Inhaled Anesthetics : Delivery Systems

Michael A. Gropper MD, PhD , in Miller's Anesthesia , 2020

Problems With Flowmeters

Flow measurement fault tin can occur even when flowmeters are assembled properly. Clay or static electricity tin cause a float to stick and misrepresent actual menstruation. Sticking of the indicator float is more common in the low-flow ranges considering the annular space is smaller. A damaged float tin crusade inaccurate readings because the precise relationship between the float and the flow tube is contradistinct. Backpressure from the animate excursion can cause a float to drop so that information technology reads less than the actual flow. Finally, if flowmeters are non aligned properly in the vertical position (plumb), readings tin be inaccurate considering tilting distorts the annular infinite. 12,34,35

The menses tube has historically been a very delicate component of the anesthesia workstation. Subtle cracks and fries may exist disregarded and can cause errors in delivered menstruation. 34 Leaks tin also occur at the O-ring junctions between the glass flow tubes and the metal manifold. Menses tube leaks are a potential hazard considering the flowmeters are located downstream from all hypoxemia rubber devices, except the breathing circuit oxygen analyzer. 33,36,37 Fig. 22.10 shows an case where an unused air menstruum tube develops a large leak. When the nitrous oxide flowmeter is in the downstream position (Fig. 22.tenA and B), a hypoxic mixture can occur because a substantial portion of the oxygen menstruation passes through the leak in the air period tube, and mainly nitrous oxide is directed to the common gas outlet. Safer configurations are shown inFig. 22.10C and D, in which the oxygen flowmeter is located in the downstream position. A portion of the nitrous oxide flow escapes through the leak, and the rest goes toward the mutual gas outlet. A hypoxic mixture is less probable because all the oxygen catamenia is advanced by the nitrous oxide (this principle is known as the Eger period sequence). It has been an industry standard that oxygen exist delivered downstream of all other gases, xx although current ISO standards require only that oxygen be at either end of a bank of flowmeters. eleven Information technology is important to retrieve that in the case of a leak in the oxygen period tube, a hypoxic mixture may result even when oxygen is located in the downstream position. 34

Proportioning Systems

Anesthesia workstations are equipped with an oxygen failure protection device in the intermediate-force per unit area section that, in response to reduced oxygen force per unit area, either proportionally reduces or completely inhibits nitrous oxide. Nonetheless, this system does not prevent the user from selecting a hypoxic gas mixture for commitment to the fresh gas outlet. On anesthesia workstations with electronically controlled gas flow, the automobile is programmed to forbid the user from selecting a hypoxic gas mixture for delivery to the fresh gas outlet. For mechanically controlled flowmeters, the business is that a user could mistakenly select oxygen and nitrous oxide flows that would result in a hypoxic mixture. According to the ISO, an warning condition is insufficient and the machine must have a system to forbid delivery of a hypoxic mixture. 11 This is accomplished by a pneumatic-mechanical interface between the oxygen and nitrous oxide flows or by mechanically linking the oxygen and nitrous oxide catamenia control valves. This way, no matter how high the operator attempts to turn up the nitrous oxide, or how low the operator tries to pass up the oxygen menstruation, when nitrous oxide is running, the automobile will automatically adjust the ratio of these flows and so that a hypoxemic gas mixture cannot be delivered. The specific devices used to achieve this control vary among manufacturers. Two examples are briefly discussed here.

Flowmeters

Robert 1000. Loeb , in Complications in Anesthesia (Second Edition), 2007

Implications

Flowmeter malfunction can present as a breathing excursion leak or an inappropriate gas composition within the breathing circuit. Although gas cannot flow retrograde from the animate excursion to a broken flowmeter, leakage of gas from a broken or missing flowmeter can quickly lead to insufficient gas volume in the breathing circuit. This manifests every bit an empty breathing bag or ventilator bellows and can lead to a misdiagnosis of the malfunction every bit a breathing excursion leak or disconnection, because these malfunctions occur more than commonly. A large leak from the flowmeter associates is a serious problem considering it prevents effective ventilation of the patient.

Flowmeter inaccuracy or a small leak from a cracked O2 flowmeter can cause the anesthesia machine to dispense a hypoxic gas mixture into the breathing circuit. The O2 analyzer in the latter is designed to discover such an occurrence. Other flowmeter problems are not liable to pb to patient injury. Leakage of a gas other than O2 should not upshot in a hypoxic mixture, considering flowmeters are bundled to preclude the preferential loss of O2 in such a state of affairs.

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Anesthesia Delivery Systems

Manuel C. Pardo Doc , in Nuts of Anesthesia , 2018

Flowmeters

Flowmeters on the anesthesia auto precisely command and measure gas flow to the common gas inlet (run into Fig. fifteen.3). 2 Measurement of the flow of gases is based on the principle that flow past a resistance is proportional to pressure. Typically, gas period enters the lesser of a vertically positioned and tapered (the cross-sectional area increases upward from site of gas entry) drinking glass catamenia tube. Gas flow into the flowmeter tube raises a bobbin or ball-shaped float. The float comes to rest when gravity is counterbalanced by the decrease in pressure caused by the float. The upper stop of the bobbin or the equator of the brawl indicates the gas flow in milliliters or liters per minute. Proportionality between force per unit area and flow is determined past the shape of the tube (resistance) and the physical properties (density and viscosity) of the gas. The flowmeters are initially calibrated for the indicated gas at the factory. Considering few gases have the same density and viscosity, flowmeters are not interchangeable with other gases. The scale accompanying an oxygen flowmeter is green, whereas the calibration for the nitrous oxide flowmeter is blue.

Gas catamenia exits the flowmeters and passes into a manifold (mixing bedroom) located at the top of the flowmeters (seeFig. 15.3). 2 The oxygen flowmeter should exist the last in the sequence of flowmeters, and thus oxygen should exist the last gas added to the manifold. This arrangement reduces the possibility that leaks in the apparatus proximal to oxygen inflow can diminish the delivered oxygen concentration, whereas leaks distal to that point result in loss of book without a qualitative alter in the mixture. Nevertheless, an oxygen flowmeter tube leak can produce a hypoxic mixture regardless of the flowmeter tube organization (Fig. 15.4). iii Indeed, flowmeter tube leaks are a run a risk, reflecting the delicate construction of this component of the anesthesia car. Subtle cracks may be overlooked and upshot in errors in delivered menstruum.

Gases mix in the manifold and flow to an outlet port on the anesthesia machine, where they are directed into either a vaporizer or an coldhearted breathing system (seeFig. 15.3). two For emergency purposes, provision is made for delivery of a large volume of oxygen (35 to 75 L/min) to the outlet port through an oxygen flush valve that bypasses the flowmeters and manifold. The oxygen affluent valve allows direct communication betwixt the oxygen high-pressure excursion and the depression-pressure circuit (seeFig. 15.3). 2 Activation of the oxygen flush valve during a mechanically delivered inspiration from the anesthesia machine ventilator permits the transmission of loftier airway pressure to the patient's lungs, with the possibility of barotrauma.

Basic Principles of Anaesthesia

Paul Flecknell , in Laboratory Beast Anaesthesia (Fourth Edition), 2016

Menstruum Meter

Separate menstruum meters are provided for each gas and a menstruation command valve controls the menstruum of gas. Every bit the valve is opened, a bobbin or ball moves upwards the flow meter. The menstruation of gas is read from the position of the height of the bobbin or the heart of the ball (Fig. 1.five). The period control valves are frail, and should merely be opened and airtight by hand. Some basic anaesthetic machines utilize turret-type menstruum meters (Fig. one.6) in which the gas leaves the menstruum meter from the bottom of the unit of measurement. These tin also be purchased combined with a pressure reducing valve and regulator and used on a compressed gas cylinder as a elementary and inexpensive means of supplying oxygen.

Effigy 1.5. The gas flow rate is read from the position of the acme of the bobbin of the flow meter. In flow meters with a ball, rather than a bobbin (right), the reading is taken from the center of the brawl.

Figure i.vi. Turret-blazon flow meters can exist used as a uncomplicated ways of providing a controlled source of oxygen, during both anaesthesia and recovery.

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Skin Flap Physiology and Wound Healing

Paul W. Flintstone Md, FACS , in Cummings Otolaryngology: Caput and Neck Surgery , 2021

Laser Doppler Flowmeters

Laser Doppler flowmeters take been used extensively in studies. 43,232–234 Although the measurement is useful, movement of the probe tin can affect the absolute value. Other variations of Doppler flowmeters have been described, such as those with a colored signal output or an implantable probe. 44 Plethysmography is an older method being studied, with improved instrumentation as a measure of tissue perfusion. 235–238 A recent study showed that an implantable venous flow coupler monitor had college sensitivity than an internal arterial Doppler for identifying flaps that needed take-dorsum; venous thrombosis is a more mutual cause of flap failure than arterial occlusion. 239 Other monitoring methods have as well been proposed. 225,240,241

Systems

John Cyganowski , ... Pietro Perrone , in Ultrafiltration for Bioprocessing, 2015

9.9 Flow meters and compressed gas supply

Flow meters are commonly used to monitor feed and filtrate menstruation rates. Sometimes they are located on the retentate (later on the UF modules) rather than on the feed (before the UF modules).

Coriolis-effect-based mass catamenia meters are preferred for the feed/retentate recirculation loop because they mensurate mass flow charge per unit independently of fluid properties. Coriolis-effect-based mass flow meters for sanitary applications employ a straight menses tube to facilitate cleaning. The drawback of this type of meter is that information technology is expensive and can be bailiwick to system vibration, which may crusade error conditions.

Magnetic (Mag) flow meters measure truthful volumetric flow rate. They are low in price with a low parasitic force per unit area driblet. However, in social club to role, the fluid being measured must have some threshold conductivity and then they will not function with water for injection (WFI). This tin can exist overcome by using dilute sodium hydroxide or dilute table salt-based buffer solutions to make measurements during preparation and post-utilise operations. 'Mag' meters are also subject area to secondary flow regimes, such as Taylor vortices, and therefore they require some minimum length of straight tubing upwardly and downstream of the sensor.

Integrity exam requires the measurement of air diffusion through the membrane. Automated integrity exam measurements typically use a differential temperature-type gas mass flow meter. They report airflow in standard cubic centimetres per infinitesimal (sccm). These meters can be implemented either on the filtrate side of the membrane or on the feed/retentate side (encounter Figure 9.9). However, this type of meter is sensitive to liquid and implementation on the filtrate line requires additional consideration.

Figure ix.9. UF integrity testing: (a) Upstream implementation and (b) downstream implementation.

A good quality cocky-relieving regulator should be used on the gas supply to accurately set the integrity test pressure.

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Sensor Instrumentation for Catamenia Measurement

Chao Tan , Feng Dong , in Reference Module in Biomedical Sciences, 2021

Positive displacement menstruum meter

Positive displacement flowmeter uses a measuring bedchamber, unremarkably rotating around a shaft, to divide the continuous fluid into a series of discrete portions with known volume in the measuring chamber. The total book of fluid can be calculated co-ordinate to the times that the measuring chamber is repeatedly filled and discharged. Theoretically, the rotation speed of measuring sleeping accommodation is straight proportional to the flow charge per unit, but in exercise, this linear relationship only exists in a certain range and there would be leakage that increases or decreases the estimation and creates uncertainties. Fig. 4 is the picture of a positive displacement flowmeter. According to the structure of mechanical construction, the positive displacement flowmeter can be categorized into oval gear flowmeter, scraper flowmeter, double rotor meter, rotary piston meter, reciprocating piston meter, disc meter, liquid seal rotary cylinder meter, helical rotor meter, etc.

Fig. 4

Fig. four. Typical positive displacement flowmeter.

The positive displacement flowmeter generally measures one-direction flow, then the bodily flow direction shall be consistent with the management indicated on the flowmeter shell (Morton et al., 2011). If necessary, a check valve should be installed downstream to avert backflow damaging the flowmeter. For rotor type meter, the rotor shaft should be parallel to the ground (except for the rotor shaft of vertical structure). Other models are mostly installed horizontally according to the requirements of functioning instructions. High viscosity liquids more often than not need to be heated before they can flow, especially when the flowmeter is stopped and restart. This will decrease the fluid viscosity to avoid the rotating structure being stuck and cause damage to the flowmeter and process.

In multiphase flow, positive deportation flowmeter tin measure the total flowrate of wet gas, gas-liquid and oil-water two-phase flow. The advantages of positive displacement flowmeters are high precision, large regulation ratio, no requirement of front straight pipe section, and linear proportional of point output and menstruation charge per unit. However, in practice, it is easy to cause the pulsation of the fluid due to its blocking issue. Likewise, the positive displacement flowmeter with complex structure and huge volume is easy to be corroded or jammed by the flowing objects. The measurement accurateness is afflicted when the fluid contains particles and dirt.

More often than not, the manufacturer specifies the menstruum range according to the type of media to be measured (mainly based on different viscosity), measuring characteristics (such as continuous or intermittent use) and measurement accuracy. In order to maintain the good functioning and extend the useful lifespan, the maximum flow rate is recommended to be 80% of the upper limit flow rate of the flowmeter. The accurateness of the positive deportation flowmeter is indicated by the basic error obtained nether laboratory conditions (Morton et al., 2018). However, in practice, the measuring conditions frequently deviate from the laboratory conditions, resulting in additional errors. The bodily error should be the sum of the basic mistake and the boosted error based on operating weather condition. Usually, the existence of intermittence volition cause the fluid leakage through the gap of meter (Morton et al., 2014b).

The positive deportation flowmeter relies on the fluid to button the measuring accessory, so the pressure loss is higher than that of other types of flow meters with the aforementioned quotient and flow rate (Morton et al., 2014a), generally effectually 20–100   kPa in liquid flow. When liquid with loftier vapor force per unit area is measured, excessive pressure level drop volition lead to cavitation erosion.

By and large, the viscosity of diverse gases is similar, which has no effect on the performance of flowmeter, while the viscosity of liquid varies greatly requiring a large menstruum gap of the metering structure. Withal, it is less affected by viscosity than differential pressure level flowmeter, float flowmeter and turbine flowmeter. Viscosity by and large affects the measurement error, pressure level loss and menstruum range of the displacement flowmeter.

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URODYNAMIC EVALUATION OF THE PATIENT WITH PROLAPSE

Sender Herschorn , in Female person Urology (Third Edition), 2008

Urine Menstruation Meters

Catamenia meters are usually of 1 of iii types: weight, electronic dipstick, or rotating disc. 45 The first measures the weight of the collected urine; the second measures the changes in electrical capacitance of a dipstick mounted in the collecting chamber; and the third measures the power required to keep a disc rotating at a abiding speed while the urine, which tends to irksome it down, is directed toward information technology. All three can provide high sensitivity and reproducibility of information. A commode chair with uroflow measuring appliance is shown in Figure 57-3.

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Physics and Modeling of the Airway

D. John Doyle , Kevin F. O'grady , in Benumof and Hagberg's Airway Direction, 2013

E Flowmeters

Most flowmeters measure out the drop in pressure that occurs when a gas passes through a resistance and correlate this pressure drop with period. The pressure drib depends on gas density and viscosity. If the resistance is an orifice, resistance depends primarily on gas density. For laminar menstruum through a tube, viscosity determines resistance (Hagen-Poiseuille equation). Some flowmeters utilize a floating ball or bobbin supported by the stream of gas in a tapered tube. The bladder is fluted so that it remains in the center of the catamenia. At depression flow, the device depends primarily on laminar catamenia, and every bit the bladder moves up the tube, the resistance behaves progressively more like an orifice.

The density of a gas changes, of course, with barometric pressure, but the viscosity changes piffling, existence primarily dependent on temperature. Gas period through an orifice is inversely proportional to the foursquare root of gas density: As the density falls, period increases (orifice size being constant). Therefore, at high altitude, the bodily flowmeter flow is greater than that indicated by the float position:

(45) Bodily flow = Nominal catamenia × 760 mm Hg P B

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Medical Gas Systems

William Frank , in Clinical Applied science Handbook, 2004

Secondary Equipment

While hoses, flow meters, and vacuum regulators are not part of the pipeline arrangement, they can contribute substantially to gas and vacuum consumption. These items should exist checked equally part of routine inspection procedures. Oft, no single department seems to exist responsible for inspecting them for either leakage or function. An emulsion of soap, water, and glycerin can be used to bank check for leaks in flow meters and fittings.

Anesthesia hoses are non considered office of the anesthesia car'south preventive maintenance, and are not ordinarily tested during motorcar maintenance. Information technology is not uncommon to find vacuum regulator seals completely missing, with the regulator pulling 3 cfm through the outlets.

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