Methods of experimental and clinical assessment of the relative measurement accuracy of an intracranial pressure transducer. Technical note

The assessment of the actual measurement accuracy of an intracranial pressure (ICP) transducer is imperative but still very difficult in practice. The authors tested the Codman MicroSensor ICP transducer experimentally. Additionally, a bedside test for assessment of measurement accuracy was used before and after clinical monitoring. For laboratory testing, seven new transducers were examined for measurement accuracy at increasing pressures ranging from 0 to 75 mm Hg. Drift was evaluated for 10 days at six different pressure levels ranging from 0 to 50 mm Hg. Temperature drift was assessed over a temperature range from 20 to 45 degrees C. The percentage absolute difference was used in the assessment of measurement accuracy. For clinical testing, measurement accuracy was assessed intraoperatively just before the insertion of the transducer and in an open water bath after its explantation, at 10 cm H2O both times. The maximum percentage absolute difference was 9% at a pressure of 10 mm Hg, and declined toward 2.3% at a pressure of 75 mm Hg. The maximum drift over 10 days was -4 mm Hg. Within the range of 30 to 40 degrees C, temperature drift was negligible. Over a period of 2 years, between June 1997 and June 1999, 40 ICP transducers were implanted in 35 patients by one surgeon. Of these devices, a malfunction was detected in two of them by testing them in a water bath before insertion. Experimental and clinical results indicate that this miniature strain-gauge transducer measures accurately; however, control readings for the probe by means of measurement in an open water bath just before insertion are strongly recommended.     

A proposal method for the assessment of cerebrospinal fluid pressure in the cerebral ventricles

This work presents a possible method of overcoming difficulties arising during the recording of intracranial pressure (ICP) from the cerebral ventricles of patients suffering from head injuries or intracranial space occupying lesions. The main difficulties in obtaining these measurements are the risk of infection and measurement errors caused by artefacts from head movements. These two problems arise when a cannula inserted into the cerebral ventricle is connected outside the skull to an external transducer measuring pressure. We propose a method utilizing a modified flushing shunt connected to a strain gauge epidural transducer. The characteristic (voltage against pressure) of the device was tested in vitro. This calibration gave satisfactory results both in the linearity of the characteristic and in the performance of the device.     

Direct arterial pressure measurements during operation to assess adequacy of arterial reconstruction in lower limb ischaemia

Over a 3-year period 164 patients undergoing arterial reconstruction for lower limb ischaemia had direct arterial pressure measurements during the operation. The arterial pressure was measured by direct puncture of the vessel with a No. 23 gauge needle connected, via a cannula, to a pressure transducer. The pressure was measured proximal and distal to anastomoses or endarterectomies, and note was made both of the absolute pressure and waveform. At 3 months 95 per cent were patent. In nine patients (5 per cent), there was a significant fall of pressure across the reconstructed artery and in eight of these procedures were carried out to correct the fault. The technique of direct arterial pressure measurement is quick and easy to perform, with equipment that is used routinely for monitoring by the anaesthetist. There has been no evidence of damage due to the arterial puncture itself. It provides a simple, objective assessment of adequacy of an arterial reconstruction and may lead to the early correction of otherwise unrecognized faults, and thus prevent early graft occlusion.     

New Technique for Retrograde Cerebral Perfusion During Arch Aneurysm Repair

Many techniques are used to reduce brain damage during surgery for dissecting aneurysms of the ascending aorta and arch. Recently, new techniques of protection were proposed, consistent with hypothermic circulatory arrest in association with retrograde cerebral perfusion via superior vena cava. We propose a simple, time-saving method, which does not require any manipulation of the heart. We use a multilumen cannula for cardioplegia (D 860-DIDECO FUNDARO') with pressure transducer. This cannula is inserted in superior vena cava by means of a simple purse-string, and linked to the arterial line with a “Y” derivation, allowing retrograde perfusion of the brain and monitoring the perfusion pressure at every moment. The superior vena cava placed downstream from the cannula is closed by a small vascular clamp, to avoid blood reflux in the right atrium. This method is timeand money-saving, is readily available, and can be prepared whenever necessary, also in the middle of the surgical procedure.     

Monitoring arterial,central venous and pulmonary capillary wedge pressure

Arterial blood pressure can be monitored non-invasively by mercury manometer, automated oscillotonometer or continuously by a Finapres based on the Penaz technique. Insertion of a cannula into an artery allows continuous beat-to-beat blood pressure monitoring with pressure transmitted along a column of saline to a piezo-resistive strain gauge transducer. Continuously monitoring blood pressure aids optimization of adequate organ perfusion and further information gained from the waveform can be used to guide treatments. Central venous pressure is the pressure within the right atrium and great veins of the thorax. In a healthy adult, it is between 0 and 8 cm H₂O, varying with respiration. It is measured via a cannula inserted into the superior vena cava (usually via internal jugular or subclavian veins) and uses a pressurized transducer set to produce a reading of central venous pressure and venous waveform. Venous bloods and central venous gases can also be taken and drugs and infusions (particularly if irritant) can be administered. Serial readings are useful for assessing progress and response to treatment. Pulmonary capillary wedge pressure represents left atrial filling pressure and therefore left ventricular end-diastolic pressure and allows more accurate assessment of left-sided heart function. It is measured by floating a pulmonary artery catheter and wedging a balloon into a pulmonary artery branch. It has a complication rate of 10% and, as studies have shown it to have no clear evidence of benefit, alternative less invasive methods such as oesophageal Doppler or arterial pulse contour analysis are now common alternatives.     

Chronic measurement of epidural pressure with an induction-powered oscillator transducer.

An induction-powered oscillator transducer (IPOT) was designed for the chronic measurement of epidural pressure. The transducer was completely implantable so all pressure measurements were made through the intact skin. The IPOT had a linear pressure range from -50 to +200 cm H2O, was sensitive to 1 mm H2O and had a zero drift of less than 1 mm H2O/day under full load. Zero drift was minimized by using a hermetically-sealed metal bellows transducing element which was chemically treated to prevent corrosion and creep. The correlation between epidural pressure and intraventricular pressure was determined during the first 24 hours after implantation in six dogs. Epidural pressure was found to be a linear function of intraventricular fluid pressure. Epidural pressure and intraventricular pressure were essential equal provided the epidural wedge pressure was minimized by proper insertion of the transducer. The correlation between epidural pressure and intraventricular pressure was determined after chronic implantation in five dogs. Epidural pressure was a linear function of intraventricular pressure in the chronically implanted dogs, but epidural pressure was not equal to intraventricular pressure. After chronic implantation, the epidural pressure transducer was not responsive to changes in intraventricular pressure because of mechanical changes in the dura. The dura became stiff and non-compliant. Maximum correlation between epidural pressure and intraventricular fluid pressure in chronic implantations will depend on judicious material selection and mechanical design at the transducer-dura interface.     

直接气道压力监测的可行性及有效性

目的 探讨光纤压力传感器(FOPT)行直接气道压力监测的可行性及有效性.方法 30例择期肺部手术患者,麻醉诱导插管后将FOPT插入双腔气管导管的左右腔内,所测压力分别为P_1和P_2,同时记录呼吸机压力表所监测的气道压P_3.比较三种气道压力的变化.结果 P_3在FOPT插入后较插入前略增加;FOPT的不同插入深度所测得的P_1和P_2差异无统计学意义;诱导后患者仰卧位双肺通气所测得P_1、P_2明显低于P_3(P_1约为P_3的60.33%,P_2约为P_3的68.19%)(P<0.05).结论 FOPT直接监测能更准确、及时反映实际的气道压力.     

Variation of method for measurement of brachial artery pressure significantly affects ankle-brachial pressure index values.

BACKGROUND: Mmeasurement of ankle brachial pressure indices (ABPI) is important in the assessment of patients with peripheral vascular disease. METHODS: Thirty-one hospitals with a vascular surgeon were selected at random. A telephone questionnaire was completed to assess the method used for the measurement of ABPI. Following the survey, 14 patients with peripheral vascular disease had their ABPI measurement done by two observers, a pre-registration house officer and a clinical nurse practitioner. Observers were blinded to their own and each other's results. Brachial systolic pressures were obtained using a DINAMAP(TM)(Critikon, Tampa, U.S.A.) automated blood pressure monitor, the Korotkoff method (12 cm cuff, parallel wrap) and an 8 MHz Doppler probe (Huntleigh) and sphygmomanometer. Ankle systolic pressures were obtained using the Doppler probe. The results were analysed using the Wilcoxon signed rank test. RESULTS: The survey demonstrated that at the majority of centres with vascular laboratories the brachial artery systolic pressures were measured using a Doppler probe. In contrast, at centres where the house officers performed the routine measurements, over 60% used the Korotkoff method to obtain this reading. One in four nurse practitioners used the Korotkoff method. When the ABPI values were calculated, the DINAMAP produced significantly higher median values than the Korotkoff (0.79 vs 0.72, p=0.003) and Doppler methods (0.79 vs 0.70, p<0.0001). The nurse had a higher median ABPI value of 0.76 compared with the doctor (0.71, p=0.01). CONCLUSION: This study shows that measurement of ABPI varies in different vascular units. The technique for ABPI measurement should be standardised.     

Clinical evaluation of two methods of subdural pressure monitoring

Recordings from two different types of subdural pressure monitor with simultaneous intraventricular pressure (IVP) tracings are compared in 20 head-injured patients. In the first 10 patients a fluid-filled catheter was placed subdurally and connected to an external transducer, and in the second 10 the Gaeltec model ICT/b solid state miniature transducer was used. The latter system has the advantage that both zero and calibration checks can be carried out after insertion. Only 44% of the fluid-filled catheter readings corresponded with IVP in series of 10-mm Hg ranges, while 53% of readings were lower; this tendency was more marked at higher pressures. With the Gaeltec transducer, 72% of subdural pressure readings corresponded with IVP, while only 9% were lower and 19% were higher than IVP. The differences may have been due to technical causes or to true pressure differentials. The subdural catheter appears too unreliable for routine clinical use, but the Gaeltec transducer may be a satisfactory alternative to ventricular pressure monitoring.     

A simple and reliable technique to monitor intracranial pressure in the rat: technical note.

A technique for intracranial pressure (ICP) monitoring in the rat that uses a permanent cisterna magna cannula is described. The cannula is placed into the subarachnoid space through the atlanto-occipital membrane with the operating microscope and is secured with cement. The distal end is connected to a pressure transducer and a polygraph recorder. To study the consistency of this technique, 12 anesthetized adult rats were subjected to baseline ICP measurements 2 days after placement of the cannula. Baseline pressures ranged between 1.0 and 10.0 cm H2O, with a mean of 5.6 cm H2O. Respiratory variations were detected in all tracings, and manual abdominal compressions (Valsalva maneuver) correlated with immediate transient rises in ICP in all rats. While CSF pressure was being continuously monitored, rats were subjected to subarachnoid hemorrhage induced by transclival basilar artery puncture. Of the 12 rats, 10 showed a moderate transient rise in cerebrospinal fluid pressure, which peaked approximately 2 minutes after subarachnoid hemorrhage (mean peak change, 10.5 cm H2O; range, 0-32.5 cm H2O). Reliable pressure tracings were obtained in three of five animals examined 3 days after subarachnoid hemorrhage (ICP range, 4.0-4.5 cm H2O; mean, 4.2 cm H2O). We conclude that this cannula is easy and inexpensive to construct and that it provides reliable ICP tracings during experimental procedures in the rat.     

Trans-septal monitoring of left atrial pressure after cardiac surgery. An alternative method

A simple, safe method for trans-septal insertion of a left atrial pressure line in 25 patients is presented. Complications have not been noted. Monitoring of left atrial pressure is often desirable after cardiac surgery. The conventional lateral approach is associated with the risk of haemorrhage after the cannula has been withdrawn. In an oversensitive heart manipulation might not be tolerated, precluding placement of the cannula.     

Ankle blood pressure measured by automatic oscillotonometry: a comparison with Doppler pressure measurements.

Oscillotonometry using Dinamap machine was investigated for the measurement of ankle and brachial blood pressures in our vascular practice. It was validated by comparison with intra-arterial pressure measured by transducer. Systolic ankle and brachial pressures in 43 patients were compared using the Dinamap and Doppler techniques, and significant correlation was found. Ankle and brachial systolic, diastolic and mean pressures and ankle/brachial pressure indices for all three pressures were measured in a group of 12 normal subjects supine at rest, and after treadmill exercise, and a range of normal values defined. The main limitation of the Dinamap is its failure to measure pressures below 50mmHg. The Dinamap technique is a noninvasive, simple, reproducible and quick method of measuring ankle and brachial pressures in vascular surgical practice.     

Safety and accuracy in the clinical recording of cerebrospinal fluid pressure

Summary A pressure transducer with a disposable, presterilized fluid chamber was developed to increase the safety margin when recording cerebrospinal fluid (CSF) pressure. With this system the fluid compartment is completely separated from the transducer proper, reducing electrical hazard and the risk of infection. The operational range is form — 20 mm Hg to 300 mm Hg with good static accuracy. The transducer with stopcocks has a flat frequency response of up to 75 Hz, showing that the two-part principle is compatible with good dynamic performance.The transducer system was easy to handle and well suited to the recording of ventricular fluid pressure, detection of spinal subarachnoid block, the investigation of CSF absorptive capacity, as well as blood pressure recording. When the transducer was connected to standard disposable stopcocks, needles, and tubing as for these recordings, the dynamic accuracy of the whole apparatus was satisfactory for analysis of the pulsatile wave-form. A total of 185 CSF pressure recordings was performed without complication.     

An experimental technique for renal intrapelvic access

Summary This study describes an experimental technique for insertion of an indwelling cannula into the canine renal pelvis, and demonstrates its use in the measurement of pelviureteric pressure and motility, creatinine clearance, Whitaker tests, nephrostograms and nephrostomy drainage. The system, which consists of cannula, Heplock and plastic fixation disc, has been inserted into 53 canine kidneys (48) dogs, and percutaneous needle passage into the subcutaneously placed Heplock was used to gain access to the renal pelvis. This technique was successful in virtually all cases (131 separate procedures), and there were few complications. The indwelling intrapelvic canula is a simple, cheap and reliable method of gaining longterm renal intrapelvic access in the dog, and has many experimental applications.     

Haemodynamic assessment of the femoropopliteal segment: comparison of pressure and Doppler methods using ROC curve analysis

Combined aorto-iliac and femoropopliteal vascular disease remains a problem in vascular surgery. Arteriography does not provide information on the relative contributions of the two lesions to the presenting symptoms. Aorto-iliac haemodynamics may reveal occult aorto-iliac disease but does not show whether combined proximal and distal reconstruction will be required to provide symptomatic relief. Haemodynamic assessment of both segments may help in this respect. A haemodynamic assessment of the femoropopliteal segment of 72 limbs in 38 patients is reported. The segmental pressure drop between a common femoral arterial cannula and a below-knee occlusion cuff is compared with a non-invasive Doppler method combining both transit time and damping factor. Comparison is made using receiver operating characteristic (ROC) curve analysis. The measurement of segmental pressure drop is more accurate than the Doppler method in detection of femoropopliteal stenoses of greater than 50 per cent of the luminal diameter (P less than 0.05).     

Laboratory evaluation of pressure transducer domes containing a diaphragm

Pressure transducers are being used with increasing frequency in patient care. The recent availability of sterile transducer domes containing a diaphragm provides a means for reducing the risk of bacteremia when using these devices. The present studies measured the effect of diaphragm domes on the accuracy of pressure measurement. The sensitivity and frequency responses of 5 pressure transducers utilizing diaphragm and nondiaphragm domes were compared. The results indicate that the type of dome utilized and the method of dome application can significantly modify pressure transducer performance. The data are useful in providing optimal accuracy and reliability of pressure measurments when using a diaphragm dome, pressure transducer combination.     

Laboratory evaluation of the pressure flow characteristics of venous cannulas during vacuum-assisted venous drainage

Vacuum-assisted venous drainage (VAVD) is frequently used to increase venous return through smaller venous cannula and reduced venous line size. The purpose of this study is to determine the maximal flow rates achieved through various venous cannula with varying amounts of negative pressure applied during VAVD. The first test circuit consisted of 25 different venous cannulae connected to 3/8 inner diameter tubing on the positive side of a centrifugal pump. Flows through the cannula were measured at a line pressure of 20, 40, 60, and 80 mmHg. The second circuit consisted of the test cannula connected to 3/8-in inner diameter tubing, a hard-shell venous reservoir, and flow probe placed on the venous line. Using a vacuum regulator and venous line pressure monitor, flow rates were measured through each cannula at -20, -40, -60, and -80 mmHg of vacuum. The results show a mean correlation of 0.989 between the two methods of flow measurement (p < .001). Mean flow rates were established at 40 and 80 mmHg using both methods. For each 40 mmHg increase using vacuum, blood flow was increased by 42.08 +/- 0.09%. These data have been incorporated into a chart to allow the perfusionist to identify the negative pressure necessary to achieve venous return when using specific venous cannula during VAVD.     

Measuring intracranial pressure with a fontanelle palpation transducer (author's transl)

The accuracy and reliability of a non-invasive method for the measurement of intracranial pressure through the fontanelle without puncture was tested during 12 or even 24 hours recordings. Wealthall and Smallwood modified aplanation transducer was first used. A metal frame had to be developed in order to secure the transducer rigidly over the fontanelle so that the recording could take place with the infant in any position and completely free of its movements. Simultaneous recordings of extradural or intraventricular pressure have shown the measurements were accurate to about 1 or 2 cm H2O in a neurosurgical environnement (i.e large fontanelles with pressure over 10 cm H2O). For smaller fontanelles or low or even negative intracranial pressure, a smaller transducer seems to be needed. The preliminary test of a plane strain gauge transducer that partially fills this need is shown.