An essential 'health check' for all medical devices

In order to safeguard patient safety, all new or modified medical devices must be assessed for their safety and performance before they are used routinely in clinical practice. Most devices will carry a CE (Confirmity European) mark to demonstrate their safety, but many devices will require an alternative method of assessment. In this article, we discuss the procedures already in place, the significant gaps that exist in the system and the risk management issues. We consider the impact on research and clinical practice, and describe our comprehensive risk management system for objectively assessing the safety of any medical device.

The light still shines, but not that brightly? The current status of perinatal near infrared spectroscopy

Efforts have been made to find new, non-invasive methods for assessing tissue oxygenation and haemodynamics, particularly in the brain of the fetus and the newborn infant. Near infrared spectroscopy (NIRS) is a developmental technique that provides just such a method, allowing calculation of variables such as cerebral blood flow and cerebral blood volume. It can also measure peripheral oxygen consumption. This review is based on our long experience of using NIRS. Basic principles, techniques, validation, and clinical applications are highlighted. Although more than two decades have passed since its introduction, NIRS remains very much a developmental technique, despite technical progression. A great deal more research is required for NIRS to become a routine clinical tool.

Effect of a change in global metabolic rate on peripheral oxygen consumption in neonates

AIM

To evaluate the effect of an induced change in global metabolic rate on peripheral oxygen consumption (VO(2)) in healthy full term neonates.

SUBJECTS AND METHODS

Twenty four healthy full term neonates were studied. Peripheral VO(2) was measured by near infrared spectroscopy (NIRS) using arterial occlusion and measurement of the oxyhaemoglobin (HbO(2)) decrement slope. Global VO(2) was measured by open circuit calorimetry. Global and peripheral VO(2) were measured in each neonate before and after a routine bath. Abdominal and forearm skin temperatures were also recorded.

RESULTS

Nineteen neonates completed the study. Global VO(2) increased by 30.7% (p = 0.001), and peripheral VO(2) by 23.1% (p = 0.001). A correlation between the fractional changes in global and peripheral VO(2) was apparent (r = 0.76, p = 0.001). Abdominal skin temperature decreased by 0.8 degrees C (p = 0.001), and forearm skin temperature by 0.6 degrees C (p = 0.04).

CONCLUSIONS

Measurement of peripheral VO(2) using NIRS with arterial occlusion is responsive to conditions that increase global metabolic rate. Any change in global VO(2) must be taken into consideration during the interpretation of peripheral VO(2) measurements in neonates.

Effect of limb cooling on peripheral and global oxygen consumption in neonates

AIM

To evaluate peripheral oxygen consumption (VO(2)) measurements using near infrared spectroscopy (NIRS) with arterial occlusion in healthy term neonates by studying the effect of limb cooling on peripheral and global VO(2).

SUBJECTS AND METHODS

Twenty two healthy term neonates were studied. Peripheral VO(2) was measured by NIRS using arterial occlusion and measurement of the oxyhaemoglobin (HbO(2)) decrement slope. Global VO(2) was measured by open circuit calorimetry. Global and peripheral VO(2) was measured in each neonate before and after limb cooling.

RESULTS

In 10 neonates, a fall in forearm temperature of 2.2 degrees C (mild cooling) decreased forearm VO(2) by 19.6% (p < 0.01). Global VO(2) did not change. In 12 neonates, a fall in forearm temperature of 4 degrees C (moderate cooling) decreased forearm VO(2) by 34.7% (p < 0.01). Global VO(2) increased by 17.6% (p < 0.05).

CONCLUSIONS

The NIRS arterial occlusion method is able to measure changes in peripheral VO(2) induced by limb cooling. The changes are more pronounced with moderate limb cooling when a concomitant rise in global VO(2) is observed. Change in peripheral temperature must be taken into consideration in the interpretation of peripheral VO(2) measurements in neonates.

Blood perfusion of subcutaneous tumours in mice following the application of low-level direct electric current

Electrotherapy with low-level direct electric current has been proved to be an effective local treatment of solid tumours. In the presented study an attempt was made to evaluate the effect of a single treatment with electrotherapy on blood perfusion of solid subcutaneous fibrosarcoma Sa-1 tumours in A/J mice. The tissue-staining method with Patent blue-violet dye, the rubidium extraction technique, and the noninvasive near-infrared spectroscopy method were used for this purpose. Results of all methods indicate that perfusion and subsequently oxygenation of tumours were reduced due to application of electrotherapy.

Measurement of peripheral oxygen utilisation in neonates using near infrared spectroscopy: comparison between arterial and venous occlusion methods

The aim of this study was to develop an arterial occlusion method and compare it with the venous occlusion method for measurement of peripheral oxygen utilisation in neonates using near infrared spectroscopy (NIRS). Twenty healthy neonates were studied. Arterial occlusion was produced by inflating a neonatal blood pressure cuff to 100 mmHg for 30-40 s and oxygen utilisation (VO(2)) was calculated using the HbO(2) decrement slope following occlusion. Venous occlusion was produced by inflating the cuff to 30 mmHg for 15-20 s and VO(2) was calculated by: VO(2)=HbTx4x(SaO(2)-SvO(2)), where SaO(2) is the arterial oxygen saturation measured by pulse oximetry and SvO(2) is the venous oxygen saturation measured by NIRS. Each baby had a minimum of three arterial and three venous occlusions. Criteria were developed for acceptance/rejection of an occlusion. Using the arterial method, the mean VO(2) was 1.12 mM cm(-1) O(2)/min (S.D.=0.25), (95% CI=1.00-1.24 mM cm(-1) O(2)/min). The coefficient of variation was 6.6+/-4.1%. Using the venous method, the mean VO(2) was 1.60 mM cm(-1) O(2)/min (S.D.=0.48), (95% CI=1. 38-1.82 mM cm(-1) O(2)/min). The coefficient of variation was 12. 6+/-5.7%. The correlation between the two methods was weak (r=0.28 and r(2) was 0.08). The mean difference between the two methods was 0. 47 mM cm(-1) O(2)/min (S.D.=0.51). The limits of agreement were -0. 53 to 1.47 mM cm(-1) O(2)/min. The arterial method gives more consistent results.

A comparison of pulse oximetry and near infrared spectroscopy (NIRS) in the detection of hypoxaemia occurring with pauses in nasal airflow in neonates

OBJECTIVE

The aim of this study was to compare the ability of NIRS and pulse oximetry to detect changes in cerebral oxygenation occurring in response to a pause in nasal airflow (PNA).

METHODS

Twenty-one recordings of cerebral oxygenation index by NIRS together with oxyhemoglobin saturation by pulse oximetry were measured on 17 preterm infants with a history of apnoea. Photoplethysmography was used to confirm the accuracy of the pulse oximetry data. PNA events were defined as pauses of greater than 4 seconds in a thermistor trace measuring nasal air flow.

RESULTS

Baseline variability in oxygenation index (Hbdiff) was found to be from -0.12 to +0.13 micromol 100 g brain(-1). A fall in Hbdiff or SpO2 was defined as a decrease of greater magnitude than 2 standard deviations from the baseline, i.e., -0.12 micromol 100 g brain(-1) and 3% respectively. In 68% of 468 PNA events a fall in oxyhemoglobin saturation (SpO2) was detected and in 56% a fall in Hbdiff was detected. In 20% of events there was no fall in cerebral oxygenation despite a fall in SpO2. In 8% of PNA episodes we recorded a fall in cerebral oxygenation but no fall in SpO2. When a fall in cerebral oxygenation was recorded, the fall was greater when the event was also associated with a fall in SpO2 (median (interquartile range (IQR)) 0.32 (0.21-0.69) vs. 0.25 (0.16-0.43) micromol 100 g brain(-1), p < 0.05). When all the PNA episodes were reviewed no close correlation was shown between the magnitude of change in cerebral oxygenation and the change in SpO2 for small changes in both indices. However, large falls (>1.5 micromol 100 g brain(-1)) in cerebral oxygenation were closely associated with large changes in SpO2.

CONCLUSIONS

We conclude that both techniques are sensitive to changes in oxygenation during PNA. Small changes in cerebral Hbdiff and arterial SpO2 do not always correlate for physiological reasons. A change in Hbdiff of >0.3 micromol 100 g brain(-1) is likely to be physiologically significant and is associated with a change in SpO2 of 12%.

Fast algorithm to determine optical properties of a turbid medium from time-resolved measurements

After analytical expressions for the time-resolved reflectance are introduced from the diffusion approximation under the three most commonly used boundary conditions, a novel algorithm is demonstrated for determining the reduced scattering and the absorption coefficients from time-resolved reflectance (or backscatter) measurements at two positions on the surface of biotissue. The algorithm is straightforward and fast and involves only some simple mathematical operations, avoiding complicated iterative nonlinear fitting to the time-resolved curve. The derived reduced scattering coefficient is not affected by whatever boundary condition is applied. The algorithm was verified with time-resolved data from the Monte Carlo model. Both a semi-infinite medium and a turbid slab medium were tested. In contrast to the nonlinear fitting method, this algorithm allows both the scattering and the absorption coefficients to be determined to a high accuracy.

Investigation of neonatal brain cytochrome redox by NIRS

Near Infrared Spectroscopy (NIRS) has been used to detect changes in cerebral blood and tissue oxygenation. Redox state of enzyme cytochrome aa3 (Cyt aa3 is thought to give an indication of tissue oxygenation. This information would be of value in exploring the aetiology of cerebral ischaemic lesions in preterm infants. However we have found that the change in cerebral cytochrome redox in infants following desaturation was inconsistent and insignificant.

Fibre-optical sensor based on fluorescent indicator for monitoring physiological pH values

A fibre-optical sensor has been developed based on the fluorescent indicator 8-hydroxyl-1,3,6-pyrene trisulphonic acid trisodium salt (HOPSA) for monitoring physiological pH values. Dowex-1 strongly basic anion exchange resin is used as the medium to immobilise the indicator on the end of an optical fibre. Polyurethane is coated on the sensor as the permeable membrane for H+. The properties of HOPSA in solution, as well as when immobilised on the sensor, are studied and found to have the potential for monitoring physiological pH values.

Is allopurinol beneficial in the prevention of renal ischaemia-reperfusion injury in the rat?: evaluation by near-infrared spectroscopy

1. The role of allopurinol in the protection of kidney function following ischaemia-reperfusion injury has been investigated using the novel technique of near-infrared spectroscopy. 2. An in vivo model of rat kidney ischaemia was used, with the expected falls in blood and tissue oxygenation seen and confirmed by near-infrared spectroscopy. 3. Allopurinol infusion increased the rate of reperfusion of oxygenated blood seen in control rats (P < 0.05). 4. Allopurinol enhanced the rate of tissue oxygenation during early reperfusion (P < 0.01). 5. This study provides further evidence for the proposed benefits of allopurinol in ischaemia-reperfusion injury. Furthermore, the potential of near-infrared spectroscopy as a technique of value in interventional studies of this nature is confirmed.

Effects of positive and negative pressure ventilation on cerebral blood volume of newborn infants

The effects of intermittent positive airway and continuous negative extrathoracic pressure ventilation on cerebral blood volume in preterm infants were studied using near infrared spectroscopy. In 12 infants continuous negative extrathoracic pressure caused a median decrease in cerebral blood volume of 0.14 ml/100 ml brain (95% confidence intervals (CI) 0.035-0.280) compared with no respiratory support. Oxygenated and deoxygenated haemoglobin also decreased, implying increased venous drainage as the main effect. In 17 infants intermittent positive pressure ventilation also caused a median reduction in cerebral blood volume of 0.06 ml/100 ml brain (95% CI 0.010-0.115) compared with endotracheal positive airway pressure. Deoxygenated haemoglobin increased by 0.07 ml/100 ml brain (95% CI 0.010-0.100) while oxygenated haemoglobin decreased by 0.10 ml/100 ml brain (95% CI 0.005-0.175). The increase in deoxygenated haemoglobin implies decreased venous drainage and the decrease in oxygenated haemoglobin implies that other factors may also be significant. Heart rate, blood pressure and oxygen saturation were monitored continuously and remained stable.

Near infrared spectroscopy used to observe changes in fetal cerebral haemodynamics during labour

Near Infrared Spectroscopy (NIRS) has been used in the neonate to observe changes in the cerebral haemodynamics and concentration of oxygenated haemoglobin. Specific changes have been demonstrated in response to spontaneous bradycardias and alterations in the inspired oxygen concentration. We report here changes in fetal cerebral haemodynamics using NIRS in response to rapid spontaneous delivery and the "topping up" of a maternal epidural with bupivacaine. NIRS offers a new way of observing changes in cerebral haemodynamics in the fetus during labour.

Effect of fetal hemoglobin on the determination of neonatal cerebral oxygenation by near-infrared spectroscopy

Near-infrared spectroscopy has been applied to the study of cerebral oxygenation and hemodynamics in the newborn. Fetal Hb is present in high concentrations in these infants. Because spectral absorption curves in the near-infrared range for fetal Hb are not identical to those for adult Hb, there is a potential for the measurements to be affected. This possibility has not previously been investigated. A quantitative analysis of cerebral oxygenation was undertaken in six newborn infants. The analysis used near-infrared spectroscopy multiplier coefficients derived from the absorption coefficients of both fetal and adult deoxygenated and oxygenated Hb. The results of both analyses were compared. It was concluded that the error in near-infrared spectroscopy analysis of neonatal cerebral oxygenation arising from the use of absorption coefficients derived from adult Hb is inconsequential, even in the presence of high concentrations of fetal Hb.

Fetal and neonatal cerebral oxygen monitoring with NIRS: theory and practice

Near infra-red spectroscopy (NIRS) is a comparatively new method for monitoring the oxygenation in blood and tissue in the brain of the fetus and the neonate. Absorption of light in the wavelength range 700-1000 nm through such tissue is measured, which is then used to calculate changes in the concentration of cerebral oxygenated and de-oxygenated haemoglobin (HbO2 and Hb) and hence cerebral blood volume (CBV). Studies carried out on several groups of newborn babies have shown clear changes in HbO, Hb and CBV with hypoxia and bradycardia. These changes may well have implications in the occurrence of hypoxic/ischaemic brain injury. Intra partum NIR measurements on the fetal brain have demonstrated clear changes in HbO2, Hb and CBVm, coinciding with the onset of contractions.

Plethysmographic validation of near infrared spectroscopic monitoring of cerebral blood volume

The validation of measurement of cerebral blood volume (CBV) and cerebral blood flow (CBF) using near infrared spectroscopy (NIRS) against jugular venous occlusion plethysmography is described. Repeated measurements in six infants were made using both techniques simultaneously. A close relationship between the two measurements of change in CBV was obtained in five infants. There was also a close relationship for measurement of CBF in four infants. This study confirms the possibility of using NIRS to monitor CBV continuously in the premature infant. This parameter may prove to be of greater clinical value than the intermittent measurement of CBF.

Cyclical fluctuations in cerebral blood volume

Cyclical fluctuations of cerebral blood flow velocity have been reported previously using Doppler ultrasound. The same phenomena was detected during investigations of changes in cerebral blood volume using near infrared spectroscopy. Rhythmic fluctuations of cerebral blood volume at a frequency of 3.5 cycles/minute is reported here.

Effects of hypoxaemia and bradycardia on neonatal cerebral haemodynamics

Near infrared spectroscopy has been used to assess the effects of bradycardia and hypoxia on the cerebral circulation in the premature neonate. The technique is well tolerated and can be applied in almost any infant. Continuous monitoring of changes in cerebral oxygenated, deoxygenated, and total haemoglobin is possible. Total haemoglobin is analogous to cerebral blood volume; thus information on circulatory changes as well as oxygenation state can be obtained. Twenty five babies had cerebral monitoring carried out using this technique. During episodes of hypoxia, both spontaneous and induced, impairment of haemoglobin oxygenation within the brain was detected together with an overall increase in the total mean haemoglobin concentration, which was 0.8 x 10(-2) mmol/l. Bradycardia with apnoea also led to impairment of cerebral oxygenation, and to a rapid fall in the concentration of total mean haemoglobin to 1.4 x 10(-2) mmol/l, which was followed in some cases by an increase to above the resting value on recovery of the heart rate to a mean of 0.7 x 10(-2) mmol/l. These disturbances to total haemoglobin concentration represent abnormalities of cerebral blood volume that may be implicated in the pathogenesis of neonatal cerebral injury.

The non-invasive monitoring of cerebral tissue oxygenation

The instrument drift was found to be less than 0.004 OD/hour and from measurements on glass filters of 8 optical density units, a coefficient of variation of 0.01 over the 30 second averaging time was observed. The instrument is sufficiently sensitive to enable monitoring of changes in the cerebral oxygen saturation level of haemoglobin and to enable changes in the concentration of cyt aa3 (oxidised form), to be measured with reasonable confidence. It is of the utmost importance in NIRS investigations to be certain of the specificity of the technique, and it is vital that reliable determinations of the amount of cytochrome aa3 in brain are made in addition to measurements of the extinction coefficients. This is still a matter of considerable debate, not only in the isolation and properties of the multisubunit structured membrane bound protein, which many enzymologists have been investigating for the last 50 years, (Keilin and Hartree, 1939; Brunori et al, 1981) but also in relating to in-vivo versus in vitro comparisons. An additional point for consideration is the validity of employing multiplier coefficients derived from the rat brain, as several groups have done (Wyatt et al 1986, Ferrari et al 1985,) and applying them to the human brain and to the infant brain. There may be significant differences in the activity of the enzyme, and the profound physiological effects which will arise during the end point fluorocarbon studies: the presence of fetal haemoglobin must also be considered (Carta et al., 1987). The clinical determination of the optical path length is a considerable problem (Cope etal., 1988,) and once again estimates of the pathlength correction factor made in animals and dead fetuses may not be valid for living human tissue. Results from our animal studies indicate that NIRS can be used to monitor changes in the oxygenation level of Hb and, in extreme hypoxia, changes in the level of the redox state of cyt aa3 can be reliably measured and are well within the sensitivity of the instrument. The results indicate that under small change in saturation the redox state of cyt aa3 appears to be unaltered. It may be that under normal physiological conditions the redox state of aa3 appears to be apparently unchanged under episodes of mild hypoxia of short duration.