Dynamic model of oxygen transport for transcutaneous PO2 analysis

A dynamic model of oxygen transport through the outer skin layers and a polarographic sensor was developed for the analysis of transcutaneous oxygen tension (tcPO2). It provides a basis for quantifying the factors that determine the relationship between tcPO2 and arterial oxygen tension (PaO2). Model simulations show the importance of stratum papillare metabolic oxygen consumption; the oxygen permeability of the skin relative to that of the sensor membrane and electrolyte; and temperature and the oxyhemoglobin dissociation curve. These simulations were consistent with experimental data obtained by using microcathode transcutaneous oxygen sensors, which were placed on the skin of 10 healthy adults. Furthermore, the model indicates that accurate evaluation of arterial oxygen tension by using transcutaneous measurements requires continuous estimation of skin perfusion. On the basis of tcPO2 measurements made during arterial occlusion, simulations indicate that quantitative evaluation of the metabolic oxygen consumption of the viable skin tissues is possible only when the oxygen permeabilities of the skin and sensor are known.

A comparison of two transcutaneous monitors for the measurement of arterial PO2 and PCO2 in neonates

We examined the ability of two transcutaneous devices (Fastrac, Sensormedics Corporation, Yorba Linda, California, U.S.A. and Hewlett Packard M1018A, Hewlett Packard Component Monitoring System, Hewlett Packard, North Hollywood, U.S.A.) to measure arterial PCO2 and PO2 in neonates. Thirty-seven neonates had transcutaneous oxygen measured with the Hewlett Packard (HPO2 group), 38 neonates had transcutaneous carbon dioxide measured with the Hewlett Packard (HPCO2 group) and the Fastrac was used on 27 neonates (FTCO2 group). Both devices were operated with electrode temperatures of 43.5 degrees C although an additional ten subjects were studied using the Fastrac with an electrode temperature of 43.0 degrees C. The mean differences (transcutaneous--arterial) and upper and lower limits of agreement were calculated for each group. For the HPO2 group they were 3.78 mmHg (-12.23 to 19.80 mmHg), for the HPCO2 group they were 0.40 mmHg (-4.50 to 5.30 mmHg) and for the FTCO2 they were -0.96 mmHg (-7.85 to 5.92 mmHg). For the Fastrac group at an electrode temperature of 43.0 degrees C the mean difference and limits of agreement were -1.00 mmHg and -4.58 mmHg to 2.58 mmHg. The average sensitivity and specificity for both machines for the detection of hypocarbia were 82% and 92% respectively while for hypercarbia they were 90% and 94% respectively. For hypoxaemia, the sensitivity and specificity were 40% and 94% while for hyperoxaemia the sensitivity and specificity were 83% and 97%. We conclude that both machines provide a useful supplement to arterial PCO2 measurements and the Fastrac performs better at 43.0 degrees C. The measurement of PO2 is less accurate but is still of clinical use.

Paradoxical transcutaneous oxygen response to cutaneous warming on the plantar foot surface: a caution for interpretation of plantar foot TcPO2 measurements

We examined the effect of cutaneous warming on the transcutaneous oxygen measurement (TcPO2) at standard locations below the knee, on the dorsal foot, on the plantar right great toe, and on the plantar surface under the second metatarsal head of 656 diabetic and 16 nondiabetic subjects. All subjects underwent a directed medical history, physical examination, and neurovascular lower extremity evaluation and assessment of autonomic neuropathy. Associations between autonomic neuropathy and TcPO2 were tested using two-way analysis of variance and multiple linear regression. An unexpected, statistically significant fall in TcPO2 occurred with cutaneous warming from 37 degrees C to 44 degrees C on the plantar great toe surface that did not differ by presence of autonomic neuropathy, or the presence of diabetes (mean change: -8.82 mmHG, 95% confidence interval [CI]: -7.70 to -9.93). The TcPO2 measured at the plantar metatarsal head site also fell with cutaneous warming from 37 degrees C to 44 degrees C (mean change: -9.67 mmHG, 95% CI: -7.75 to -11.59). As expected, the TcPO2 increased with cutaneous warming from 37 degrees C to 44 degrees C on the dorsal foot site (mean rise:= 35.61 mmHg, 95% CI: 34.18 to 37.04). The mean TcPO2 at any site did not differ by presence of autonomic neuropathy at either 37 degrees C or 44 degrees C. We conclude that cutaneous warming leads to a paradoxical fall in TcPO2 on the plantar foot surface that does not depend on the presence of autonomic neuropathy or diabetes. Caution in interpretation of ambient versus warmed plantar TcPO2 values is recommended.

Transcutaneous measurement of partial pressure of oxygen and carbon dioxide

Transcutaneous monitoring is noninvasive and relatively simple to use. In neonates and small infants, this monitoring technique may provide very useful clinical information. Transcutaneous gas monitoring, using conventional electrochemical techniques, provides a means of trending the values of PaO2 and PaCO2 in most patients with relatively normal cardiovascular function. In patients with compromised cardiopulmonary function and in many adults, because of different skin structure, transcutaneous gas monitoring will not accurately reflect arterial blood gas tensions. Because transcutaneous gases depend on skin perfusion, however, it may be useful in monitoring tissue perfusion, especially in patients with peripheral vascular disease and tissue flaps. The heating of the monitoring probe necessitates frequent site changes to avoid thermal injury, which make it more labor intensive than other noninvasive monitoring methods.

Simultaneous in vivo comparison of two-versus three-wavelength mixed venous (Svo2) oximetry catheters

OBJECTIVE

Venous oximetry catheters provide useful realtime information about mixed venous hemoglobin saturation (Svo2). Currently available systems utilize either two or three wavelengths of light to obtain these measurements. Previous animal and clinical studies have attempted to compare the accuracy of these two devices under similar circumstances. However, the relative accuracy of the two-wavelength versus three-wavelength systems has never been assessed under identical conditions. For this purpose, we designed an animal model for simultaneous measurement of Svo2, over a wide range of physiologic and pathologic states.

METHODS

Seven anesthetized swine underwent simultaneous placement of two- and three-wavelength catheters. Paired data points consisted of values obtained from a reference oximeter and from each of the catheters. Observations were obtained every 15 min during the following manipulations: (1) eucarbic hypoxia induced by reducing FiO2 to 0.18, 0.15, and 0.12 for 15 min each; (2) stimulated surgical manipulation; and (3) hypovolemic shock produced by hemorrhage to a mean arterial pressure of 50 torr for 1 hr. Data were analyzed by calculation of mean error (bias) and precision for each system in comparison with the oximeter.

RESULTS

The overall error of the two-wavelength system was +0.15%, with a precision of +/- 2.52%. The three-wavelength system had an overall error of +3.71%, with a precision of +/- 2.30%. Overall correlation between catheter Svo2 and oximeter values was the same for both devices (r = 0.99).

CONCLUSIONS

Both currently available in vivo spectrophotometric systems are capable of producing satisfactory results over wide ranges of Svo2. In contradistinction to older reports, we found that the two-wavelength Svo2 system produced results equivalent to those obtained from the three-wavelength device. In this regard, there is no detectable advantage in accuracy to measuring in vivo Svo2 with three rather than with two wavelengths.

[Cerebral oximeter INVOS-3100]

A new device is described: cerebral oximeter intended for monitoring cerebral oxygenation and early diagnosis of cerebral ischemia and hypoxia. The advantages and shortcomings of the proposed method are analyzed in comparison with other current methods for the early diagnosis of cerebral ischemia.

Transcutaneous PO2 measurement in compound fractures

The transcutaneous assessment of tissue oxygenation has become a useful method for evaluating microcirculatory disorders in different diseases. In this clinical experimental pilot study, the impact of compound fractures on tissue oxygenation was examined. Using a special probe placed directly into the muscle, tissue oxygenation was measured at the site of tissue injury as well as at one non-injured site within the same fractured extremity. A non-injured extremity was also measured as a control. At the injured site tissue oxygenation was significantly increased during the first 4 days after trauma compared with the non-injured extremity (40.7 +/- 1.8 mmHg versus 23.5 +/- 4.1 mmHg). At the same time the non-injured site of the fractured extremity appeared to be significantly less oxygenated compared with the non-injured extremity (13.2 +/- 2.3 mmHg versus 23.5 +/- 4.1 mmHg). After day 4 until the end of the study (day 10) a complete return to normal values was noted at both sites. In conclusion, it is most likely that, due to a change of microcirculation within the fractured extremity, the injured site is temporarily oversaturated by oxygen and this is associated with low oxygenation at the non-injured site on the same extremity. As the microcirculatory status may be important in making decisions about trauma surgery (e.g. whether or not to amputate), transcutaneous oxygen measurement is a practical and useful method in trauma care.

Subcutaneous oxygen tension: a useful adjunct in assessment of perfusion status

OBJECTIVES

Using a new fluorescence-quenching optode which, unlike earlier oximeters, neither consumes oxygen nor generates heat, we sought to determine the effects of hemorrhage and resuscitation on subcutaneous PO2. Additionally, we compared the effects of resuscitation with diaspirin crosslinked hemoglobin, an oxygen-carrying solution, on subcutaneous PO2 to that of traditional resuscitative fluids. We also compared mean arterial pressure and central venous oxygen saturation, indirect indices of perfusion, to subcutaneous PO2, a direct index of perfusion.

DESIGN

Prospective trial, randomized for selection of treatment regimen.

SETTING

Shock-trauma laboratory of a medical university.

SUBJECTS

Male Sprague-Dawley rats, weighing 260 to 380 g.

INTERVENTIONS

Rats were bled 22 mL/kg and resuscitated, 1 min later, with either 66 mL/kg of lactated Ringer's solution, 22 mL/kg of human serum albumin, 22 mL/kg of blood, or 22 mL/kg of diaspirin crosslinked hemoglobin. A fifth group of animals was not resuscitated after hemorrhage. Subcutaneous PO2 and mean arterial pressure were monitored continuously throughout the experiment, while central venous oxygen saturation was measured intermittently.

MEASUREMENTS AND MAIN RESULTS

Subcutaneous PO2 decreased in response to hemorrhage and, although it did increase after resuscitation with each fluid, no treatment was able to restore subcutaneous PO2 to baseline within 2 hrs postresuscitation. Subcutaneous PO2 continued to decrease after hemorrhage in the unresuscitated animals. In contrast, mean arterial pressure was restored to baseline values in only blood- and diaspirin crosslinked hemoglobin-treated animals, although this effect was lost within 30 mins in the blood-treated group. Only blood restored the central venous oxygen saturation to baseline values in the early postresuscitation period.

CONCLUSIONS

The fluorescence-quenching optode consistently followed changes in subcutaneous PO2 during hemorrhage and after resuscitation. Diaspirin crosslinked hemoglobin performed as well as blood in restoring peripheral perfusion, as measured by subcutaneous PO2, while both of these fluids were superior to either lactated Ringer's solution or albumin. Both whole blood and diaspirin crosslinked hemoglobin restored mean arterial pressure to baseline, although the effect of the latter was of a longer duration. The pressor effect of the crosslinked hemoglobin did not affect peripheral perfusion, as reflected by the values for subcutaneous PO2. Subcutaneous PO2 is a useful adjunct in assessment of the adequacy of peripheral perfusion and may help redefine targets for resuscitation.

Effects of dynamic exercise on subcutaneous oxygen tension and temperature

Exercise influences blood flow distribution and may thereby influence tissue oxygen. Given that experimental evidence suggests exercise improves wound healing, this study was undertaken to determine the effect of exercise on subcutaneous tissue oxygen tension (PscO2). Subjects performed a treadmill test to maximum oxygen consumption (VO2) while PscO2 and subcutaneous temperature (Tsc) were measured using a tonometer and optode/thermocouple system. There were no significant differences in PscO2 between measurement points. Mean Tsc had increased 4.8% at maximum VO2, and 6.3% as the cool-down period ended and differed significantly across the exercise period. Changes in PscO2 and Tsc were not correlated. Oxygen availability was neither enhanced nor significantly reduced by exercise. However, PscO2 did not increase as expected in relation to increases in Tsc. Further study will determine the purported beneficial effects of exercise and the mechanism by which it may affect wound healing.

[Usefulness of transcutaneous oximetry in vascular surgery]

The assessment is presented of the value of transcutaneous oximetry in vascular surgery and in choosing the level of ischaemic limb amputation. Transcutaneous measurements of oxygen pressure (tcp02) were performed in 172 patients operated on for chronic ischaemia of the lower limbs before and three weeks after the operation. The studied subjects had the following operations done: bifurcated or unilateral aortofemoral bypass, femoropopliteal bypass, restoration of patency of the iliac and femoral arteries, lumbar sympathectomy, lower limb amputation. The studies were carried out using Hellige SM 361 oxymonitor and a similar Polish device. For oximetric measurements superficial skin sensor (Clark's polarographic electrode) was used, which was applied into the dorsum of the foot and medial crural surface. Transcutaneous oximetry seems to be a useful method for the assessment of the results of operations restoring blood flow in lower limb arteries. It makes possible to evaluate the results of lumbar sympathectomy. Transcutaneous measurements of oxygen pressure are also a useful method for the assessment of results of amputation of chronically ischaemic lower limb.

Noninvasive oxygen monitoring techniques

As this article demonstrates, tremendous progress has been made in the techniques of oxygen measurement and monitoring over the past 50 years. From the early developments during and after World War II, to the most recent applications of solid state and microprocessor technology today, every patient in a critical care situation will have several continuous measurements of oxygenation applied simultaneously. Information therefore is available readily to alert personnel of acute problems and to guide appropriate therapy. The majority of effort to date has been placed on measuring oxygenation of arterial or venous blood. The next generation of devices will attempt to provide information about living tissue. Unlike the devices monitoring arterial or venous oxygen content, no "gold standards" exist for tissue oxygenation, so calibration will be difficult, as will interpretation of the data provided. The application of these devices ultimately may lead to a much better understanding of how disease (and the treatment of disease) alters the utilization of oxygen by the tissues.

[The use of the Gasthmatic System in studying bronchial hyperreactivity]

The Gasthmatic System is a new method for monitoring transcutaneously pO2 and pCO2 changes prior to, during and after bronchial provocation. Unlike normals, asthmatic and atopic subjects reproduce the same gas patterns: immediate transient hypocapnia, during inhalation of ultrasonically nebulised distilled water (UNDW) (dead space effect) and delayed more persistent hypoxia, after end of UNDW exposure. This pattern is suggestive of an uneven distribution of the alveolar ventilation and ventilation/perfusion ratio and is noted both in asymptomatic asthmatics and in nonasthmatic atopic rhinitis subjects FEV1-non-responders to UNDW. This distributional pattern, therefore, appears to be able to screen very early stages of airway disease, when conventional parameters of lung volumes and mechanics fail to reveal any significant changes in hyper-reactive response. The Gasthmatic model is simple, painless, effortless, and allows transcutaneous measurements during all the phases of the hyperreactivity test, including the actual stimulation phases. It proves, therefore, to be particularly useful for children too young to cooperate with common lung function tests.

Accuracy of nine commercially available pulse oximeters in monitoring patients with chronic respiratory insufficiency

9 pulse oximeters (BIOX-III, Nellcor 250, Jaeger CNS, Micro2), Pulsox-8, Oxycount, PulsOXImeter, PalcoLabs and Pulsox-7) were studied in 90 COPD patients. The transcutaneus oxygen saturation (SaO2%) was compared with arterial oxygen saturation calculated simultaneously with drawn blood samples (AVL-995). The measurement of error distribution and the cumulative distribution function (CDF) of measurement errors allows ranking of the pulse oximeters, which can be divided in 3 groups. We conclude that the accuracy of the tested nine pulse oximeter does not enable precise absolute measurements, specially at lower oxygen saturation ranges.

A triple electrode for simultaneous investigations of transcutaneous oxygen tension, laser-Doppler flowmetry and dynamic fluorescence video microscopy

A newly designed triple probe is introduced for measurements of transcutaneous oxygen tension, laser Doppler flowmetry (LDF) and microangiodynamics of skin capillaries by dynamic video microscopy with and without fluorochromes. The performance of the triple probe was checked in 9 healthy volunteers (6 women, 3 men; mean age: 34 years) and 9 patients (5 women, 4 men; mean age: 67 years) with peripheral arterial occlusive disease (PAOD). The mean Doppler ankle/arm pressure ratio was 0.54 +/- 0.30. Six patients suffered from severe claudication, 2 from rest pain and 1 patient had toe and forefoot necrosis. The foot dorsum was selected as measuring site. After recording baseline values of skin surface PO2 (ssPO2) at 37 degrees C, LDF and capillary images, a suprasystolic compression at the ankle level was performed for 4 min. Thirty seconds before cuff opening 0.2 ml/1 l blood volume of 20% sodium fluorescein was injected in an antecubital vein. Sodium fluorescein arrival times, filling times and maximum fluorescent light intensity times were measured, and ssPO2 and LDF were recorded continuously during postocclusive reactive hyperemia (PORH). The results indicate an adequate function of the triple probe. The mean resting ssPO2 was 2.0 +/- 1.9 mm Hg in PAOD patients and 4.0 +/- 3.9 mm Hg in controls (p = 0.185). Maximum ssPO2 during PORH was significantly reduced (p = 0.008) in patients (3.1 +/- 2.1 mm Hg) compared to healthy subjects (11.8 +/- 7.7 mm Hg). Resting LDF values were 6.5 +/- 6.4 perfusion units (PU) in PAOD patients versus 10.3 +/- 8.2 AU in controls (p = 0.295). Peak LDF during PORH was significantly reduced (p = 0.005) in patients (19.5 +/- 6.4 PU) versus healthy subjects (33.8 +/- 11.5 PU.(ABSTRACT TRUNCATED AT 250 WORDS)

Noninvasive monitoring of oxygenation in infants and children: practical considerations and areas of concern

OBJECTIVE

To review the technical principles of transcutaneous PO2 (tcPO2) monitors and pulse oximeters and to discuss methodological and practical issues related to the use of these devices in infants and children.

DESIGN

Literature review and summary of personal experience.

RESULTS

tcPO2 measurements are influenced by skin thickness, sensor temperature, amount of contact gel used, and state of peripheral perfusion. The average in vivo response time to a rapid decrease in PaO2 is approximately 16 seconds. Sensitivity to both hypoxemia (PaO2 < 50 mm Hg) and hyperoxemia (PaO2 > 80 to 100 mm Hg) is approximately 85%. Pulse oximeters require careful sensor placement (to avoid optical shunts) and adequate pulse pressures (> 20 mm Hg). They are prone to movement artifact. There are considerable differences in bias and precision between different brands, resulting, for example, in quite different upper alarm limits required to detect hyperoxemia reliably. The normal range of oxygen saturation measured by pulse oximetry, measured with one brand of pulse oximeter (Nellcor) during regular breathing, is 95% to 100% in preterm infants and 97% to 100% in full-term infants and children.

CONCLUSIONS

Because both devices have their specific shortfalls, they should ideally be used in combination, particularly in critically ill preterm neonates. Where this is not feasible or necessary, clinicians must be aware of the limitations of the device they are using.

Capnometer transport delay: measurement and clinical implications

The sidestream capnogram is delayed behind real time by transport delay (TD; time to aspirate gas through the sampling tubing) and by the dynamic response (DR) of the measurement cuvette. In six capnometers, we measured TD and DR by plunging the end of the sample tubing into a flask containing CO2 and then digitally analyzing the capnogram. TD ranged from 0.6 to 5.0 s and accounted for 89% or more of the total response time (TD + DR) of the capnometer. TD was generally not reported in the manufacturers' specifications. TD was further prolonged by low aspiration rates or by sampling tube extensions. During a series of quick breaths after endotracheal intubation, long TD can delay the appearance of CO2 and result in a false diagnosis of esophageal intubation. Also, long TD can prolong DR, which can result in underestimation of end-tidal PCO2 during rapid ventilation in pediatric anesthesia.

[Basic principles and characteristics of transcutaneous "reflective" oximetry]

The papers considers the main methods of mathematical description of light-biological tissue interactions in the context of problems of transcutaneous reflective oximetry. Provides a review of the most significant studies which have been recently performed in this field. By using them as an example, the paper analyzes the most serious problems arising in designing reflective oximeters and the main ways to solve them. The paper also discusses the advantages of the reflective oximeter and prospects for their development.

[The potentials of polarography in predicting the effect of a reconstructive operation in patients with severe ischemia of the lower extremities]

Possibilities to prognose the effect of reconstructive operation on the lower extremity vessels are shown on the basis of transcutaneous of oxygen on the dorsal surface of the foot in complex with ultrasonic Dopplerometry in patients with severe ischemia of lower extremities. Effectiveness of the test with the orthostatic growth of Po is proved. An examination of 60 patients with severe ischemia of lower extremities and 10 patients without pathology of main vessels (control group) was performed. Operations were performed on 48 patients, in 35 of them reconstructive operations were made. No correlation was found between preoperative value of Po and a result of the reconstruction. A correlation was found between the value of the orthostatic growth of Po and outcome of the reconstructive operation. The average growth of Po in the group of patients with a negative outcome of the reconstruction made up 9.93 + 2.78 mm Hg, with a positive outcome it was 14.137 + 2.30 mm Hg (p < 0.05). In patients with malleolus index of the regional systolic pressure > 0.3 with the orthostatic growth of Po > 20 mm Hg the probability of positive prognosis of the reconstruction made up 88.8%.