What Do FOT Measurements Mean? Understanding R5, R20, X5, AX and Fres

If you have an FOT report in front of you, the numbers may not mean much at first glance. What do R5, R20, R5–R20, X5, AX and Fres actually tell you?

Each measurement looks at a different part of the respiratory response to the pressure oscillations used during the test. Some describe respiratory resistance, while others describe respiratory reactance and the mechanical behaviour of the respiratory system.

Knowing what these FOT measurements mean can make your report easier to follow. You can understand what each number represents, why the values are considered together, and what the overall pattern can tell you about your respiratory response. This guide breaks down the main FOT parameters in simple terms, so you can make better sense of the information in your FOT report.

What Do the Main FOT Measurements Tell You About Your Respiratory System?

FOT measurements show how the respiratory system responds to the pressure oscillations applied during the test. Some FOT parameters describe resistance to airflow, while others describe how the lungs and chest wall respond through their elastic and inertive properties.

Together, these measurements show how respiratory resistance and reactance change across different oscillation frequencies. They can help a pulmonologist look at the respiratory response from different angles rather than relying on one number alone.

The main FOT parameters in an FOT report include:

• R5: Respiratory resistance measured at 5 Hz, reflecting resistance across the respiratory system.
• R20: Respiratory resistance measured at 20 Hz, with a greater contribution from the central airways.
• R5–R20: The difference between R5 and R20, showing how resistance changes with frequency.
• X5: Respiratory reactance measured at 5 Hz, reflecting the elastic and inertive properties of the respiratory system.
• AX: The area under the respiratory reactance curve across a defined frequency range.
• Fres: The frequency at which respiratory reactance reaches zero.

But what does each of these FOT parameters actually tell you? Let's look at them one by one.

What Does R5 Measure in an FOT Test?

R5 is the respiratory resistance measured at 5 Hz during an FOT test. It tells clinicians how much the respiratory system resists the movement of air while the patient breathes normally.

Think of it like breathing through a tube. Air moves more easily when the tube is open and becomes harder to move when the tube gets narrower. The same basic idea applies to the airways.

“A higher R5 means greater resistance to airflow, while a lower R5 means less resistance during the measurement.”

So, when looking at R5, the important question is not simply whether the number is high or low. Clinicians compare it with the patient's appropriate reference values and consider factors such as age, body size, symptoms, and other FOT measurements.

The 5 Hz measurement also gives R5 a broader view of respiratory resistance. Low-frequency oscillations can reach further into the respiratory system, so R5 can reflect contributions from both larger airways and smaller, more distant airways.

The R5 value is generated during normal breathing and reported as one of the key FOT parameters by oscillometry machines in hospitals. However, R5 cannot show exactly where increased resistance is coming from or identify its cause on its own.

That is why clinicians may compare R5 with R20. Looking at the two measurements shows whether respiratory resistance changes when the oscillation frequency changes, which leads us to the next important FOT parameter.

What Does R20 Measure in an FOT Test?

R20 is the respiratory resistance measured at 20 Hz during an FOT test. It shows how much the respiratory system resists airflow when the oscillations are applied at this higher frequency.

Just like R5, a higher R20 means greater resistance to airflow during the measurement. However, the frequency used for R20 changes what the measurement tells us about the respiratory system.

At 20 Hz, the response is more strongly influenced by the larger, central airways. This makes R20 useful for looking at respiratory resistance where these airways make a greater contribution to the measured response.

That difference becomes more useful when R20 is compared with R5. That’s because R5 reflects resistance measured at 5 Hz, while R20 reflects resistance at 20 Hz. The difference between these two values is called R5–R20, which shows how much measured resistance changes between the two frequencies.

During normal breathing, oscillometry machines in India measure and report R20 as part of the frequency-based FOT measurements. R20 is therefore not meant to be read as a standalone number. Pulmonologists consider it alongside R5, R5–R20, other FOT parameters, appropriate reference values, and the patient's clinical findings.

What Does R5–R20 Tell You About Respiratory Resistance?

R5–R20 is the difference between respiratory resistance measured at 5 Hz and 20 Hz. It shows how much respiratory resistance changes when the frequency of the oscillations changes during an FOT test. In simple terms, pulmonologists can look at the difference like this:

• Smaller R5–R20: Resistance at 5 Hz and 20 Hz is more similar.
• Larger R5–R20: Resistance changes more between the two frequencies.

What Is R5–R20?

R5–R20 is calculated by comparing the resistance measured at 5 Hz with the resistance measured at 20 Hz. The value is therefore not another separate resistance measurement. It describes the difference between two resistance measurements taken at different frequencies.

This matters because R5 and R20 do not reflect exactly the same part of the respiratory system. R5 is measured at a lower frequency and can reflect resistance across more of the respiratory system. R20 is more strongly influenced by the larger central airways.

Why Does R5–R20 Matter?

The difference between R5 and R20 can show how respiratory resistance changes as the FOT frequency changes. A larger difference means that the measured resistance at 5 Hz is further from the resistance measured at 20 Hz.

This frequency-dependent change can provide information about the peripheral parts of the respiratory system, where resistance may behave differently from the larger central airways. Oscillometry machines in hospitals can measure these changes across different frequencies, making R5–R20 one of the FOT parameters available in the test.

A higher R5–R20 does not automatically indicate small-airway disease. Uneven airway narrowing, tissue properties, upper-airway effects, and other factors can also influence the measurement. Pulmonologists therefore consider it alongside other FOT measurements, appropriate reference values, and the patient's clinical findings.

What Does a Higher R5–R20 Mean?

A higher R5–R20 means there is a greater difference between respiratory resistance at 5 Hz and 20 Hz. This can indicate that resistance is more frequency-dependent during the FOT test.

However, a higher R5–R20 does not automatically mean that a person has small-airway disease. Uneven airway narrowing, tissue properties, upper-airway effects, and other factors can also affect the measurement.

Pulmonologists therefore do not interpret R5–R20 alone. They consider it alongside R5, R20, other FOT measurements, appropriate reference values, and the patient's clinical findings.

In short, R5–R20 tells you how much respiratory resistance changes between 5 Hz and 20 Hz. The difference can provide additional information about the respiratory system, but its meaning depends on the wider pattern of FOT findings.

What Does X5 Tell You During An FOT Test?

X5 is the respiratory reactance measured at 5 Hz during an FOT test. It shows how the respiratory system responds to the pressure oscillations based on its elastic and inertive properties.

At 5 Hz, elastic properties have a greater influence on reactance. X5 can therefore provide information about how the lungs and chest wall respond to the pressure changes during the measurement.

X5 is often negative at this frequency. A more negative X5 can reflect changes in the mechanical behaviour of the respiratory system. However, its meaning depends on the overall FOT pattern and should not be interpreted as a standalone finding.

This makes X5 an important FOT parameter when assessing respiratory reactance. Pulmonologists may consider it alongside other FOT measurements, such as AX, to understand how reactance changes across frequencies and what the overall FOT report shows.

What Does AX Tell You in an FOT Test?

AX, or reactance area, measures the area under the respiratory reactance curve across a defined range of oscillation frequencies. It shows the overall change in respiratory reactance across that frequency range during an FOT test.

To put that simply, FOT plots respiratory reactance at different frequencies to create a curve. AX represents the area under that curve, giving pulmonologists a way to look at the overall reactance response rather than focusing on X5 alone.

A larger AX generally means a greater overall reactance response across the measured frequency range. Changes in AX can therefore provide information about changes in the mechanical behaviour of the respiratory system, including changes that may be associated with the peripheral airways.

Note that Pulmonologists always consider AX alongside X5, R5, R5–R20, other FOT parameters, and the patient's clinical findings to understand the overall pattern shown in the FOT report.

What Does Fres Tell You in an FOT Test?

Fres, or resonant frequency, is the frequency at which respiratory reactance reaches zero during an FOT measurement.

But what causes respiratory reactance to reach zero? Two properties of the respiratory system influence it:

• Elasticity
• Inertance

The elastic properties come mainly from the lungs and chest wall. They describe how these structures stretch and recoil when pressure changes.

Inertance relates to the movement of air within the airways. It reflects how the mass of moving air responds to changes in pressure and airflow.

The influence of these two properties changes as the FOT frequency changes. At lower frequencies, elastic effects usually have a greater influence on respiratory reactance. As the frequency increases, inertive effects become more prominent.

These effects balance each other at Fres, causing respiratory reactance to reach zero. Fres can vary with factors such as age and respiratory disease, so oscillometry machines in hospitals can provide this measurement as part of a wider assessment of respiratory mechanics.

Pulmonologists therefore consider Fres alongside appropriate reference information and other FOT measurements. A single Fres value cannot provide a diagnosis. It is one measurement that helps explain how the respiratory system responds to the oscillations used during FOT.

What Do These FOT Measurements Tell You Together?

Together, R5, R20, R5–R20, X5, AX, and Fres show how resistance and reactance change across different oscillation frequencies. Looking at these measurements together can help a pulmonologist understand how the respiratory system responds to the oscillations during an FOT test.

Each parameter answers a different part of the same question:

How is the respiratory system responding to the oscillations during the test?

Why Does the Pattern Matter?

An FOT report does not give clinicians one number that answers every question about respiratory function. Instead, the measurements generated by oscillometry machines in India are interpreted in relation to one another.

For example, a clinician may first look at R5 and R20 to see how respiratory resistance behaves at different frequencies. The difference between them, represented by R5–R20, can then provide additional information about frequency-dependent changes in resistance.

The reactance measurements add another part of the assessment. X5, AX and Fres describe different features of respiratory reactance, helping the clinician examine how the respiratory system responds to the oscillations from a mechanical perspective.

This means two reports could have a similar R5 value but show different patterns across the other measurements. The interpretation may therefore depend on what the other FOT parameters show, rather than R5 alone.

Conclusion

Understanding FOT measurements becomes more useful when those numbers can be captured clearly and interpreted as part of the wider respiratory picture. That is where the right testing system matters. alveoflow is an oscillometry machine designed for hospitals to bring FOT and spirometry together in one system. It allows pulmonologists to assess respiratory mechanics alongside conventional pulmonary function testing.

With quiet-breathing FOT and spirometry available in the same sitting, alveoflow supports a more complete approach to respiratory assessment.

So, are you looking for an oscillometry machine for your hospital or clinic? Explore alveoflow for advanced respiratory testing.