Pressure Transducer

In contrast, nasal pressure transducers detect airflow through the nasal cavity past detecting fluctuations in nasal pressure during the breathing cycle.

From: Encyclopedia of Sleep , 2013

Cardiovascular Monitoring

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

Transducer Setup: Zeroing and Leveling

Prior to use, force per unit area transducers must be zeroed, calibrated, and leveled to the appropriate position, a maneuver accomplished by exposing the transducer to atmospheric force per unit area and performing the zero procedure every bit defined by each device manufacturer. It is important to recognize that the zero pressure locus should be positioned appropriately given the specific clinical context, that it is positioned relative to the patient, that the nothing reference point is local atmospheric force per unit area, and that the zero should be checked and re-zeroed periodically. 105 Occasionally, a faulty transducer, cablevision, or monitor volition crusade the zero baseline to drift, introducing significant mistake until the zero reference is reestablished. 111,112 Considering current disposable pressure level transducers meet accuracy standards established by the AAMI and the American National Standards Plant, formal bedside transducer calibration is no longer routinely performed. Nevertheless, it remains good exercise to routinely compare pressures obtained via a newly placed arterial catheter with a blood force per unit area obtained via other means. 111,112

Choosing the appropriate level at which to plant the cypher point must exist done with respect to the patient and the clinical context. Notation that transducer zeroing and leveling are singled-out and carve up. Zeroing establishes the nil reference signal as ambient atmospheric pressure, while leveling aligns this reference betoken relative to the patient's body, determining where the value "0" will be. This is even more of import when monitoring values for which the physiologic range is pocket-size, such every bit cardinal venous or intracranial pressure level. In such cases, pocket-size zeroing or leveling errors may translate to large relative errors in measurement.

In about cases, arterial pressure transducers should be placed to best estimate aortic root pressure. In full general, the best position for this is approximately 5 cm posterior to the sternal edge. 113,114 However, a more conventional location for the reference level used for all hemodynamic monitoring, including central venous and pulmonary artery pressures (PAPs), is the mid-thoracic level, which corresponds most closely to the mid-left atrial position and is located halfway between the inductive sternum and the bed surface in the supine patient. 115,116 The well-nigh critical point, regardless of where the clinician chooses to assign the reference level, is that it is consistently maintained throughout the monitoring period.

In specific circumstances, though, clinicians may cull to place the transducer at a level dissimilar from the standard. For instance, during a sitting neurosurgical process, information technology may be more than informative to place it at the level of the patient's ear to judge the level of the Circle of Willis. In such cases, the blood force per unit area at the level of the brain is being measured and displayed rather than that of the aortic root, which will be significantly higher. Fixing the transducer to a pole rather than the bed risks introducing error when the bed height or position is changed.

Basic principles and limitations of hemodynamic monitoring

Richard Teplick , in Foundations of Anesthesia (2d Edition), 2006

TRANSDUCERS

Pressure transducers convert pressure into an electrical signal by exposing 1 side of a diaphragm or piezoelectric crystal within the transducer to the pressure level to be measured and the other to atmospheric pressure. This pressure difference causes the diaphragm or crystal to flex, and the resultant displacement is converted into an electrical signal either past changing the length or belongings of a material so that its resistance changes or, for piezoelectric crystals, by generating a small voltage. The pressure-induced displacement causes a change in a voltage, termed the excitation voltage, that is applied to the transducer. The relationship betwixt applied pressure level and changes in the excitation voltage defines the sensitivity of the transducer. Almost transducers used for hemodynamic monitoring are standardized to produce a change of l μV per volt of excitation per cmHg of pressure. That is, their sensitivity is fifty μV/5/cmHg and, different older transducers, does not vary appreciably amongst transducers. Considering the diaphragm is stiff (typically, 100 mmHg of pressure causes a book displacement of 0.001 mm iii), its movement can track even speedily changing portions of pressure level pulses.

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Physics of Ultrasound

Carol M. Rumack MD, FACR , in Diagnostic Ultrasound , 2018

Transducer

A transducer is any device that converts i form of energy to another. In ultrasound the transducer converts electrical energy to mechanical energy, and vice versa. In diagnostic ultrasound systems the transducer serves two functions: (1) converting the electric energy provided by the transmitter to the acoustic pulses directed into the patient and (2) serving as the receiver of reflected echoes, converting weak force per unit area changes into electrical signals for processing.

Ultrasound transducers usepiezoelectricity, a principle discovered by Pierre and Jacques Curie in 1880. 5 Piezoelectric materials have the unique ability to respond to the action of an electric field by changing shape. They likewise accept the property of generating electrical potentials when compressed. Changing the polarity of a voltage applied to the transducer changes the thickness of the transducer, which expands and contracts as the polarity changes. This results in the generation of mechanical pressure waves that tin be transmitted into the torso. The piezoelectric issue besides results in the generation of small potentials beyond the transducer when the transducer is struck by returning echoes. Positive pressures crusade a small polarity to develop across the transducer; negative pressure during the rarefaction portion of the acoustic wave produces the opposite polarity across the transducer. These tiny polarity changes and the associated voltages are the source of all the information candy to generate an ultrasound prototype or Doppler display.

When stimulated past the awarding of a voltage difference across its thickness, the transducer vibrates. The frequency of vibration is determined by the transducer material. When the transducer is electrically stimulated, a range orband of frequencies results. The preferential frequency produced by a transducer is determined past thepropagation speed of the transducer material and its thickness. In thepulsed wave operating modes used for almost clinical ultrasound applications, the ultrasound pulses contain boosted frequencies that are both higher and lower than the preferential frequency. The range of frequencies produced by a given transducer is termed itsbandwidth. More often than not, the shorter the pulse of ultrasound produced past the transducer, the greater is the bandwidth.

Most modern digital ultrasound systems employ broad-bandwidth engineering.Ultrasound bandwidth refers to the range of frequencies produced and detected past the ultrasound system. This is important because each tissue in the body has a feature response to ultrasound of a given frequency, and different tissues respond differently to dissimilar frequencies. The range of frequencies arising from a tissue exposed to ultrasound is referred to every bit thefrequency spectrum bandwidth of the tissue, or tissue signature.Broad-bandwidth technology provides a means to capture the frequency spectrum of insonated tissues, preserving acoustic information and tissue signature. Broad-bandwidth axle formers reduce speckle artifact by a process offrequency compounding. This is possible because speckle patterns at different frequencies are independent of ane another, and combining data from multiple frequency bands (i.due east., compounding) results in a reduction of speckle in the concluding image, leading to improved contrast resolution.

Evaluation and Monitoring of Respiratory Office

Reena Mehra , Kingman P. Strohl , in Slumber Disorders Medicine (Third Edition), 2009

Intranasal Pressure Transducer

Intranasal pressure transducers provide an indirect measurement of airflow by detecting pressure changes, with an excellent response to airflow profile, and are capable of detecting airflow limitation. Irresolute pressures require a transducer that can respond to rapid changes. Nasal force per unit area transducers provide a significantly more than sensitive measure of airflow than temperature-based transducers, and many believe that pressure transducers may provide a mensurate of upper airway resistance as inspiration and expiration provide transducer bespeak fluctuations similar to airflow. 12 In 1 study, the nasal cannula/pressure transducer was found to be a noninvasive, reproducible detector of all events in SDB—in particular, it detected the same events as esophageal manometry (respiratory effort-related arousals [RERAs]) with an intraclass correlation coefficient of 96. 13, 14 The newer transducers provide boosted information for scoring hypopneas (Fig. 14-4). If used for evaluation of slumber-related animate disorders, the new level of sensitivity may pb to scoring of many more events than are typically scored with other methods of airflow detection (Figs. 14-v and 14-6). These events may be as meaning as conventionally scored apneas, only at present near all of the clinical literature is based on temperature-based airflow transduction.

Nasal pressure monitoring is not recommended for patients who are predominantly mouth breathers or accept nasal obstruction, in which case airflow may exist underestimated. 15 Nasal pressure sensors connected to the nose via nasal prongs are more accurate than thermoelements in detecting hypopneas. xv Even so, nasal pressure is falsely increased in the presence of nasal obstruction, and there is a nonlinear relation between nasal pressure level and nasal flow. Square root linearization of nasal pressure greatly increases the accuracy for quantifying hypopneas and detecting flow limitation. 16, 17 Mouth animate can bear upon the measurement, merely pure mouth breathing is uncommon. 18, 19

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Urodynamic and Video-Urodynamic Evaluation of the Lower Urinary Tract

Alan W. Partin Physician, PhD , in Campbell-Walsh-Wein Urology , 2021

Signal Transmission and Transducers

Transducers are the hardware that allows pressure in the patient to exist measured and transferred to the UDS system. External strain judge transducers located "betwixt" the patient and the urodynamic auto take been popular for years. Pressurized tubing (to avoid dampening or dissipation of the pressure) extends from the pressure level transducer to the catheters placed in the patient. An electronic cable or "wireless transmission" brings the signal from the transducer to the urodynamic automobile. Traditionally, a water-filled organization was used in which the entire system from transducer to patient is filled with water. Because this system depends on the transmission of pressure level through fluid (water), it is crucial that there are no air bubbles in the transducer or tubing. The pressurized tubing transmission lines should be clear-cut to allow for like shooting fish in a barrel recognition of air in the line. The transducers are normally set at the level of the patient's bladder (symphysis pubis) at the offset of the study. This is of import because if the patient changes position during the exam (due east.1000., standing to sitting), the elevation of the transducer can exist adjusted so that it remains at the level of the bladder.

More recentlyair-charged catheters (T-Doc, Wenonah, NJ) have become popular for pressure measurement. Air-charged catheters utilize a miniature, air-filled balloon placed circumferentially around a polyethylene catheter. External forces on the airship of the catheter are transmitted to the air-filled catheter lumen and communicated to an external semiconductor transducer. The technology of the balloon system allows circumferential measurement readings. The catheters are disposable and intended for single use. Air-charged catheters have several practical advantages over fluid-filled pressure lines because there is no fluid connection betwixt the patient and the urodynamic equipment—just air. This means at that place is no hydrostatic force per unit area result to account for, so there is no need to position anything at the level of the symphysis pubis, and at that place is no need to flush the organization through to exclude air (essential when using a fluid-filled organisation). As well, there are no artefactual fluctuations in force per unit area produced when the patient moves. Information technology must besides be remembered thatmany UDS nomograms and other "standards of measurement" were adamant using fluid-filled systems. In that location is comparative evidence for the utilise of air-charged catheters to measure urethral force per unit area and Valsalva leak indicate pressure. One study showed comparable performance between air-charged and microtip catheters (Pollak et al., 2004). Another written report concluded that they cannot exist used interchangeably equally air-charged catheters showed systematically higher readings (Zehnder et al., 2008).

In an experimental model, Cooper et al. showed thatair-charged and h2o-filled catheters answer to pressure changes in dramatically different ways (Cooper et al., 2011). Water-filled catheters acted equally an underdamped system, resonating at 10.xiii ± 1.03 Hz and attenuating signals at frequencies college than xix Hz. They demonstrated significant motion and hydrostatic artifacts. Air-charged catheters acted as an overdamped organization and attenuated signals at frequencies higher than three.02 ± 0.13 Hz. They demonstrated significantly less movement and hydrostatic artifacts than water-filled catheters. The authors point out that about urodynamic signals occur below 3 Hz, and, as such, air-charged systems could be beneficial because well-nigh of the higher-frequency dissonance is dampened. However, urodynamic signals can accept frequency components greater than three Hz, specially when utilizing rapidly changing signals, such as coughs. The authors concluded that "cognition of the characteristics of the pressure level-measuring system is essential to finding the best lucifer for a specific application."

Intracranial Pressure Monitoring

Marek Czosnyka , in Essentials of Neuroanesthesia and Neurointensive Care, 2008

Intraventricular Drains

An external pressure transducer continued to a catheter catastrophe in the ventricular arrangement that allows direct pressure measurement is yet considered the "gilt standard" for measurement of ICP. Boosted advantages include the capability of periodic external calibration and CSF drainage. However, insertion of a ventricular catheter may be difficult or impossible in patients with advanced brain swelling, and the risk of infection is increased significantly after three days of monitoring. When measurement is performed by open up extraventricular drainage (EVD), care should exist taken to not interpret ICP readings as valid while the EVD is open ( Fig. xl-ii). EVD should be airtight for measurement (for at least 15 minutes), or an independent intraparenchymal transducer should be used.

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Functional Beefcake of the Spine

B. Vibert Md , H.Due north. Herkowitz Physician , in Orthopaedic Physical Therapy Secrets (Third Edition), 2017

12 How does the spine receive loads in unlike postures? What is the event of a backrest or lumbar back up?

Nachemson measured intradiscal pressure with force per unit area transducers placed at L3-L4 in normal patients at different postures. His research showed that the to the lowest degree loaded status is lying supine. In vivo, loads increase sequentially with lying on the side, standing, sitting in a chair, continuing with flexed spine, sitting in a chair with flexed spine, standing with flexed spine carrying a weight, and sitting in a chair with flexed spine carrying a weight. The relative loads are as follows:

•

Lying on the side—25%

•

Continuing—100%

•

Seated—145%

•

Standing with forward curve—150%

•

Seated with frontwards bend—180%

The loads on the lumbar spine are lower during supported sitting than during unsupported sitting considering part of the weight of the upper torso is supported by the backrest. Backward inclination and use of a lumbar back up further reduce the loads.

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Physiological control

Michael C. Stevens , ... Francesco Moscato , in Mechanical Circulatory and Respiratory Back up, 2018

Clinically available implantable transducers

In that location are 2 clinically available pressure transducers that may be utilized for physiological control purposes. CardioMEMS (FDA P100045) (St. Jude Medical, Minnesota, US) is the only FDA-approved (as of May 2014) clinically utilized implantable pressure transducer. The CardioMEMS system is a pulmonary artery (PA) pressure level sensor that is implanted into the distal PA via correct centre catheterization. The sensor uses a deformable capacitor circuit whose resonant frequency is related to the PA pressure level. The sensor is electromagnetically coupled via the inductor component, eliminating the need for a battery in the transducer. Pressure information is wirelessly transmitted back to a receiver unit of measurement, which can be uploaded by the user into a database [ 93].

The HeartPOD (St. Jude Medical, Minnesota, United states of america) is an implantable left atrial pressure sensor that is implanted via the femoral artery or subclavian venous channel (Fig. 20.12).

Fig. 20.12. HeartPOD left atrial pressure level sensing system.

 Reprinted with permission from Troughton RW, Ritzema J, Eigler NL, Melton IC, Krum H, Adamson PB, et al. Direct left atrial force per unit area monitoring in astringent heart failure: long-term sensor performance. J Cardiovasc Transl Res 2011;4:iii–13. https://doi.org/10.1007/s12265-010-9229-z.

The sensing module consists of four piezoresistive strain gauge elements affixed to a titanium diaphragm. The sensor uses a pb fixed to a coil antenna that sits in a subcutaneous pocket. A handheld monitor inductively powers the sensor every bit it is being interrogated. The organization has undergone two clinical trials simply has not yet been FDA approved [84]. Neither the CardioMEMS or HeartPOD have been incorporated into an LVAD physiological control system.

Just one company has produced an LVAD that features a menstruation transducer. The HeartAssist-v (HA5) and the more recent aVAD (ReliantHeart Inc., Houston, Texas, USA) are axial flow VADs that are approved for implantation in Europe, with the HA-5 also FDA approved for pediatric use in the USA (Fig. 20.13). These are the only clinically available VADs that feature a depression-power Transonic (Transonic Systems, Ithaca, NY, Usa) ultrasonic period probe attached to the VAD outflow graft. The sensor measures flow charge per unit using an ultrasonic transit time method. Data is recorded in the device controller and sent via a cellular network to the monitoring middle at set intervals [94].

Fig. 20.13. The ReliantHeart aVAD, with a Flow Accurate menstruum probe effectually the outflow grant for existent-time flow measurement.

 Image courtesy of ReliantHeart.

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Tracheotomy and Upper Airway Obstruction

John E. Heffner MD , in Critical Intendance Secrets (5th Edition), 2013

xiv Should tracheostomy tube cuff pressures be directly measured periodically in patients undergoing mechanical ventilation?

Frequent monitoring of gage pressure with a pressure gauge provides the but measure out to prevent tracheal injury at the cuff site. Other techniques, such equally finger palpation of the external inflation seedling, minimal occlusive volume, and the tension felt by the operator on an inflating syringe, do non substitute for straight measurements of intracuff pressure.

Fundamental Points

Maintaining Appropriate Intracuff Pressures

ane.

Maintain intracuff pressures betwixt 18   mm Hg and 25   mm Hg.

2.

Measure out pressures routinely with a force per unit area gauge.

3.

Check intracuff pressures when patients become for anesthesia.

4.

Intracuff pressures tin can increase markedly when patients travel by air during helicopter ship.

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Overview of the Microcirculation

Paul C Johnson , in Microcirculation, 2008

I.v.4.2 Impedance Microelectrode Technique

With evolution of electronic amplifiers and pressure transducers it became possible to couple a micropipette with a strain gauge or capacitance transducer only direct measurement of microicirculatory pressure was not feasible due to the compliance of the transducer membrane, compressibility of water and hydrodynamic resistance of the pipette tip. The solution to this problem involved a clever approach which measured the electrical impedance of a full-bodied electrolyte solution in a pipette inserted into the vessel and a servo system that adapted the pressure level in the pipette to maintain the pipette impedance constant, essentially preventing blood plasma from entering the pipette [124] This method made it possible to directly measure force per unit area in the microcirculation and with a frequency response sufficient to monitor the arterial pulse in the arterioles. The utilise of micropipettes with tip diameters equally pocket-size as 1 micron i.d. has made it possible to mensurate pressure level in microcirculatory vessels down to the capillary level [125]. This methodology has been used extensively to determine the force per unit area in microcirculatory vessels under a multifariousness of circumstances although successful application of the method requires practice and patience.

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