Thoracic Gas Volume (VTG), Lung Compliance (CL), Airway Resistance (Raw), Airway Conductance (Gaw)
Thoracic gas volume (VTG) composes all the air contained within the thorax, whether or not it is in ventilatory communication with the rest of the lung. The measurement of VTG through body plethysmography is an application of Boyle law, which states that for a gas at constant temperature, pressure and volume vary inversely (P1V1 = P2V2).
Compliance of the lung (CL) is an indication of its elasticity, and increases in obstructive diseases (e.g., emphysema) and decreases in restrictive processes (e.g., interstitial lung disease).
Raw is a measurement of the resistance to airflow in the tracheobronchial tree (which is a hyperbolic function). Raw increases with decreased lung volumes and decreases with higher lung volumes in a nonlinear, hyperbolic fashion.
Airway conductance (Gaw) is the reciprocal of Raw, decreasing in a linear fashion as Raw increases.
Have the patient sit in the plethysmograph (a glass-walled boxlike chamber). Fit with nose clips and have the patient breathe through a mouthpiece/filter (bacterial/viral) combination connected to a transducer.
Ensure that the plethysmograph door is secured. Delay the test for a few minutes to allow the box pressure to stabilize due to temperature changes.
Instruct the patient to perform a panting maneuver while holding the cheeks rigid and the glottis open against a closed shutter located within the transducer assembly. Plethysmograph box and mouth pressures are displayed on a monitor for subsequent determination of the VTG.
Ask the patient to breathe rapidly and shallowly. Plethysmograph box pressure changes versus flow are displayed on a monitor for subsequent determination of the Raw.
To determine CL, pass a balloon catheter through the nose into the patient’s esophagus. Typically, the balloon catheter is lightly coated with a topical anesthetic (e.g., lidocaine jelly) for patient comfort. Ensure that the inflated balloon is connected to a transducer and instruct the patient to breathe normally. Changes in intraesophageal pressure during normal respiration (which mimic changes in intrapleural pressure) are recorded for determination of the CL.
See Chapter 1 guidelines for intratest care.
An increased VTG demonstrates air trapping, consistent with obstructive pulmonary disease (e.g., emphysema).
An increased Raw or decreased Gaw demonstrates increased resistance to airflow through the tracheobronchial tree; this is seen in asthma, emphysema, bronchitis, and other forms of obstruction.
An increase in CL (i.e., lung is more distensible) is seen in obstructive diseases.
A decrease in CL (i.e., lung is stiffer) is seen in fibrotic diseases, restrictive diseases, pneumonia, congestion, and atelectasis.
Pretest Patient Care
Explain the purpose and procedure of the test.
Assure the patient that although the chamber is airtight, the test only takes a few minutes. A technician will be in constant attendance to open the door should that be necessary. There is also a handle inside of the body box should the patient feel anxious and need to open the door. Assess for ability to comply with test requirements and instructions. Assess for predisposition to claustrophobia, panic attacks, or other similar responses.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Allow the patient time to rest quietly if necessary.
Review test results; report and record findings. Modify the nursing care plan as needed.
Explain the possible need for follow-up testing and treatment.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.