Understanding Pressure Transducers and Pressure Flow Signals

By Rick Swanson, RPSGT, CRTT

A sensor or electrode acts as a transducer by taking one form of energy and converting it to another form of energy (i.e., a signal that is put into sleep data recording systems).  A snore sensor takes the vibrations of snoring and produces an electrical signal of that vibration.  Similarly, a pressure transducer measures the pressures of breathing and puts out an electrical signal showing the flow of breathing.

How does a pressure transducer do that?

Image1 Understanding Pressure Transducers and Pressure Flow SignalsImage2 Understanding Pressure Transducers and Pressure Flow Signals

Courtesy Maxim Integrated Products

The figure above on the left is of a Wheatstone bridge circuit. The figure above on the right depicts a pressure transducer chamber. The Wheatstone bridge circuit is etched directly onto the pressure transducer chamber, and when it is in use a voltage is applied to the circuit.  The pressure coming into the pressure transducer chamber deforms, or bulges, the side of the chamber.

The resisters shown in the figure above on the left (R1, R2, R3, and R4) are variable resisters.  Their values change as the transducer wall deforms when pressure is introduced to the pressure chamber.  The signal output value is changed by the changes in the resister values and a pressure signal is produced.

Pressure transducers used in sleep can be broken down by power source and by transducer type. In sleep testing today there are two types of power sources for pressure transducers:  1) replaceable batteries; 2) a piezo power source.  There are also two types of pressure transducers:  1) a differential transducer (two ports); 2) a gauge transducer (one port). The differential transducer is usually battery-powered and the single port is piezo-powered.  The differential transducer measures the pressure difference between the two input ports. The gauge measures one pressure against ambient pressure.


Differential Gauge
Power source Replaceable batteries Piezo crystals
Inputs Two One
Advantages

Measure patient flow on CPAP
Compatible with more systems

Small size
No batteries to replace
Disadvantages Batteries to be replaced Compatibility issues

Often the type of recording system can dictate which type of pressure transducer can be used.
System compatibility should be determined with your system provider or the pressure transducer manufacturer.

Filter settings

To record the signal from the pressure transducer accurately, the flow channel must have a long time constant. This is a very low setting on the low frequency filter (high pass filter).  This will allow the signal to remain above baseline and show the possible narrowing of upper airways. The result is a prolonged or stretched waveform that appears as flattening when compared to previous, “untroubled” breathing waveforms.

To see all the details in the signal, use the following settings:

  • Low Frequency Filter (High Pass Filter): 0.05 – 0.01 Hz
  • Time Constant: 3 – 5 seconds
  • High Frequency Filter (Low Pass Filter): 15 – 70 Hz
  • Sampling Rate: 25 Hz or higher

Assuming a normal/average respiratory rate of 12 breaths per minute, 12/60seconds = 0.2 Hz.
The high frequency filter can be set very low without affecting the pressure waveform.

The Technical Specifications of AASM Guidelines (pg 19) has a Low Frequency Filter of 0.1 Hz for all respiration channels.  Using that filter setting will make the pressure signal look like a thermal signal.  Using the correct low frequency filter is not a violation of the Guidelines.

The Low Frequency Filter (LFF) is also known as the Time Constant (TC).
0.16=1/2p
TC=0.16/Frequency (F)
F=0.16/TC

There is an inverse relationship between the LFF and TC. The lower the LFF, the longer the TC.
The longer the TC, the more time the signal has to display flattening. A LFF setting of 0.05 Hz is approximately three seconds. A breathing rate of twelve breaths per minute allows five seconds for each breath cycle. The TC is the time it takes for the signal/waveform to return to baseline. If the TC is set too short the signal will not be able to show flattening. Conversely, if the LFF is set too high the signal will not be able to show flattening above baseline.

ScreenShot115 Understanding Pressure Transducers and Pressure Flow Signals

Cannulas

The AASM Guidelines call for a nasal pressure signal. The thermal device measures both nasal and oral airflow.  A nasal cannula is less obtrusive than a nasal/oral cannula. Nasal/oral cannulas may be subject to a loss of signal due to pressure going through the oral prong.

When placing the nasal cannula on a patient, the nasal prongs may be trimmed, as needed, for patient comfort. The prongs should be placed at the opening of the nostrils to measure the pressure of flow.  The cannula should be securely positioned to a thermal device placed against the upper lip.   This should help minimize the tendency for the cannula to roll into the nostril and “bottom out” against the inside of the nose.

A cannula is a column of air. The column has the capacity to change pressure and transmit the pressure to the pressure transducer. There is no flow through the cannula. It changes pressure and this pressure change is transmitted to the pressure chamber.

Image3 Understanding Pressure Transducers and Pressure Flow Signals

On inhalation the nasal prongs experience a negative pressure due to the Venturi effect of the air flowing around the nasal prongs into the nostril. On exhalation they act as pitot tubes, accepting a positive pressure from the air pressing in on the nasal prong openings.  These pressure changes are transmitted to the pressure chamber.

Square root transformation

The Guidelines of the AASM state under the Respiratory Rules for adults: “Nasal air pressure transducer with or without square root transformation of the signal.”  For children, the Guidelines state: “Nasal air pressure transducer without square root transformation of the signal.”

What is this square root transformation of the signal?  It is a method by which the electrical output from the pressure transducer is put through a circuit that would electronically apply a square root mathematical expression to the signal.  The objective of this is to make the signal more linear.  The signal is generated by the deformation of the pressure chamber and is not always linear.

If your sleep facility tests both children and adults, use pressure transducers without the square root transformation of the signal.  There are no pressure transducers currently available with the square wave transformation.

There have been a number of articles published showing that this transformation does not appreciably change results. All measures of sleep-disordered breathing remain the same whether the square root transform is used or not. One of the papers validating this is:

“Validation of Nasal Pressure for the Identification of Apneas/Hypopneas during Sleep” Steven J. Heitman, Raj S. Atkar, Eric A. Hajduk, Richard A. Wanner and W. Ward Flemons Departments of Medicine and Sociology, Foothills Hospital and University of Calgary, Calgary, Alberta, Canada

This paper is also available online at:  http://ajrccm.atsjournals.org/cgi/content/full/166/3/386#TBL1

Anatomy of the pressure waveform

Nasal pressure waveforms contain a lot of information about a patient’s upper airway status and can look very different from thermal signals.  A thermal signal is nice and linear, as the elements heat and cool. The pressure waveform, however, can be jagged and spiky. This is because it is measuring pressure which can have sudden changes, especially as someone breathes.

z1 Understanding Pressure Transducers and Pressure Flow Signals

The figure above shows a typical pressure airflow waveform labeled with the actions of breathing that it depicts.

z2 Understanding Pressure Transducers and Pressure Flow Signals

The diagram above shows a peculiarity that often appears and is called the end expiratory pause.  There are three parts to breathing: 1) inhale; 2) exhale; 3) pause.  For some people this pause will show up in the pressure flow waveform.

z3 Understanding Pressure Transducers and Pressure Flow Signals

This is a depiction of the flattening seen in the pressure flow waveform due to snoring or upper airway resistance syndrome (UARS).  The narrowing of the upper airways during a snore or UARS prolongs the inhalation. This is because the same volume that was previously moved in and out of the lungs now must go through a smaller opening, reducing the pressure and taking longer to move the air into the lungs.

z4 Understanding Pressure Transducers and Pressure Flow Signals

The above figure shows more detail of the anatomy in the pressure waveform during a snoring event.

z5 Understanding Pressure Transducers and Pressure Flow Signals

The above figure shows why the low frequency filter settings are important. The bottom waveform clearly shows the flattening of UARS using a DC input from the pressure transducer. The middle waveform also displays this flattening in an AC channel with a long Time Constant (low filter set at 0.05 or 0.01 Hz). At the top a decrease in amplitude is visible. It is very difficult, however, to see flattening in the waveform because the short Time Constant (higher low filter setting ~0.1Hz) does not allow the signal to remain above baseline long enough to properly display flattening.

All of the included figures are representations of possible waveforms seen with pressure transducers. The figures are suggestive of settings to expect while using pressure flow signals. If a patient does not display a signal exactly like these illustrated, it does not mean that the patient isn’t having these events.

Using pressure airflow in conjunction with thermal airflow affords the technologist the ability to see apneas and hypopneas with the specific equipment developed for those events.  The signals from the two technologies look different because they are two different technologies measuring the same parameter. Neither technology is more right than the other.  Pressure is more sensitive to hypopneic events and thermal is more sensitive to apneic events. Together they provide a more complete picture of the patient’s airflow.

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