When you visit a pulmonologist complaining of shortness of breath, a chronic cough, or exercise intolerance, the clinical conversation almost always leads to the same question: what is your lung function actually doing? The answer doesn't come from a stethoscope. It comes from objective physiologic testing — carefully designed measurements that tell your physician exactly how your lungs are moving air, exchanging gas, and responding to stress.
At PulmoCrit, we perform this testing in-house at our PC Physiology & Metabolic Center — a dedicated facility offering the full spectrum of pulmonary physiology studies, from standard spirometry to advanced cardiopulmonary exercise testing (CPET). This means you don't need a separate referral or a separate trip to a hospital-based lab. Your physician orders the test, our respiratory therapists perform it, and your doctor interprets it — all within the same practice, often on the same day as your clinic visit.
This article explains every test we offer, what each one measures, why it matters, and who typically needs it.
PC Physiology & Metabolic Center — located at our Encino office (16260 Ventura Blvd., Suite 600), serving patients from Northridge, Encino, and Thousand Oaks. Testing is physician-ordered, certified-therapist-performed, and physician-interpreted — no outsourcing, no delays.
Why Objective Physiologic Testing Matters
Many respiratory conditions are invisible to a physical exam. A patient with moderately severe COPD may have a completely normal chest X-ray. An asthma patient in between attacks may breathe normally in the office and have unremarkable lung sounds. Someone with unexplained exertional dyspnea may look entirely fine at rest.
Pulmonary physiology testing fills that diagnostic gap. These are standardized, reproducible measurements — not impressions or estimates — and they provide information that cannot be obtained any other way. They answer questions like: Is there airflow obstruction? Is it reversible? Is the lung stiff or weak? How much oxygen is crossing from the air into the blood? Why does this patient get breathless on a flight of stairs when their resting tests look normal? No imaging, no blood test, and no clinical examination can answer those questions as precisely.
Equally important: objective data tracks change over time. A single spirometry result is useful. Serial spirometry over three, five, or ten years tells you whether your lung function is stable, slowly declining, or improving in response to treatment. That longitudinal picture is often the most important data your pulmonologist has.
Spirometry
What It Is
Spirometry is the cornerstone of pulmonary function testing. You breathe into a mouthpiece connected to a flow sensor, following precise instructions from our respiratory therapist: take the deepest breath you can, then blast the air out as hard and as fast as possible, and keep blowing until your lungs are completely empty. The maneuver is repeated at least three times to ensure reproducibility.
What It Measures
The two most important values from spirometry are:
- FEV₁ (Forced Expiratory Volume in 1 second) — how much air you can forcibly expel in the first second of exhalation. This is the primary marker of airflow obstruction. Normal FEV₁ is roughly 80% or more of the predicted value for your age, sex, and height.
- FVC (Forced Vital Capacity) — the total volume of air you can exhale after a maximal inhalation.
- FEV₁/FVC ratio — the ratio of these two values. A low ratio (typically below 0.70) is the defining feature of obstructive lung disease, meaning air flows out too slowly. A normal or high ratio with a reduced FVC suggests a restrictive pattern — the lungs hold less volume than they should.
Who Needs It
Spirometry is the first test ordered for virtually any patient with respiratory symptoms. It is the primary diagnostic tool for COPD and asthma, a baseline measurement for anyone on inhaled medications, and a pre-operative assessment for patients undergoing lung or major surgery. It is also used to monitor progression of known lung disease and to screen high-risk individuals (smokers, occupational exposures) before symptoms develop.
Bronchodilator Reversibility Testing
What It Is
Bronchodilator reversibility testing is almost always performed as part of the same session as spirometry. After the baseline spirometry is completed, you receive two to four puffs of a short-acting bronchodilator (albuterol) via a metered-dose inhaler. You wait 10–15 minutes, then repeat the spirometry maneuvers. The before-and-after values are compared.
What It Tells Your Physician
A significant response — defined as an improvement in FEV₁ of at least 12% and 200 mL — is called "bronchodilator reversibility" and is characteristic of asthma. It means the airflow obstruction is at least partially reversible with medication, which distinguishes asthma from COPD (where the obstruction is largely fixed, though partial improvement can still occur). The presence or absence of reversibility also guides medication selection and predicts how well a patient will respond to inhaled bronchodilators long-term.
Full Pulmonary Function Tests (PFTs) — Lung Volumes and DLCO
What They Are
A "full PFT" panel extends beyond spirometry to measure two additional dimensions of lung physiology: lung volumes (how much air the lungs can hold) and diffusion capacity (how efficiently the lungs transfer gas to the blood). Together with spirometry, these three measurements provide a complete physiologic fingerprint of the respiratory system.
Lung Volumes — What They Measure
Lung volume testing requires a body plethysmograph — a sealed, transparent chamber about the size of a phone booth that you sit inside while performing breathing maneuvers. The plethysmograph uses pressure changes to calculate the amount of air remaining in your lungs when you breathe normally and after a full exhalation.
Key measured values include:
DLCO — Diffusing Capacity of the Lung for Carbon Monoxide
DLCO measures how efficiently gas crosses from the air sacs (alveoli) into the bloodstream. You take a slow, deep breath of a gas mixture containing a trace amount of carbon monoxide and hold it for 10 seconds, then exhale slowly. The difference between the concentration of CO inhaled and CO exhaled tells us how effectively the lungs are transferring gas.
DLCO is reduced when the alveolar-capillary membrane is damaged, thickened, or destroyed — as occurs in pulmonary fibrosis (where scarring thickens the membrane), emphysema (where alveolar walls are destroyed), and pulmonary vascular disease (where reduced blood flow through the lungs limits gas exchange). A normal FEV₁ with a significantly reduced DLCO is a classic pattern of early emphysema or interstitial lung disease.
DLCO is also used to monitor response to treatment in interstitial lung disease, to guide oxygen prescribing, and to assess surgical risk before major thoracic or cardiac surgery.
FeNO — Fractional Exhaled Nitric Oxide Testing
What It Is
FeNO is a non-invasive breath test that takes less than five minutes and requires no special effort from the patient. You breathe out slowly and steadily through a mouthpiece at a constant flow rate while a sensor measures the concentration of nitric oxide in your exhaled breath. That's the entire test.
Why Nitric Oxide?
Nitric oxide is produced by the cells lining the airways in response to eosinophilic inflammation — the type of airway inflammation that drives allergic and eosinophilic asthma. When airways are inflamed, FeNO levels rise. A value above 40 ppb is considered elevated and strongly suggests eosinophilic airway inflammation. Values above 50 ppb are associated with an excellent response to inhaled corticosteroids and predict which patients are likely to benefit from biologic medications targeting the eosinophilic pathway (such as dupilumab, mepolizumab, or benralizumab).
What FeNO Changes in Clinical Practice
FeNO is particularly useful in three scenarios. First, it confirms eosinophilic asthma in a patient with equivocal spirometry — a high FeNO in a patient with borderline airflow obstruction tips the diagnosis toward asthma rather than another cause of symptoms. Second, it guides steroid prescribing: a low FeNO in a patient with asthma-like symptoms suggests that inhaled corticosteroids may provide little benefit, prompting evaluation for other diagnoses. Third — and increasingly importantly — it is a prerequisite for biologic therapy selection. All major guidelines now recommend FeNO testing as part of the workup for patients being considered for add-on biologic therapy for severe asthma.
Methacholine Challenge Testing
What It Is
A methacholine challenge is a provocation test — meaning it intentionally tries to trigger a mild, controlled asthma response in order to test whether your airways are hyperresponsive. You inhale progressively increasing concentrations of methacholine (a substance that causes airway smooth muscle to contract) while spirometry is repeated after each dose. If your FEV₁ drops by 20% or more at a concentration of 8 mg/mL or less, the test is considered positive for airway hyperresponsiveness.
When Is It Used?
Methacholine challenge is most valuable when asthma is clinically suspected but spirometry is normal. A patient with a history of episodic cough, chest tightness, or exertional wheeze — but normal spirometry in clinic — may have asthma that is simply in remission at the time of testing. The methacholine challenge can unmask the underlying airway hyperresponsiveness that characterizes asthma even in the absence of active symptoms. A negative methacholine challenge (no significant drop in FEV₁ even at high methacholine doses) has a very high negative predictive value — effectively ruling out active asthma. The test is always supervised by a respiratory therapist with a bronchodilator immediately available to reverse any bronchospasm at the end of the procedure.
Cardiopulmonary Exercise Testing (CPET) — The Complete Picture
What It Is
Cardiopulmonary exercise testing — commonly called CPET or VO₂ max testing — is the most comprehensive non-invasive physiologic assessment available. Where spirometry evaluates the lungs at rest, CPET evaluates how the heart, lungs, muscles, and circulation work together as an integrated system under the physical stress of exercise. It is performed on a stationary cycle ergometer (or, less commonly, a treadmill) while you wear a fitted mask connected to a breath-by-breath gas analyzer.
The test begins at a very low workload and increases in a controlled ramp protocol until you reach your maximum voluntary effort or a clinical endpoint is reached. Throughout the test, a continuous stream of physiologic data is captured every breath. The resulting dataset is rich enough that an experienced physician can identify — with precision — whether a patient's exercise limitation is primarily pulmonary, cardiac, vascular, muscular, or a combination.
What CPET Measures — The Key Variables
What CPET Can Diagnose That Nothing Else Can
The diagnostic power of CPET lies in its ability to reveal the mechanism behind symptoms, not just their presence. Consider a patient who becomes severely breathless walking to their car but has a normal spirometry, a normal echocardiogram, and normal resting oxygen levels. Every individual resting test is normal. CPET integrates all of these systems simultaneously under load — and can show, for instance, that while the heart and lungs both appear normal at rest, the oxygen pulse plateaus at low workload (suggesting reduced cardiac reserve), or that SpO₂ drops to 87% only during peak exercise (suggesting pulmonary vascular disease), or that ventilation hits its maximum capacity at an unusually low work rate (suggesting occult airflow limitation or chest wall restriction). These patterns are invisible to any resting test.
Common Reasons CPET Is Ordered at PC Physiology & Metabolic Center
- Unexplained dyspnea — breathlessness that can't be fully explained by resting tests alone
- Pulmonary arterial hypertension (PAH) — serial CPET tracks disease course; VO₂ peak and VE/VCO₂ slope are established prognostic markers
- Pre-surgical evaluation — major thoracic, cardiac, or abdominal surgery; VO₂ max predicts peri-operative risk and complications
- Heart failure — peak VO₂ guides therapy escalation decisions, including LVAD and transplant listing criteria
- COPD and pulmonary rehabilitation — establishes individualized training prescription and quantifies functional reserve
- Pulmonary fibrosis — monitors functional decline and guides supplemental oxygen prescribing
- Disability evaluations — provides objective, reproducible quantification of functional work capacity
VO₂ Max — What It Actually Means
VO₂ max — formally written as VO₂ peak in clinical settings — is the rate at which your body can consume oxygen during maximal exercise, expressed in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). It is the gold standard measure of cardiorespiratory fitness and functional capacity.
For healthy, sedentary adults, VO₂ max typically ranges from about 25–35 mL/kg/min. Elite endurance athletes can reach values above 70–80 mL/kg/min. In clinical populations, a VO₂ max below 10 mL/kg/min represents severe impairment; values between 10–16 are moderate impairment; values above 20 are generally associated with preserved functional capacity.
What makes VO₂ max clinically important is not just the number itself, but the shape of the curve leading up to it and all the data points alongside it. A VO₂ max of 14 mL/kg/min reached with a normal VE/VCO₂ slope, normal oxygen pulse, and preserved breathing reserve tells a very different story than the same number reached with a VE/VCO₂ of 45, a flat oxygen pulse, and exertional desaturation. The former might reflect deconditioning or mild cardiac limitation; the latter strongly suggests pulmonary vascular disease. The number without the context is only half the answer.
The Six-Minute Walk Test (6MWT)
What It Is
The six-minute walk test is a simple, standardized exercise test performed in a hallway. You walk at your own pace for six minutes while oxygen saturation and heart rate are monitored. The total distance walked, SpO₂ nadir, and degree of perceived exertion are recorded and compared to predicted values for your age, sex, and height.
What It Measures — and How It Differs from CPET
The 6MWT measures submaximal functional capacity — what you can actually do in day-to-day life, rather than your absolute physiologic maximum. It doesn't provide the detailed breath-by-breath data of CPET, but it is simpler, less expensive, and validated as a prognostic marker in diseases like pulmonary fibrosis, COPD, and pulmonary hypertension. It is also used to identify exertional desaturation in patients who maintain normal resting oxygen levels.
In practice, the 6MWT and CPET are often complementary rather than interchangeable. The 6MWT tells you how far someone walks; CPET tells you why they stop. Both are performed at PC Physiology & Metabolic Center, and many patients receive both during the same visit when a complete functional assessment is needed.
How the Tests Work Together — A Clinical Perspective
The true value of an in-house physiology lab is integration. Consider what a complete physiologic evaluation looks like for a patient referred for unexplained exertional dyspnea:
- Spirometry and bronchodilator response — establishes whether there is airflow obstruction and whether it is reversible. If obstruction is found, the severity is graded and a diagnosis of COPD or asthma is established or refined.
- FeNO testing — if asthma is on the differential, FeNO helps classify it as eosinophilic or non-eosinophilic, guiding the choice of controller medication or biologic eligibility.
- Full PFTs with lung volumes and DLCO — establishes whether restriction is present (ruling in interstitial lung disease or chest wall disease), quantifies the degree of air trapping and hyperinflation (relevant in emphysema), and assesses gas exchange efficiency (a reduced DLCO with otherwise preserved mechanics points toward vascular disease or emphysema even before CT shows it).
- CPET — if resting tests don't explain symptoms, or if a complete characterization of exercise limitation is needed, CPET provides the integrative view. It identifies whether the limitation is pulmonary, cardiac, vascular, or deconditioning — and quantifies severity in ways that inform prognosis and guide intervention.
- 6MWT — provides a real-world functional baseline, identifies exertional desaturation, and serves as a serial monitoring tool.
At most community practices, this complete workup requires referrals to two or three separate facilities, weeks of scheduling, and results fragmented across different medical records systems. At PulmoCrit's PC Physiology & Metabolic Center, it is coordinated in a single location, with all results reviewed and integrated by your own physician.
How to Prepare for Physiology Testing: On the day of your visit, avoid smoking for at least 4 hours, hold short-acting bronchodilators for 4 hours (unless told otherwise), avoid caffeine before CPET, and wear comfortable clothes and shoes. If you use a rescue inhaler, bring it. Your physician's office will provide specific preparation instructions when scheduling your test.
Why In-House Testing Matters
Pulmonary physiology testing is only as good as the quality of its execution and interpretation. Poorly performed spirometry — insufficient effort, submaximal exhalation, inadequate coaching from the technician — produces unreliable results that can lead to misdiagnosis. At PC Physiology & Metabolic Center, all testing is performed by credentialed respiratory therapists following ATS/ERS standardization guidelines, and every result is personally reviewed and interpreted by your PulmoCrit physician, not an outside reading service.
In-house testing also creates clinical efficiency that genuinely changes how quickly patients get answers and start treatment. When spirometry, FeNO, and a full PFT are all performed on the same day as your consultation appointment, your physician has objective data in hand during your visit — enabling a diagnosis and treatment plan to be established in a single encounter rather than over multiple return trips.
Schedule Testing at PC Physiology & Metabolic Center
Whether your physician has already ordered pulmonary function testing, CPET, or FeNO — or you're experiencing unexplained breathlessness, exercise intolerance, or a known lung condition that needs objective monitoring — our in-house physiology center provides the complete evaluation your care requires.
Request a Testing Appointment → Call (844) 428-5864PC Physiology & Metabolic Center · 16260 Ventura Blvd., Suite 600, Encino · Mon–Fri, 9 AM–5 PM