AtCor Medical

The SphygmoCor System uses a pressure probe to record the pressure wave at the radial artery, which is then calibrated with the brachial blood pressure. The system then derives the pressure wave at the ascending aorta utilising a generalised transfer function. The generalised transfer function essentially describes the properties of the brachial artery and the effect it will have on the pressure wave as it travels along it. By applying this transfer function, to the calibrated radial pressure wave, it allows the derivation of the calibrated aortic pressure wave.

Has the system been validated? Is it reproducible?

The SphygmoCor System has been validated against invasive recordings of the aortic pressure wave. The latest validation paper showed the derivation of the central pressures to be accurate to within 1mmHg. All reproducibility papers have shown the system to be very reproducible. The validation and reproducibility papers are listed in the Scientific Publications.

Does the system come with a computer?

The SphygmoCor System does not come with a computer. It can be installed on any PC or laptop that meets the minimum computer requirements.

What type of computer do I need?

The computer to be used with the system must meet the following specifications: IBM compatible PC or notebook computer with Pentium Processor P4 or greater, 1 GB RAM, 1024 x 768 256-colour XGA display, 60 GB initial free hard disc space, CD-ROM drive, Windows standard printers drivers, dedicated USB port, Windows 2000, XP (Prof) or Vista (Business). The SphygmoCor EM3 is not supported on Windows NT/95/98/ME.

Can I install the system on a Mac?

No, the system is not compatible with Mac computers.

How does the probe work? Is it a doppler probe?

The probe is a pressure tonometer. It measures the pressure wave via applanation tonometry. Applanation tonometry is the process of flattening (but not occluding) the artery against underlying bone. Applanating the artery allows for accurate recording of the intra-arterial pressure without penetrating the skin or blood vessels.

How sensitive is the probe?

The probe has been validated against intra-arterial pressure recording and shown to be very accurate, particularly across the higher frequency components (Kelly et al, J Vasc Med Biol Vol1 No3 1989). The probe is therefore able to detect even slight changes in the radial pressure waveform.

How durable is the probe? What maintenance is required?

The probe is very durable and has a long life span, provided care is taken with it's use. The probe doesn't require any maintenance, but should be kept clean with an alcohol wipe.

How do I back up the data?

Open "Windows Explorer" (or "My Computer") and select "C:\Program Files\AtCor\Sphygmocor CvMS". In this directory you will find the folders "Data" and "System". Highlight these folders and select "Copy". Now select the drive you wish to back up the data on (eg. network drive, zip drive, CD burner), and select paste.

Troubleshooting

When the software is loading an error message pops up saying “Electronics Module Not Found”

This is usually because the electronics module is not switched on, or isn't connected to the computer. Check that the "Power" light is on and that the cables are properly connected, then click "Yes".
If you are still receiving this message, try selecting a different COM number on the SphygmoCor Configuration Window (click the Settings option under the System menu).
If you are using a USB to Serial Converter, please click here.
If you are using an EM3 module, please click here.

I am having trouble taking a reading, any tips?

1. The patient's arm needs to be stable. Ensure that the arm is rested either on a table or bed, and the wrist is supported and slightly flexed (either with a small rolled-up towel, or the operator's hand).
2. Make sure that the wrist is flat and not being held on an angle.
3. The operator's entire arm should also be supported. Elbow should be resting on the table or bed, and the hand should be resting on the patient's thumb.
4. The tonometer should be held perpendicular to the wrist.
5. Hold the tonometer close to the tip, similar to holding a pen.
6. Place the probe over the area with the strongest pulse, which is generally closer to the bony section of the wrist.

How do I know if I have taken a good reading?

You should always check the Quality Control Section of the report. The following key quality control parameters are given: Average Pulse Height - is the average height of all the pulses, and hence a measure of pulse strength. Ideally, this number should be greater than 100, but numbers greater than 80 are considered acceptable. Pulse Height Variation - is the amount of variation there is in the pulse heights (expressed as a percentage). The variation should be no greater than 5%. Diastolic Variation - amount of variation in the diastolic portion of the pulse wave. This value should also be no greater than 5%. Shape Deviation - amount of variation in shape during systole. This value should be no greater than 4%. If these values are within the respective quality control limits, they will appear in green. If they are outside the limits, they will appear in red. On the "Clinical Report", you will also find the Operator Index. The Operator Index is a number out of 100 that is derived from the above quality control parameters. If the number is over 80, the reading is acceptable. If it is between 75-79, it is borderline. If it is less than 75, the reading is unacceptable.

I don't understand why the reading hasn't met quality control.

If a reading hasn't satisfied quality control, it is either because the probe wasn't directly on the artery, or there was too much variation in the downward pressure applied with the tonometer. This can be determined by looking at the pulse overlay in the quality control section. When the probe is positioned directly on top of the artery the waveforms will all have the same shape, and be very "smooth". If the probe is on the side of the artery, there will be a lot more variation between each waveform and the signal will be more "noisy". If every waveform in the pulse overlay has the same shape, but there is a wide spread, it indicates that the probe was directly on the artery, but the downward pressure on the probe was varied during recording.

Why does my PWA report display the label "Note"?

The PWA report may display a "Note" for three reasons:
1. Peripheral and Central T1 (time to the first peak) are outside the physiological range (80 to 150 msec).
2. The ratio between Peripheral T1 and Central T1 is more than 70%, which is called 'Correspondence of Aortic and Peripheral T1’.
3. Low Operator Index.
The reason for the "Note" is displayed on the PWA Clinical Report Screen.
More information can be found here.

Why does my PWA report display a "Note" when I have a high Operator Index?

The high Operator Index indicates that the pressure waveforms captured over the 10 second period are consistent and do not vary considerably from beat to beat, however this does not provide a reflection of physiological features of the waveform. The timing of the first peak observed in the waveforms (Peripheral and Central T1) are physiological features of the waveform and the SphygmoCor software will indicate when these are detected outside the physiological range. Hence it is possible to have a "Note" label on a highly reproducible set of waveforms.

Should a report be discarded if it displays a "Note" label?

If the report displays a "Note" it is advisable to repeat the measurement. If on repeated measurements the "Note" label is still present for the same reason (as mentioned above); care should be taken when analysing the results.

Why can repeated measurements of AIx (Augmentation Index) on the same subject vary?

AIx, like blood pressure varies through the day, and can be affected by coffee, cigarettes, heavy meals and activity. These conditions will change your heart rate and blood pressure and consequently your AIx (though may not be permanently altered). To ensure reproducible and accurate measurements, all subjects therefore should be measured under similar conditions. It is important to take SphygmoCor measurements when the subject is relaxed (either lying or sitting down) and has abstained from coffee, cigarettes, heavy meals or exercise prior to the measurement, unless these are the conditions specific to the study. All subjects have to be under similar conditions when measured, to insure reproducible and accurate measurements.

What does the ‘out of range dp/dt’ message mean? And should I disregard reports that produce such a message?

Max dp/dt is the highest slope during early systole. The SphygmoCor software measures this on the radial pressure waveform and will display the message ‘out of range dp/dt’ when this value is very low or very high. There are a number of reasons that a high or low Max dp/dt may be observed one of which is height of the pulse in mVolts in the raw recording, as opposed to being associated with Pulse Pressure. That is, the max dp/dt may be out of range due to technical reasons rather than physiological reasons.
When the message of ‘out of range dp/dt’ is observed, care should be taken when analysing the results from the measurement. Duplicate measurements are always recommended particularly in instances when measurements have cautionary messages displayed.

Why does it take so long to calculate a report?

There are a number of factors that can influence SphygmoCor's performance. If you are experiencing difficulties, we recommend the following steps:
1. Perform a database pack.
2. Ensure there are no other applications running at the same time as SphygmoCor.
3. Use a dedicated SphygmoCor computer.

I am experiencing problems using SphygmoCor with Windows Vista.

SphygmoCor must be configured correctly on your computer to execute reliably on the Microsoft Windows Vista Operating system. Please follow the instructions here.

Pulse Wave Velocity

Can I purchase the PWV system on it’s own?

No, the Pulse Wave Velocity system (SphygmoCor CPV) is sold as an add-on to the Pulse Wave Analysis System (SphygmoCor CP).

There are 4 algorithms available for pulse wave velocity. What is the algorithm for, and which one should I use?

These algorithms are used to determine the point at which diastole ends and systole begins. A technical sheet with further information on these algorithms can be found here. If you do not have a preference for the algorithm to be used, we recommend using "3. Intersecting Tangents" as this is the algorithm the system uses for pulse wave analysis.

The ECG recording is quite poor, how can I improve it?

Ensure that the ECG pads you are using have not dried out. Try cleaning the skin and repositioning the electrodes. The system uses the Lead II ECG configuration, make sure that each pad is placed in the correct location and attached to the correct lead, as displayed in the figure below.

How do I know if my PWV recording satisfies quality control?

The pulse wave velocity (m/s) of the measurement is presented with the standard deviation (PWV +/- SD). The standard deviation should be equal to or less than 10% of the velocity eg. If the PWV = 8.0m/s, the SD should be < 0.8. Also, the standard deviations of the mean timing data from ECG to Site A and ECG to Site B should be equal to or less than 6% of the mean time. These values can be found in the table at the bottom of the report screen, and appear in green if they are equal to or less than 6%, and red if they are greater than 6%. More information can be found here.

Is it possible to measure PWV in patients with a pacemaker and, if so, will it affect the quality of the measurement?

It is possible to measure PWV on most patients with a pacemaker. The main determinant will be the quality and morphology of the ECG signal. If the ECG signal displays a square wave (due to the Pacemaker itself) then this will affect the QRS detection, which is essential for measuring PWV with SphygmoCor. In these instances, care should be taken when analysing the results. Not all pacemakers have the square pulse, however, and in these instances good quality PWV measurements should be possible.

When performing a carotid to femoral PWV measurement, why do we subtract the carotid-sternum distance from the sternum-femoralis distance to get the total distance travelled by the pulse?

"The proximal and distal pulse waves may be recorded from two different arterial sites, where pulse waves propagate in opposite directions, such as for the carotid-femoral PWV measurement. In fact, the pulse wave generated by the left ventricle contraction is propagated throughout the aorta, iliac and femoral arteries in the opposite direction than through the carotid artery. In this case, evaluating the covered distance by superficial measurement ... presents some margin of error.." (Ref)

In other words, using the superficially measured carotid to femoral distance to calculate PWV will produce an error because of the opposite direction of the pulse propagation of both arterial sites. To compensate for that, you need to subtract the carotid-sternum distance from the sternum-femoralis distance and use the result as the distance for calculating PWV.

Ref: Asmar R, "Arterial Stiffness and Pulse Wave Velocity: Clinical Applications", 1999 Editions scientifiques et medicals Elsevier SAS, 1999 Paris.