Spirometry

Pulmonary Function Testing (PFT) is a broad term to describe several tests.  It requires specification by the prescribing physician for the individual components:

  • Spirometry- FEV1 and FVC.
    • FEV1 is the forced expiratory volume in 1 second.   A normal person would be able to expire 80% of their forced vital capacity in 1 second.  Individuals with obstruction will have a reduced FEV1 volume.  Forced vital capacity is the volume an individual can expire in 6-8 seconds.  An FEV1/FVC ratio <0.70 signifies obstruction.
  • Lung volumes- TLC, RV, ERV.
    • At Temple this is measured by body plethysmography.  RV and TLC are normal if values are between 80-120%.  RV and TLC percentages >120% signify gas trapping and hyperinflation, respectively.  Percentages <80% signify restriction.
  • Diffusion capacity- DLCO, DLCO/VA
    • Diffusion capacity is measured through inhalation of carbon monoxide.  DLCO/VA will correct for the patients alveolar volume.
  • Methacholine challenge
    • Increasing amounts of methacholine will be given to a patient to see if it will cause a reduction in FEV1.  FEV1 drop of 20% will lead to immediate discontinuation of the test.  Patients cannot take their bronchodilator the day of the test.
  • 6 minute walk test

 

PFT’s and Oxygen Delivery

James Brown gave us an excellent talk on “PFT and Oxygen Delivery,” elucidating the various oxygen delivery modalities available to our hypoxemic patients.

PFT’s

  1. Look at the loop. Does it look obstructed or restricted?
  2. FEV1/FVC (post bronchodilator). If less than 70, they have obstruction. Then check FEV1 to determine the severity of obstruction.
  3. FVC: If FVC low, then be suspicious of restriction
  4. Check the volumes. TLC < 75 = restriction, TLC >110 = hyperinflation. RV > 120 = air trapping
  5. DLCO and DLCO/Va (corrected for volume) tells you the diffusion capacity

Oxygen Delivery in Hypoxemic Patients

Too much O2 can result in worsening V/Q matching in COPD patients (oxygenating areas that have difficulty with gas exchange)

Nasal Cannula: Your standard run-of-the-mill nasal cannula offers up to 6L/min of O2.  Anything flow above that is limited by the narrow tubing that feeds it.  So if your patient is still hypoxic on 6L NC, then seek another method of oxygenation.

Face mask: can go up to 10L.

Nasal Pendant: appears like a nasal cannula except it also has a disk chamber that fills during a patient’s expiratory phase. During inspiration, the patient breaths in the oxygen that has filled the chamber.  Unclear how much oxygen is actually delivered, and it varies depending on respiratory rate (ie – shorter expiratory phase for the disk chamber to fill with oxygen).  Typically can offer somewhere in the range of 6-12L/min of O2.

Venti-Mask: Like a face mask except that it has a dial to adjust the FiO2. One of the few oxygen delivery modalities in which you can definitely state the percent FiO2 the patient is receiving.

Non-Rebreather Mask (NRB): in theory a NRB offers 100% FiO2.  Since at least one of the tabs on the front of the mask is usually removed, it ends up providing about 80% FiO2.

A non-rebreather should only be used in an emergency.  Otherwise, use other oxygen delivery modality.

BIPAP: is used for work of breathing and hypercapnia. Top number is Inspiratory Positive Airway Pressure (IPAP) and bottom number is Expiratory Positive Airway Pressure (ie –  PEEP). Increasing the difference between the two numbers allows for better treatment if hypercapnia is an issue.

  • CPAP is just EPAP/PEEP, but no IPAP.  So there’s just one number listed (eg – 8cm H2O).  It is used for OSA/OHS and also helpful in pulmonary edema.
  • Note: for patients in respiratory distress, do not use CPAP.  It does not provide the offloading of respiratory muscles during inspiration that would help both alleviate their distress and decrease O2 consumption by the muscles.

High flow nasal cannula (at Temple the brand is Aquinox) can be used when flows greater than 10L are needed. They can go up to 60L and can also titrate the FiO2. The high flow also allows for a PEEP effect (but only if the mouth is closed) to keep the airway open in very mild OSA and to recruit more alveoli. Dead space can still be cleared if the mouth of the patient is open.

Ventilator: We’ll save this discussion for another lecture, but in essence it provides up to 100% FiO2, ability to add Heliox, offloads the patient’s respiratory muscles, multiple possible respiratory settings, etc.