ECG Instrumentation PDF

ECG INSTRUMENTATION With a little bit of fizzics! AIMS & LEARNING OUTCOMES Aims To develop an understanding of the scientific principles used for an electrocardiograph Learning Outcomes To describe the technical parameters that define Electrocardiograph performance To state the AHA spec...

ECG INSTRUMENTATION With a little bit of fizzics! AIMS & LEARNING OUTCOMES Aims To develop an understanding of the scientific principles used for an electrocardiograph Learning Outcomes To describe the technical parameters that define Electrocardiograph performance To state the AHA specifications for an Electrocardiograph CORRECT TERMINOLOGY Electrocardiograph Electrocardiogram Tracing produced by an Instrument for recording the electrical activity of Electrocardiograph the heart  Measurand Unit Abbreviation SI UNITS Distance Metre M Frequency Hertz Hz Current Amps I Potential Difference Volts V Resistance Ohms Ω Conductance Siemens G Capacitance Farads F Time Secs S Weight Newtons N Mass Kilograms Kg SCIENTIFIC NOTATION Exponent 1012 109 106 103 10-3 10-6 10-9 10-12 Name Tera Giga Mega Kilo Milli Micro Nano pico Symbol T G M K m μ n p Example TVolts GHz MOhms KHz mVolts microAmp nFarad pfarad BIOPOTENTIAL SYSTEMS Transducer Processor Display  ECG Measurements Amplitude (voltage) Time  EQUIPMENT STANDARDS AND SPECIFICATIONS WHAT IS THE HEART RATE? Paper Speed = 5mm/s Paper Speed = 25mm/s Paper Speed = 50mm/s  AHA SPECIFICATIONS Equipment Specific Operator Adjustable Input Impedance Sensitivity Common Mode Paper speed Rejection Ratio Frequency Response Linearity Time Constant Patient isolation EQUIPMENT SPECIFIC INPUT IMPEDANCE A measure of the impedance to current flow offered by an Electrocardiograph CIRCUITS (OHM’S LAW) V=IR Rtotal = R1 + R2 + R3 (series)  High Impedance Electrode Low Impedance Electrode Electrocardiograph  Low Impedance Electrode High Impedance Electrode Electrocardiograph  INPUT IMPEDANCE (Z) AHA state: input impedance should be greater than 0.5M Effect if not met: The ECG will display with reduced amplitude What should I remember?: Input Impedance should be high What should I do?: Nothing – This cannot be changed  Common Mode Rejection Ratio Electrodes pick up noise including common-mode voltage The ability of the ECG amplifier to reduce common mode voltage noise is known as the Common Mode Rejection Ratio or CMRR CMRR Noise Signal Output Noise 75V 25mV 1.5mV Signal 150mV Calculate the CMRR  Common Mode Rejection Ratio AHA state: CMRR should be at least 1000:1 between 45-65 Hz when imbalanced by 5K Effect if not met: The ECG will be noisy What should I remember?: CMRR should be high What should I do?: Nothing – This cannot be changed  COMMON MODE REJECTION RATIO Poor CMRR Good CMRR Note: Common mode noise mainly comes from mains interference. In the UK this is 50 cycles per second (50Hz). LINEARITY Non-Linear Output Linear Input If points on a graph lie in a straight line then the relationship is said to be linear.  LINEARITY AHA state: maximum error must be less than 5% of the chart width Effect if not met: ECG waveforms will be distorted What should I remember?: An ECG system should be as linear as possible What should I do?: Nothing – This cannot be changed  TIME CONSTANT The ability of the ECG system to process low frequency or slowly changing signals e.g. T waves Definition: time taken for the amplitude of a step change in voltage to decay to 36.8% of its initial value.  Time Constant 36.8% is 3.68mm Response Time Constant Input  TIME CONSTANT AHA state: Time constant must be greater than 3.2 seconds Effect if not met: Low frequency parts of an ECG waveform will be distorted What should I remember?: Time constant should be long enough to display an undistorted T wave What should I do?: Nothing – This cannot be changed  Patient Isolation The maximum amount of current that the system can pass to the patient  Patient Isolation AHA state: Current should be less than 10 A Effect if not met: Depends on the frequency and current but could vary from a mild sensation to burns, ventricular fibrillation and other dramatic outcomes at the other extreme 0.9-1.2mA – current perceptible 15-20mA – muscle contraction (unable to release grip) 50-100mA – ventricular fibrillation (death) 100-200mA – severe burns, constriction of muscle groups  Patient Isolation What should I remember?: Current should be as low as possible What should I do?: Check the Electrocardiograph has had an electrical safety test. Visually inspect the system before use. OPERATOR DEPENDENT  SENSITIVITY The sensitivity of the ECG may be altered to increase or decrease the amplitude of the ECG on the trace Common choices are 5 mm/mV 10 mm/mV 20 mm/mV  Sensitivity AHA state: 1mV must equal 10mm on the paper Effect if not met: The ECG waveform amplitude will be reduced or increased in size. Possible misdiagnosis.  Sensitivity What should I remember?: Sensitivity affects the size of the waveform What should I do?: Use a sensitivity of 10mm/mV Check machine setting before you start Use alternative in exceptional circumstances Clearly annotate ECG trace Additional ECG on 10mm/mV 1 mV  ECG PAPER SPEED Historically chart recorders were used to produce ECGs and hence this is the speed at which paper emerges If paper emerges at a rate of 25mm in one second then 1 second on the ECG will be represented by 25mm 1 Second  PAPER SPEED AHA state: 25mm is equal to 1 second Effect if not met: The ECG waveform will be reduced or increase in duration Heart rate will appear higher or lower ECG PAPER SPEED What should I remember?: Paper speed affects the timing of the waveform What should I do?: Use 25 mm per second Check the setting before you start Use an alternative in exceptional circumstances Annotate trace Use 25mm/s as well (record both) Paper Speed = 5mm/s Paper Speed = 25mm/s Paper Speed = 50mm/s  Frequency Response The range of frequencies over which the Electrocardiograph ‘works’ Radio 2 88 to 91 FM ECG FREQUENCY RESPONSE  Frequency response AHA state: frequency response should equal 0.05 to 150 Hz Effect if not met: The ECG waveform will be distorted 0.05 150 Frequency (Hz)  Frequency response What should I remember?: Frequency response affects waveform shape What should I do?: Switch the mains (50 Hz) filter on ONLY once all noise sources have been excluded Clearly annotate ‘Filter on’ Indicates Filter on  ECG Frequency Response 1 to 150 Hz 1 to 0.05 to 150 Hz 0.05 to 25 Hz AHA specifies a frequency response of 0.05 to 150 Hz  Summary Common Mode Rejection Ratio >1000:1 Input Impedance >0.5 MΩ Linearity 3.2 seconds Patient isolation