Sunday, August 26, 2007
Measuring The Blood Pressure
'Normal' or 'acceptable' blood pressure varies with age, state of health and clinical situation. At birth, a typical blood pressure is 80/50 mmHg. It rises steadily throughout childhood, so that in a young adult it might be 120/80 mmHg. As we get older, blood pressure continues to rise and a rule of thumb is that normal systolic pressure is age in years + 100. Blood pressure is lower in late pregnancy and during sleep.
From this, you can see that a systolic pressure of 160mmHg for an elderly man or 90 mmHg for a pregnant woman may be quite normal.
From this, you can see that a systolic pressure of 160mmHg for an elderly man or 90 mmHg for a pregnant woman may be quite normal.
Automatic non-invasive blood pressure measurement (Electronic Sphygmomanometer)
Automatic devices which essentially apply the same principle as the oscillotonometer have been produced (e.g. the 'Dinamap' made by Critikon). They require a supply of electricity. A single cuff is applied to the patients arm, and the machine inflates it to a level assumed to be greater than systolic pressure. The cuff is deflated gradually. A sensor then measures the tiny oscillations in the pressure of the cuff caused by the pulse. Systolic is taken to be when the pulsations start, mean pressure is when they are maximal, and diastolic is when they disappear. They can produce fairly accurate readings and free the hands of the anaesthetist for other tasks. There are important sources of inaccuracy, however. Such devices tend to over-read at low blood pressure, and under-read very high blood pressure. The cuff should be an appropriate size. The patient should be still during measurement. The technique relies heavily on a constant pulse volume, so in a patient with an irregular heart beat (especially atrial fibrillation) readings can be inaccurate. Sometimes an automatic blood pressure measuring device inflates and deflates repeatedly "hunting" without displaying the blood pressure successfully. If the pulse is palpated as the cuff is being inflated and deflated the blood pressure may be estimated by palpation and reading the cuff pressure on the display.
Automatic devices which essentially apply the same principle as the oscillotonometer have been produced (e.g. the 'Dinamap' made by Critikon). They require a supply of electricity. A single cuff is applied to the patients arm, and the machine inflates it to a level assumed to be greater than systolic pressure. The cuff is deflated gradually. A sensor then measures the tiny oscillations in the pressure of the cuff caused by the pulse. Systolic is taken to be when the pulsations start, mean pressure is when they are maximal, and diastolic is when they disappear. They can produce fairly accurate readings and free the hands of the anaesthetist for other tasks. There are important sources of inaccuracy, however. Such devices tend to over-read at low blood pressure, and under-read very high blood pressure. The cuff should be an appropriate size. The patient should be still during measurement. The technique relies heavily on a constant pulse volume, so in a patient with an irregular heart beat (especially atrial fibrillation) readings can be inaccurate. Sometimes an automatic blood pressure measuring device inflates and deflates repeatedly "hunting" without displaying the blood pressure successfully. If the pulse is palpated as the cuff is being inflated and deflated the blood pressure may be estimated by palpation and reading the cuff pressure on the display.
Tuesday, August 21, 2007
Monday, August 06, 2007
Try to get the datasheet of the AD620 chip and read it.
You acn now make your very own ECG machine.
You acn now make your very own ECG machine.
Labels: AD 620
Luckily, we don't have to build all of this because commercial instrumentation amplifiers are available. The AD620 has a CMRR of 100 dB with a differential gain that is adjustable up to 1,000. This is done by changing the value of a resistor, RG on the input of the chip. . By buying a commercial amplifier we also don't need to worry about the offset null because the internbal circuits is perfectly balanced in the factory.
Labels: The AD620 Instrumentation amp
AC couplingThe other problems we had were with DC offset, drift and motion artefact. All of these problems are caused by very low-frequencies (DC is zero frequency), so we need to use a high-pass filter with a very low cutoff frequency. We need to keep the cutoff frequency very low to avoid degrading the ECG signal, so the capacitor needs to be large (0.47 or 1 uF) and the resistor similarly large (e.g. 1 MW).
Labels: AC Coupling
In practice, it is difficult to precisely match resistors that are discrete components. To overcome this problem the entire circuit is put on a single integrated circuit, since IC manufacturing technology enables precise resistor ratios to be obtained. Such chips as Analog Devices AD620 find widespread use in working with low-level signals with large common-mode components in noisy environments - just the sort of situation we find in biomedical engineering.
Why would the Instrumentation be better to record the ECG ?
Unfortunately, the differential amplifier turns out to be rather limited in its performance because of the low input impedance of (R2 + R1). To improve this, two bootstrapped buffer amplifiers (which are just op-amps with unity gain) are commonly added, which results in the simple instrumentation amplifier:
Unfortunately, the differential amplifier turns out to be rather limited in its performance because of the low input impedance of (R2 + R1). To improve this, two bootstrapped buffer amplifiers (which are just op-amps with unity gain) are commonly added, which results in the simple instrumentation amplifier:
Labels: The Instrumentation Amplifier
Since the output is proportional to the difference between the two voltages, anything (e.g. noise) which is present on both inputs will be cancelled out. However, a signal (e.g. the EKG) which is different on the two inputs will be amplified, which of course is exactly what we want. The ratio of the gain of the difference gain to the common gain (usually expressed in dB) is called the Common Mode Rejection Ratio (CMRR).
CMRR = Differential Gain Common Gain
An typical differential amplifie has a CMRR of about 30,000. So, supposing we build a circuit with a differential gain of 1,000, this means that the common gain (acting on the noise) will be:
Differential Gain / Common Gain= 30,000
So, Common Gain = Differential Gain / CMRR = 1,000/30,000 = 1/30 or 0.03
In other words, instead of getting amplified, the noise will actually be attentuated 30-fold.
CMRR in dBJust to be awkward, gains and CMRR are usually quoted in dB, so for voltage gains, the equation becomes:
CMRR (dB) = 20 log (Differential Voltage Gain / Common Voltage Gain)
Thus, a typical differential amp will have a CMRR of 20 log 30,000 = 90 dB
How about going the other way? Well, the inverse of a log to base 10 is 10 raised to its power, so:
Voltage Gain ratio = 10CMRR/20
e.g. 90 dB = 1090/20 , i.e. a voltage gain ratio of 104.5 or 30,000
CMRR = Differential Gain Common Gain
An typical differential amplifie has a CMRR of about 30,000. So, supposing we build a circuit with a differential gain of 1,000, this means that the common gain (acting on the noise) will be:
Differential Gain / Common Gain= 30,000
So, Common Gain = Differential Gain / CMRR = 1,000/30,000 = 1/30 or 0.03
In other words, instead of getting amplified, the noise will actually be attentuated 30-fold.
CMRR in dBJust to be awkward, gains and CMRR are usually quoted in dB, so for voltage gains, the equation becomes:
CMRR (dB) = 20 log (Differential Voltage Gain / Common Voltage Gain)
Thus, a typical differential amp will have a CMRR of 20 log 30,000 = 90 dB
How about going the other way? Well, the inverse of a log to base 10 is 10 raised to its power, so:
Voltage Gain ratio = 10CMRR/20
e.g. 90 dB = 1090/20 , i.e. a voltage gain ratio of 104.5 or 30,000
Labels: Common mode rejection
This new design is called a differential amplifier, because it amplifies the difference between the two input voltages.
The gain is given by Gain = R2.(V2-V1) R1
The gain is given by Gain = R2.(V2-V1) R1
Labels: The Differential Amplifier
Sunday, August 05, 2007
ECG has an amplitude of only about 1 mV, so to detect it an amplifier is needed. There is a problem, though - electrical noise, or electromagnetic interference (EMI). EMI is generated by many common appliances, such as power lines, fluorescent lights, car ignitions, motors and fans, computers, monitors, printers, TVs and television, and cell phones. When the ECG is amplified, the noise is amplified too, and often swamps the ECG signal. Luckily, the noise is usually of a higher frequency than the ECG. For example, AC hum is 60 Hz, which is above the highest frequency in the ECG (about 20 Hz). So the noise can be reduced by low-pass filtering.
Labels: ECG AMPLIFIER
