PFT: spirometers and FV loop

PULMONARY FUNCTION TESTING

Introduction to Lecture #3

• Focus: Spirometers and FV Loop

Equipment Overview

• Two general types of measuring devices:

  • Measure volume.

  • Measure flow.

• Volume-measuring devices:

  • Spirometers

• Flow-measuring devices:

  • Pneumotachometers

• Characteristics of measuring devices:

  • Capacity

  • Accuracy

  • Error

  • Resolution

  • Precision

  • Linearity

  • Output

Equipment Characteristics

• Capacity:

  • Definition: The range or limits of how much the device can measure.

  • Most spirometers are designed for measuring volumes and flow rates appropriate for adults.

• Accuracy:

  • Definition: How well the device measures a known reference value.

  • Established using a super syringe for calibration.

• Error:

  • Defined as the arithmetic difference between reference values and measured values.

  • Accuracy and error are opposing terms; greater accuracy corresponds to smaller error.

Equipment Characteristics Continued

• Resolution:

  • Definition: The smallest detectable measurement by the device.

• Precision:

  • Synonymous with reliability (repeatability) of measurements and the opposite of variability.

  • Defined as having a small standard deviation, indicating high precision.

• Linearity:

  • Definition: The accuracy of the instrument across its entire range of measurement.

  • Some equipment is inconsistent at fast versus slow flow rates.

• Output:

  • Refers to specific measurements made or computed by the instrument.

Types of Spirometers

• Volume-based:

  • Direct volume measurement:

  • Collins water-sealed spirometer

  • Stead-Wells water-sealed spirometer

  • Dry rolling seal spirometer

  • Bellows spirometer

• Flow-based:

  • Measures flow, which reflects volume:

  • Wright's respirometer

  • Pressure differential pneumotachometer

  • Heat dissipation pneumotachometer

Calibration and Quality Control

• All equipment must meet ATS (American Thoracic Society) standards.

• Volume Calibration:

  • Performed using a 3L syringe (Super Syringe).

  • Accuracy is established when the range is +/- 3.5%

  • Acceptable volume range: $2.895 ext{L} - 3.105 ext{L}$

Volume Spirometers

• Collins Water-Sealed:

  • Features a metal bell suspended in water in a sealed container.

  • Operates using volume displacement.

  • Noted for being fairly accurate but very heavy.

• Stead-Wells Water-Sealed:

  • Composed of a lightweight plastic bell in water in a sealed container.

  • Requires no counterbalance; exhibits less resistance than Collins.

  • Regarded as the “gold standard” for spirometry.

• Dry Rolling Seal Spirometer:

  • Characterized by a cylinder with a piston connected via a plastic seal that rolls on itself.

  • Electrical mechanism; relatively accurate.

• Bellows Spirometer:

  • Features round or wedge-shaped bellows that open and close with breathing.

  • Opens on expiration and closes on inspiration; known for being portable.

Flow Spirometers

• Wright's Rotary Vane Respirometer (Wright's Respirometer):

  • Senses flow and indicates volume.

  • Caution: Never use for flows exceeding 300 LPM.

  • Sensitive to moisture and should not be used for PEFR (Peak Expiratory Flow Rate) or FVC (Forced Vital Capacity) due to excessive flow rates.

  • Effective in measuring VT (Tidal Volume), SVC (Slow Vital Capacity), and MV (Minute Ventilation).

• Pressure Differential Pneumotachometer:

  • Converts gas flow into an electronic signal.

  • The degree of pressure difference indicates flow, which helps derive volume.

  • Noted for being small, compact, and portable while maintaining fair accuracy.

• Heat Dissipation Pneumotachometer:

  • Uses the principle that the flow affects a heated element's cooling.

  • Determines flow based on the amount of cooling, indicating volume.

  • Noted for being small, compact, and portable with fair accuracy.

Body Plethysmography

• Also known as the “body box.”

• Capable of performing all spirometry and lung volume measurements.

• Can determine Residual Volume (RV).

• Used to accurately measure lung compliance (cL).

• Important note: must convert ATPS (Ambient Temperature and Pressure Saturated) to BTPS (Body Temperature and Pressure Saturated); many machines handle this automatically.

• Caution: if the box gets too hot, it may alter results; recommended to open the little door for a minute to regulate temperature.

Infection Control

• Low-risk procedure: PFTs (Pulmonary Function Tests) are deemed to have minimal risk of transmitting infectious microorganisms to patients and respiratory therapists (RTs).

• Risks: Potential for direct or indirect contact with saliva, mucus, or blood.

• Standard Precautions:

  • Wear gloves when handling mouthpieces, tubing, and other equipment.

  • Adhere to airborne precautions by wearing an N95 mask for patients with suspected airborne infections.

  • Wash hands and clean equipment surfaces between patients.

• Instruments Maintenance:

  • When disassembling testing instruments for cleaning and disinfecting, consider manufacturer recommendations.

  • Recalibration may be necessary.

  • Using in-line filters can help protect equipment and reassure patients.

FVC Maneuver

• The FVC (Forced Vital Capacity) maneuver generates key informational outputs:

  • FVC

  • FEV1 (Forced Expiratory Volume in 1 second)

  • FEV1/FVC ratio

  • FEF (Forced Expiratory Flow) 200-1200

  • FEF 25%-75%

  • PEFR

• According to ATS guidelines, a minimum of three tests must be conducted.

• FVC is deemed reliable if the FVC from the largest and second-largest trial vary by no more than 150 ml, and FEV1 should also vary by no more than 150 ml from acceptable trials.

Flow Volume Loop (FV Loop)

• The FV Loop consists of FVC followed by forced inspiratory volume (FIV).

• It produces two curves that allow for a comparison of flow rates and volume changes at different points during FVC and FIV.

• Graphically represents obstructive and restrictive lung disorders.

Normal FV Loop Characteristics

• Expiration Curve:

  • Typical volume measurements reach a peak expiration flow at, for example, ??? L.

• Inspiration Curve:

  • Volume measurements reflect various phases of breathing in.

• Specific Measurements:

  • PEFR

  • FEF 200

  • FEF 1200

  • FEF 25%

  • FEF 50% (Vmax50)

  • FEF 75%

  • Corresponding time intervals for each curve.

Effort Dependent and Independent Portions of FVC

• Effort Dependent Portion of FVC:

  • The first 1/3 of the expiratory curve

  • Relies heavily on large airways and patient effort.

  • Greater patient effort correlates with higher FEF 200-1200 and PEFR values.

• Effort Independent Portion of FVC:

  • The last 2/3 of the expiratory curve

  • Influences small airways; flow rate cannot be increased with muscular effort after a maximum flow is achieved.

  • Recognized as “non-effort dependent.”

Quick Interpretation of FV Loop Shapes

• Normal Loop Shape:

  • Indicates normal lung function.

• Obstructive Loop Shape:

  • Characterized by an expiratory curve that is bowed inward, resembling a “scooped” shape.

• Restrictive Loop Shape:

  • Appears as a narrowed loop reminiscent of a “witch’s hat.”

Reversibility Testing

• After classification of type and severity of obstruction, determine reversibility through pre/post bronchodilator flows.

• If FEV1 shows improvement of greater than 12% post-bronchodilator, this indicates reversibility.

Upper Airway Obstructions

• Three Main Types of Upper Airway Obstructions:

  1. Fixed Obstruction:

     • Flattened curve on both inspiration and expiration.

     • Possible causes include tracheal tumor, tracheal stenosis, and foreign body obstruction.

  2. Variable Extrathoracic:

     • Demonstrates a flattened loop pattern on inspiration.

     • Common causes are vocal cord paralysis, vocal cord polyps, and Obstructive Sleep Apnea (OSA).

  3. Variable Intrathoracic:

     • Exhibits a flattened loop during expiration.

     • Potential causes include tumors and tracheomalacia.