11.1 Spirometry Artifacts: Hesitation, Coughing, Early Termination, Glottic Closure, Leak

11.1 Spirometry Artifacts: Hesitation, Coughing, Early Termination, Glottic Closure, Leak

Diagnostic spirometry serves as the clinical cornerstone for evaluating pulmonary mechanics, quantifying airflow limitation, and monitoring respiratory disease progression. However, the physiological validity and diagnostic utility of forced vital capacity (FVC) maneuvers depend strictly upon adherence to standardized performance criteria established by the American Thoracic Society (ATS) and the European Respiratory Society (ERS). Because spirometry is an effort-dependent diagnostic test, physical artifacts, submaximal patient effort, and technical equipment malfunctions frequently distort flow-volume loops and volume-time curves. The pulmonary function technologist must possess advanced clinical pattern-recognition skills to immediately identify technical artifacts, determine their physiological impact on key spirometric indices (such as $FEV_1$, $FVC$, $FEV_1/FVC$, and $PEFR$), and apply targeted instructional interventions before concluding testing.

---

ATS/ERS Acceptability vs. Repeatability Criteria Overview

Evaluating spirometry data requires a rigorous two-step evaluation process:

1. Acceptability Criteria: Applied to individual maneuvers to ensure each effort is technically satisfactory, free from physical artifacts, and executed with maximal patient effort from full inflation to complete exhalation.
2. Repeatability Criteria: Applied across a series of acceptable maneuvers to confirm precision and consistency. For adults, the two highest $FVC$ values must agree within 0.150 L (150 mL), and the two highest $FEV_1$ values must also agree within 0.150 L (150 mL) (or within 0.100 L / 100 mL if $FVC$ is less than $1.000\text{ L}$). A minimum of 3 acceptable maneuvers must be obtained, with a maximum of 8 trials per session to prevent patient fatigue.

+-----------------------------------------------------------------------+
|                   SPIROMETRY ACCEPTABILITY CHECKLIST                  |
+-----------------------------------------------------------------------+
| 1. Explosive start of exhalation: Vbe < 0.100 L (or < 5% of FVC)       |
| 2. Free of cough within the first 1.0 second of exhalation            |
| 3. Continuous exhalation without glottic closure or hesitation        |
| 4. Satisfactory end of forced expiration (EOFE): plateau of < 0.025 L |
|    for >= 1.0 s, OR forced expiratory time >= 15 s, OR a repeatable   |
|    FVC when the patient cannot continue                               |
| 5. Free of mouthpiece obstruction (tongue/teeth) or system air leaks  |
+-----------------------------------------------------------------------+

---

Hesitation at Start of Exhalation & Back-Extrapolation ($V_{be}$)

An acceptable $FVC$ maneuver requires an immediate, maximal, and explosive effort from full inflation (Total Lung Capacity, $TLC$). When a patient hesitates, pauses, or gradually ramps up expiratory effort at the beginning of forced exhalation, the start of the test is delayed, compromising the precise timing of forced expiratory volume in 1 second ($FEV_1$).

The Standardized Zero-Time Back-Extrapolation Method

To establish an objective zero-time point ($t_0$) for timed expiratory volume measurements, ATS/ERS standards mandate the back-extrapolation technique:

• A straight line is mathematically extended back to the time axis along the steepest slope of the volume-time curve (which corresponds to the peak expiratory flow rate, $PEFR$).
• The point where this steepest tangent line intersects the baseline volume axis defines the true zero-time ($t_0$).
• The volume accumulated between the initial baseline and $t_0$ is designated as the back-extrapolated volume ($V_{be}$).

Acceptability Thresholds for $V_{be}$

• Adult Threshold: $V_{be}$ must be less than 0.100 L (100 mL) or less than 5% of the total FVC, whichever value is greater.
• Pediatric Threshold (children < 6 years): $V_{be}$ must be less than 0.075 L (75 mL) or less than 12.5% of FVC.

Physiological & Mechanical Impact

If $V_{be}$ exceeds the acceptable threshold ($V_{be} > 0.100\text{ L}$):

• The maneuver demonstrates excessive hesitation and MUST be rejected from $FEV_1$ calculation.
• Hesitation falsely alters the calculated zero-time, which can lead to overestimation or underestimation of $FEV_1$ depending on software algorithms, while spuriously lowering measured $PEFR$.
• Flow-Volume Loop Appearance: The initial ascending limb displays a rounded, convex, or non-linear slope rather than a sharp, nearly vertical spike to peak flow.
• Volume-Time Curve Appearance: A lazy, shallow S-shaped curve appears at the initiation of exhalation before rapid volume accumulation begins.

Technologist Troubleshooting & Coaching

To correct hesitation, the technologist must emphasize continuous inspiration to absolute $TLC$ followed immediately—without holding the breath at full inflation—by an explosive "blast" of air. Use dynamic, authoritative verbal commands: "In, in, in all the way... now BLAST it out! Push! Push!"

---

Coughing During the Forced Expiratory Maneuver

Coughing is an involuntary protective reflex caused by sudden airway compression and rapid vocal cord adduction-abduction cycles. In pulmonary function testing, coughing introduces violent flow and pressure oscillations that disrupt spirometric tracings.

Critical Timing Distinction: 1st Second vs. Subsequent Exhalation

Graphical Presentation of Cough Artifacts

• Flow-Volume Loop: Displays sharp, jagged, high-frequency serrations or multiple transient vertical spikes along the descending expiratory limb. If coughing occurs at peak effort, $PEFR$ exhibits multiple fragmented peaks.
• Volume-Time Curve: Shows step-like vertical jumps or stair-step fluctuations as rapid bursts of volume are intermittently expelled and interrupted.

Clinical Management

When a patient coughs consistently:

1. Provide a cup of room-temperature water to soothe airway irritation.
2. Instruct the patient to avoid hyperventilating prior to the maneuver.
3. Allow adequate rest intervals (2–3 minutes) between trials to minimize airway cough receptor activation.
4. If cough persists despite intervention, document the presence of cough artifact in the technologist notes and report $FEV_1$ from post-1-second acceptable portions if applicable.

---

Early Termination and Incomplete Exhalation

Early termination occurs when a patient stops forced exhalation before reaching an acceptable end of forced expiration - before a volume plateau, before a forced expiratory time of 15 s, and without a repeatable FVC. This artifact is extremely common in patients with severe airway obstruction, elderly individuals, or uncooperative subjects.

ATS/ERS Expiratory Duration & Plateau Criteria

To satisfy ATS/ERS acceptability standards, a forced exhalation must achieve at least ONE of the following end-of-test criteria:

1. Volume Plateau Criterion (preferred): The patient maintains continuous expiratory effort until volume changes by less than 0.025 L (25 mL) over a continuous period of at least 1.0 second.
2. Forced Expiratory Time $\ge 15$ seconds: Severe airflow obstruction can require this long; the 2019 standard treats 15 s as a stopping point rather than something to push past.
3. Repeatable FVC / Patient Cannot Continue: The patient cannot or should not continue (dizziness, presyncope, exhaustion) and the FVC obtained is within the repeatability tolerance of, or larger than, the largest previously observed FVC.

The 6-second rule is retired. "At least 6 seconds in adults, 3 seconds in children under 10" came from the 2005 standard and was removed in 2019 whenever a genuine plateau is demonstrated. A short blow with a clean 1-second plateau is acceptable; a 7-second blow still climbing is not.

Physiological Consequences of Early Termination

• Falsely Reduced $FVC$: Truncating exhalation misses the slow, low-flow volume contribution from small, high-time-constant airways.
• False Elevation of $FEV_1/FVC$ Ratio: Because $FEV_1$ is completed in the first second while $FVC$ is artificially truncated, the calculated ratio ($FEV_1/FVC$) increases mathematically. This can falsely mask an underlying obstructive defect or misclassify an obstructive pattern as normal or restrictive.

Normal Complete Exhalation:   FEV1 = 2.0 L, FVC = 4.0 L  --> FEV1/FVC = 50% (Obstructive)
Early Terminated Maneuver:   FEV1 = 2.0 L, FVC = 2.2 L  --> FEV1/FVC = 91% (Falsely Normal!)

Graphical Features

• Flow-Volume Loop: The descending limb terminates abruptly at a high flow rate, dropping vertically straight down to the zero-flow axis instead of gently tapering to zero at $RV$.
• Volume-Time Curve: Truncates sharply with a steep upward slope at the end of the trace rather than flattening into a horizontal asymptote.

Technologist Coaching

Enthusiastic, sustained coaching is required to carry patients through to a genuine plateau: "Keep blowing, keep squeezing, keep pushing! Keep going, don't stop, all the air out!" Visual incentive graphics on spirometry software can significantly improve expiratory duration.

---

Glottic Closure and Vocal Cord Adduction

Glottic closure involves the involuntary reflex closing of the vocal cords during forced expiration, completely blocking airflow while the chest wall muscles continue to contract under high intrathoracic pressure.

Graphical Signature of Glottic Closure

• Flow-Volume Loop: Flow drops instantaneously and perpendicular to zero, creating a sharp vertical wall cut-off.
• Volume-Time Curve: The volume accumulation halts abruptly, displaying an absolutely flat, horizontal line plateau while the maneuver time continues to advance.

Impact on Spirometric Values

• If glottic closure occurs within the first second ($t \le 1.0\text{ s}$), the maneuver is completely invalid and must be discarded because $FEV_1$ cannot be accurately measured.
• If glottic closure occurs after 1.0 second, $FEV_1$ remains valid, but $FVC$ is invalid because true lung emptying to $RV$ was mechanically prevented.

Differentiating Glottic Closure from Early Effort Cessation

• Glottic Closure: The patient continues straining with respiratory muscles against a closed airway; mouth pressure remains elevated despite zero airflow.
• Simple Effort Cessation: The patient relaxes respiratory muscles and opens the mouth or releases the mouthpiece; pressure drops to zero simultaneously with flow.

---

Mouthpiece Obstruction

Mouthpiece obstruction occurs when the breathing tube opening is physically occluded by patient anatomy or equipment malfunction.

Primary Causes

1. Tongue Placement: The patient places the tongue over the front orifice of the mouthpiece during exhalation.
2. Teeth Occlusion: The patient bites down tightly on a flexible silicone or rubber mouthpiece, collapsing the lumen.
3. Mouthpiece Kinking: Disposable cardboard or plastic mouthpieces become deformed or crushed.

Graphical Presentation & Diagnostics

• Flow-Volume Loop: Displays an abnormally flattened, truncated, or dampened peak expiratory flow rate ($PEFR$) with erratic flow dips or a variable plateau near peak flow, despite a smooth volume-time accumulation curve.
• Physiological Discrepancy: High measured lung volumes ($FVC$) paired with disproportionately low, flattened peak flow rates ($PEFR$) without clinical evidence of upper airway obstruction.

Technologist Remediation

• Re-instruct the patient to position the mouthpiece over the tongue, ensuring the tongue sits flat underneath the tube.
• Ensure teeth rest on the outer ridges of the mouthpiece flange rather than biting down on the central tube.
• Utilize rigid bite-block mouthpieces or disposable filtered mouthpieces with internal structural ribs to prevent tube collapse.

---

System Air Leaks (Mouthpiece, Noseclip, & Equipment Circuit)

Air leaks occur when air escapes from the measuring circuit during forced exhalation, or when ambient air enters the system during inspiration, creating volume-accounting errors.

Common Leak Locations

• Patient Interface: Loose seal around the lips, wearing loose dentures, or slipping of the noseclip.
• Equipment Hardware: Cracks in volume-displacement bellows/water seals, loose tubing connections, damaged pneumotachometer pressure lines, or ill-fitting inline viral/bacterial filters.

Graphical Recognition of Air Leaks

• Flow-Volume Loop: The loop fails to close cleanly at the zero-volume baseline. On inspiration following forced exhalation, the loop displays an asymmetrical shift or step-change.
• Volume-Time Curve: Shows a continuous downward drift of baseline volume during expiration (in volume-displacement systems) or an unexpected reduction in volume during sustained effort.
• Inspiratory Leak: Falsely elevates starting lung volume, causing calculated $V_{be}$ to spike or producing erratic volume spikes.

Troubleshooting & Leak Testing Protocols

1. Inspect Noseclip & Lips: Ensure rigid padded noseclip is firmly seated over both nares. Instruct patients with weak facial muscles (e.g., stroke, neuromuscular disease) to use two hands to seal their lips around the mouthpiece flange.
2. Daily Hardware Leak Check: Prior to testing, perform a static pressure leak test on volume-displacement spirometers by occluding the patient port and applying a constant pressure (e.g., $3.0\text{ cmH}_2\text{O}$) using a 3-Liter calibration syringe. Volume loss must not exceed 10 mL/minute.