Physiological Pressure Measurements
  • Blood pressure (BP) is the pressure exerted by the circulating blood on the walls of blood vessels.
  • Systolic pressure is the peak pressure in the arteries, when the ventricles are contracting. It occurs towards the end of cardiac cycle. Diastolic pressure is the minimum pressure on the arteries when the ventricles are relaxing.
  • The standard values of systolic and diastolic pressures for a resting healthy adult is 120mm Hg and 80mrnHg respectively.
  • Invasive measurement refers to the measurement of BP by placing an intravascular cannula needle in the artery.
  • In indirect measurements, the BP is measured indirectly and here we don’ require any direct contact with the artery.
  • Normally physicians measure blood pressure by a device called Sphygmomanometer which comprises an inflatable cuff to restrict the blood flow and is placed around the upper arm attached to a mercury manometer.
  • When the systolic pressure first exceeds the cuff pressure he can hear some crashing sounds in the stethoscope. These sound are caused by the first jet of blood pushing through the occlusion when the occlusion is reduced. These sounds are called “Korotkoff” sounds.
  • Traditionally systolic pressure is the pressure at which the first Korotkoff sound is heard and diastolic pressure is the pressure at which Korotkoffsound is not audible.
  • Double diastolic pressure is indicated as (Systolic pressure/ 1st Diastolic pressure/ 2nd Diastolic pressure) mm Hg.
  • In oscillometric pressure measurements the onset of pressure variations corresponds to the systolic pressure and the diastolic pressure correspond s to the point where the rate of amplitude decreases suddenly.
  • The invasive arterial pressure measurement with intravascular cannulae involves direct measurement of arterial pressure by placing a cannula needle in the artery.
  • There are two ways of placing the catheter in the artery. It can be either by percutaneous insertion in which the skin is punctured over the artery by using a needle to insert it to the artery. Surgical method or arterial cut down method where a surgical procedure is used to place the catheter.
  • Pacemaker is a device which uses electrical impulses delivered by electrodes to maintain and regulate the beating of heart-so the primary function of pacemaker is to maintain adequate heart rate.
  • Modern pacemakers are externally programmable and allow the cardiologist to select optimum pacing modes for individual patients.
  • Different pacing modes can be adopted depending on the condition of patients. Different pacing modes are Transcutaneous pacing (external pacing),Epicardia pacing( temporary), Tran-venous pacing, permanent pacing etc
  • There are three basic types of permanent pacemakers based on the number of chambers involved and their basic operating mechanism called Single chamber pacemaker, dual chamber pacemaker and rate responsive pace maker.
  • The timing circuit in pacemaker determines the basic timing rate of the pulse generator.
  • The stimulating pulse duration is determined by the pulse width circuit. It is triggered, by the output from the timing circuit.
  • The function of pace limiting circuit is to limit the pacing rate. The maximum pacing rate is usually selected as 120 pulses per minute.
  • The output circuit since it provides the proper input pulse to stimulate the heart and hence called heart stimulator circuit.
  • The function of R-wave sensing circuit is to detect or sense a spontaneous R-wave and to reset the oscillator when the pulse is not needed.
  • The voltage monitor circuit continuously monitors the battery voltage as the pacing rate is depending on the efficiency of battery; it has to be monitored regularly.
  • It is a special circuit which slows down the heart rate during certain conditions such as cell depletion.
  • Fibrillation is a critical condition of heart in which this synchronism of the heart muscle fibers is lost.
  • A method to return the fibers to its normal synchronized working is called defibrillation.
  • Depending on this fibrillation is classified into atrial fibrillation which is the fibrillation of atrial muscles and the fibrillation of ventricles is called ventricular fibrillation.
  • The most successful method of defibrillation was proved to be an application of electrical shock to the area of the heart. If sufficient current is applied to stimulate all musculatures of the heart for a brief period of time fibrillation can be prevented.
  • The early defibrillators used the alternating current which is transformed from nomal line voltage up to 300-1000 volts from a power socket to the exposed heart by way ofpaddle type electrodes. This method is repeated until defibrillation occurs. This method of defibrillation is called AC defibrillation.
  • The DC defibrillator is designed in such a way that the capacitor is rapidly discharged through electrodes across the chest of the patient. It can correct both atrial and ventricular defibrillation. A DC fibrillator is designed such that a capacitor is charged to a high DC voltage and then is allowed to discharge completely.
  • Numerous studies showed that a Lown waveform and a biphasic truncated waveform (BTE) were equally effective when requiring lower levels of energy to produce defibrillation. The biphasic shocks appear to achieve the same defibrillation success rates as monophasic waveforms but at significantly lower energy levels.
  • Manual External Defibrillators (MED) are used to diagnose a cardiac condition.
  • Manual Internal Defibrillators (MID), Automated External Defibrillators (AED), Semi automated External Defibrillators (SED), Implantable Cardioverter Defibrillators (ICD), Wearable cardiac defibrillators etc are the other kinds of defibrillators.
  • The wearable portable external defibrillator can be worn like a waistcoat.
  • Electromagnetic blood flow meters are based on the principle of electromagnetic induction. According to Fleming’s right hand rule, the EMF induced in the conductor under the influence of magnetic field is directly proportional to the velocity of motion of conductor. Here we consider the blood vessel carrying blood as the conductor.

Author Bio: The Author of this article, Sreejith is writing articles on Non Invasive Blood Pressure Measurement and Trivandrum Portal

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