-alveoli into the pulmonary capillaries.

DLCO= V(.)CO/(PACO2-PC(-)CO

Where:

• V(.) = uptake of CO in ml of CO at STPD conditions/min
• PACO2 is the average partial pressure of CO in alveoli
• PC(-)CO is the average partial pressure of CO in the pulmonary capillary plasma.

(Hb) has a very high affinity for CO (200 times > then 02).

Partial Pressure of CO in plasma (PC(-)) =0 when COHb is low

As a result, DLCO=V(.)CO/PACO

Increases in DLCO occur in:

• Pulmonary hemorrhage
• Polycythemia
• Exercise
• Diseases with increased blood flow
• Asthma

Decreases in DLCO occur in:

• Cardiovascular diseases
• Emphysema
• Parenchymal lung disease (Fibrosis)
• Anemia
• CRF

The routine management of mechanical ventilation in the ICU includes monitoring of peak airway pressures, plateau pressures and determining airway resistance.
When volume or pressure is pushed through an airway, a peak pressure is generated. This peak pressure is the sum of the amount of pressure necessary to get through the airways, inflate the alveoli and displace the chest wall and diaphragm. An inspiratory hold is performed on the ventilator to measure how much this pressure (plateau pressure) is actually being sensed in the alveoli once the lungs are inflated. By subtracting the plateau pressure from the peak pressure, we can calculate the resistance from the airways.
In managing mechanical ventilation, we routinely look at the plateau pressure to determine the limits to which we can increase our ventilating volumes. For the majority of patients, the chest wall and diaphragm are relatively compliant so are not a major factor in ability to ventilate patients. In cases of stiff chest wall or distended abdomens, the plateau pressure may be misleading as the pressure sensed within the alveoli is in part due to the pressures from the stiff chest wall or diaphragm.
Recently, esophageal catheters have been used to help optimize ventilation of patients with concerns re. stiff chest walls or diaphragms (distended abdomens). A catheter inserted in the esophagus is in close proximity to the pleural space. Esophageal pressures can be used as a surrogate to pleural pressures. Use of esophageal pressure monitoring can then help to differentiate between:

o pressure in the pleural space, attributable to chest wall and diaphragm and
o pressure distending the lungs (transpulmonary pressure) which might result in barotrauma

Ptpt (transpulmonary) = Paw (plateau) – Pes (esophageal)

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High Frequency Oscillatory Ventilation (HFO) is a ventilatory strategy that employs very small tidal volumes (often less than anatomic dead space) combined with very fast rates or frequencies (where 1 Hertz or Hz = 60cycles/min).

The Sensormedics 3100B high frequency oscillator consists of a continuous positive airway pressure circuit with an integrated motor-driven piston/diaphragm for generating the oscillations. There is active inspiration as well as active expiration on the oscillator.

Gas transport during HFO is thought to be as a result of several factors: molecular diffusion, direct alveolar ventilation (bulk gas flow to the proximal alveoli), net convective transport caused by asymmetric gas-velocity profiles, improved gas mixing caused by Taylor dispersion in turbulent flow, pendelluft, and cardiogenic mixing.

In HFO, alveolar ventilation (and thus CO2 elimination) is dependent on frequency and tidal volume, but relatively independent of lung volume. Oxygenation is “uncoupled” from ventilation; that is, it is proportional to mean airway pressure and lung volume.

And an interesting article from Stanford: