1. FUNCTION OF THE KIDNEYS
The main function of the kidneys is to form urine out of blood plasma, which basically consists of two processes,
(i) The removal of waste products from blood plasma
(ii) The regulation of the composition of blood plasma
The human body has two kidneys which lie in the back of the abdominal cavity just below the diaphragm, each kidney consist of about a million individual units, these tiny units are called ‘nephrons’.
The kidney work only on plasma, each substance in plasma is handled in a characteristic manner by the nephron, involving particular combinations of filtration, re-absorption and secretion.
The total blood flow through the kidneys as about 1200 ml/min. Kidneys also play an important role in maintaining the acid-base balance.
1.2 ARTIFICIAL KIDNEY
Treatment with a mechanical device like the artificial accumulation of waste products and water and thus the blood concentrations of the toxic substances are returned to normal levels. By effectively removing these materials from the blood, the dialyzer temporarily replaces the function of the natural kidneys and is able to keep the patient close to normal condition. Dialyzing unit operates outside the patient’s own body.
The dialyzing membrane has perforations which are extremely small and are invisible to the naked eye. The perforations in the dialysis membrane have an average diameter of 50 Å with an estimated range of 30 Å to 90 Å.
The artificial kidney is thus simply a membrane separation device that serves as a mass exchanger during clinical use. It is unable to perform any of the synthetic or metabolic function of the normal kidney and, therefore, cannot correct abnormalities that result from the loss of these functions.
1.3 HAEMODIALYSIS MACHINE
A haemodialysis machine is used for the production of warm dialysate which is then circulated through an external dialyzer assembly. It also controls the cycling of the blood from the patient through the artificial kidney (dialyzer) and back to the patient. It continuously monitors and controls all important parameters.
The haemodialysis machine performs five basic functions,
– Mixes the dialysate
– Monitors the dialysate
– Pumps the blood and controls administration
– Monitors the blood for the presence of air and drip chamber pressure
– Monitors the ultra-filtration
Block Diagram of Hemodialysis Machine
Mixing large amount of dialysate from dry chemicals is time consuming and laborious. Water and concentrate pumps were driven by one shaft from the same motor delivering a fixed ratio of water and concentrate into a mixing chamber, fixed proportion of water (piston) and concentrate (shaft) from different chambers. The proportion system could be of the fixed ratio or variable ratio type.
Dialysate Temperature Control and Measurement
The dialysis is normally done at the body temperature. The temperature of the dialysate is, therefore, monitored and controlled before it is supplied to the dialyzer.
Two types of circuits can be used for effecting control of temperature:
(i) A bi-metallic thermostat which would connect or disconnect supply to the heater coil depending upon the temperature of the dialysate.
(ii) A completely electronic single-term proportional controller which makes use of a thermistor for sensing the temperature and a triac for control of power to the heater.
In the modern microprocessor-based haemodialysis machine, A dual element heater assembly, which has a 150W and a 300W element, is used to bring the fluid up to and to maintain the operating temperature. When the fluid temperature rises to within 2.5℃ of the preset temperature (between 35 ℃ to 39℃ ), the 300W heater is turned off and the 150W heater is used to maintain the set temperature (Fig1.3)
(Fig 1.3) Temperature monitor and control circuit using multiple heaters
The flow is measured using a flow-thru transducer which produces a precise number of pulses per unit of flow (26,000 / liter or 108 pulses / seconds at 250 ml / min).
The conductivity monitored by a conducting cell, to verify the accuracy of proportioning. The conductivity of the dialysate changes by about 2% for every 1℃ change in temperature.
Dialysate Pressure Control and Measurement
Negative pressure upon the dialysate is created by the effluent pump. The effluent pump is a fixed flow.
Pressure adjustment should not produce any significant change in the flow rate. The pressure is measured by a strain gauge transducer connected immediately downstream of the dialysate return side. Pressures within the range 0 to 400 mmHg are generally made available and any value can be adjusted in this range.
|Constituent||Concentration (mEq/l) (g/l)|
Table: Composition of Dialysate
Venous Pressure Measurement
Venous pressure is normally measured at the bubble trap. A length of tubing connects the trap to a small plastic housing to which a strain gauge transducer is attached.
For maximum accuracy, the sensor connection should be maintained in the same altitude during dialysis.
Blood Leak Detector
In a dialysis machine, a thin membrane separates the patient’s blood from the dialysate. Normally, the pressure on the blood side of the membrane is maintained at a much higher level than the pressure on the dialysate side. This is necessary to minimize the time required for the dialysis procedure.
The ultra-filtrate monitor circuit is used to monitor the amount of fluid removed from the patient and in conjunction with the negative pressure, to control the rate at which it is removed.
UF Rate (L/hr)= Total fluid removal Required (liters) / Total fluid removal Required (liters)
The flow is measured with a flow meter which comprises a stainless steel float. The flow rate is fixed at a normal 500 ml/min. It is positioned downstream of the dialyzer, it is possible to observe a large blood-to-dialysate.
Effluent pumps are available in several design configurations. The more common types are: the diaphragm type, gear type and magnetically coupled.
The blood pump used in dialysis machine is usually of the peristaltic type. It is designed to give blood flow at a rate of 50 to 350 ml/min.
Air embolism is a serious hazard in dialysis. Air may be sucked in due to inadequate flow in the line in the pumped dialysis system. The level of blood in the venous return bubble trap may be monitored by a photo-electric cell.
These pumps are usually of the plastic syringe type, having a capacity of 30cc. The delivery of heparin from the pump is calibrated in cm/h. The pump is driven by a stepper motor.
Blood Pressure Monitor
The blood pressure monitor circuit is used to monitor the arterial and venous blood pressures. Two separate pressure transducers of the strain gauge type are used for this purpose.
The heart of the computer system is a microprocessor. The microprocessor operates with a clock frequency of 1 MHz or higher. The program is stored in EPROM with a total capacity of around 24 K bytes. The system includes a RAM circuit (2 K × 8 bits memory) powered by a built in battery.
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