Kevin K. Tremper

EFFECTS OF METHEMOGLOBINEMIA ON PULSE OXIMETRY AND MIXED VENOUS OXIMETRY

The performance of three commercially available pulse oximeters was assessed in five anesthetized dogs in which increasing levels of methemoglobin were induced. Hemoglobin oxygen saturation in each dog was monitored with three pulse oximeters (Nellcor N-100, Ohmeda 3700, and Novametrix 500) and a mixed venous saturation pulmonary artery catheter (Oximetrix Opticath). Arterial and mixed venous blood specimens were analyzed for PaO2, PaCO2, and pHa using standard electrodes. An IL-282 Co-oximeter was used on the same specimens to determine oxyhemoglobin and methemoglobin as percentages of total hemoglobin. Methemoglobin levels of up to 60% were induced by intratracheal benzocaine. As MetHb gradually increased while the dogs were breathing 100% inspired oxygen, the pulse oximeter saturation (SpO2) overestimated the fractional oxygen saturation (SaO2) by an amount proportional to the concentration of methemoglobin until the latter reached approximately 35%. At this level the SpO2 values reached a plateau of 84-86% and did not decrease further. When, at fixed methemoglobin levels, additional hemoglobin desaturation was induced by reducing inspired oxygen fraction, SpO2 changed by much less than did SaO2 (regression slopes from 0.16 to 0.32). Thus, at high methemoglobin levels SpO2 tends to overestimate SaO2 by larger amounts at low hemoglobin saturations. Plots of SpO2 versus functional saturation (oxyhemoglobin/reduced hemoglobin plus oxyhemoglobin) show an improved but still poor relationship (regression slopes from 0.32 to 0.46). The Oximetrix Opticath pulmonary artery catheter behaves similarly but provides somewhat better agreement with functional saturation than do the pulse oximeters in the presence of methemoglobinemia. Pulse oximetry data (SpO2) should be used with caution in patients with methemoglobinemia.

The Effect of Carbon Monoxide Inhalation on Pulse Oximetry and Transcutaneous Po2

Five dogs were anesthetized, intubated, and ventilated with various mixtures of oxygen, nitrogen, and carbon monoxide. Each dog was monitored with arterial and pulmonary artery catheters, a transcutaneous PO2 analyzer, and two pulse oximeters. An IL-282 Co-oximeter was used to periodically measure arterial oxyhemoglobin (O2Hb) and carboxyhemoglobin (COHb) as percentages of the total hemoglobin. The PaO2, PaCO2, and pHa were measured in the same blood specimens using standard electrodes. When the inspired oxygen concentration was reduced in the absence of COHb, the pulse oximeter saturation (SpO2) estimated O2Hb with reasonable accuracy. COHb levels were then varied slowly from 0-75% in each dog. As the COHb level increased and oxyhemoglobin decreased, both pulse oximeters continued to read an oxygen saturation of greater than 90%, while the actual O2Hb fell below 30%. In the presence of COHb, the SpO2 is approximately the sum of COHb and O2Hb, and, thus, may seriously overestimate O2Hb. The pulse oximeter, as the sole indicator of blood oxygenation, should, therefore, be used with caution in patients with recent carbon monoxide exposure. On the other hand, transcutaneous PO2 falls linearly as COHb increases, and reaches about one-fifth of its initial value at the highest COHb levels despite the maintenance of constant arterial PO2.

Changes in cardiac output after acute blood loss and position change in man

Thoracic bioimpedance cardiac output (Qtbi) was measured at 1-min intervals in 27 volunteers before, during, and after withdrawing 500 ml (3.7 to 8.5 ml/kg; mean 5.8) of blood. The effects of passive leg raising (PLR) and standing on Qtbi were measured before and after blood withdrawal. Arterial oxygen saturation (SaO2), transcutaneous oxygen tension (PtcO2), mean arterial BP (MAP), and heart rate (HR) were also measured before and after blood withdrawal. Thoracic bioimpedance cardiac index (CI) decreased 18% (0.8 +/- 0.1 L/min.m2, p less than .0001) and stroke volume index (SI) decreased 22% (14.8 +/- 2.7 ml/beat.m2, p less than .0001) after blood withdrawal. HR, MAP, SaO2, and PtcO2 were not significantly different after blood withdrawal. Before blood withdrawal PLR increased CI 6.8% (0.3 +/- 0.1 L/min.m2, p less than .0001); after blood withdrawal PLR increased CI 11.1% (0.4 +/- 0.1 L/min.m2, p less than .0001). PLR can increase stroke volume and cardiac output in hypovolemic humans.

Noninvasive monitoring of carbon dioxide: a comparison of the partial pressure of transcutaneous and end-tidal carbon dioxide with the partial pressure of arterial carbon dioxide.

This study compares two noninvasive techniques for monitoring the partial pressure of carbon dioxide (Pco2) in 24 anesthetized adult patients. End-tidal PCO2 (PETCO2) and transcutaneous Pco2 (PtcCO2) were simultaneously monitored and compared with arterial Pco2 (PaCO2) determined by intermittent analysis of arterial blood samples. PETCO2 and PtcCO2 values were compared with PaCO2 values corrected to patient body temperature (PaC02T) and PaCO2 values determined at a temperature of 37°C (PaCO2). Linear regression was performed along with calculations of the correlation coefficient (r), bias, and precision of the four paired variables:PETCO2 versus PaCO2 and PaCO2T (n = 211), and PtcCO2 versus PaCO2 and PaCO2T (n = 233). Bias is defined as the mean difference between paired values, whereas precision is the standard deviation of the difference.The following values were found forr, bias, and ± precision, respectively.PetCO2 versus PaCO2: 0.67, −7.8 mm Hg, ±6.1 mm Hg;PETCO2 versus PaCO2T: 0.73, −5.8 mm Hg, ±5.9 mm Hg;PETCO2 versus PaCO2: 0.87, −1.6 mm Hg, ±4.3 mm Hg; PtcCO2 versus PaC02T: 0.84, +0.7 mm Hg, ±4.8 mm Hg.Although each of thesePCO2 variables is physiologically different, there is a significant correlation (P < 0.001) between the noninvasively monitored values and the blood gas values. Temperature correction of the arterial values (PaCO2T) slightly improved the correlation, with respect toPETCO2, but it had the opposite effect for PtcCO2. In this study, the chief distinction between these two noninvasive monitors was thatPETCO2 had a large negative bias, whereas PtcCO2 had a small bias. We conclude from these data that PtcCO2 may be used to estimate PaCO2 with an accuracy similar to that ofPetCO2 in anesthetized patients.

Dermal heat transport analysis for transcutaneous O2 measurement .

Heat from a transcutaneous oxygen electrode is transmitted locally to the blood beneath it causing a shift in the HbO2 dissociation curve. This increases the local PO2, and allows a measurable PO2, at the skin surface. The temperature effect on the HbO2, curve must be accounted for in in vivo calibration of Ptco2, data. To do this, the capillary blood temperature beneath the electrode must be known. A heat balance is written around the capillary blood with heat being conducted in from the electrode and carried out by two means: conduction to deep tissue; and transport away by the flowing capillary blood. The following equation is the steady state solution of the heat transport problem:

Hemodynamic profile of adverse clinical reactions to Fluosol-DA 20%.

Hemodynamic changes of 2 patients to a 0.5-ml test-dose infusion of Fluosol-DA 20% are presented. The first patient had the following symptoms and signs approximately 2 min after receiving the test dose: normotensive bradycardia of 30 beat/min, a 35% drop in cardiac output, a 50% increase in systemic vascular resistance, and a 100% increase in pulmonary artery systolic and diastolic pressures. The patient complained of shortness of breath and diffuse pressure pain of the chest. All the signs and symptoms gradually resolved without treatment over the following 3 min. The second patient complained of mild, vague chest and abdominal pressure 2 min after the test dose. The only associated hemodynamic change was a slight increase in pulmonary artery systolic pressure. A 74% drop in the neutrophil count returned to the pretest value in 10 min. In contrast to anaphylactic or anaphylactoid-type reactions, these patients did not have urticaria and had an increase in systemic vascular resistance. Their reactions were reminiscent of those occurring during systemic complement activation. The possible mechanisms and prevention of these reactions are discussed.

Transcutaneous PO2 measurement

SummaryTranscutaneous PO2 sensors have been developed over the past ten years from the same basic electrodes used in conventional blood gas machines. The skin is heated to enable the skin surface sensors to respond quickly to the gas tensions beneath them. PtcO2 is a variable which reflects the PO2 in the peripheral tissue. PtcO2 has its own range of normal values and it responds to cardiopulmonary changes which affect tissue oxygenation. In the majority of patients, those without decreased cardiac output, PtcO2 follows the trend of the arterial gas tension, and the PtcO2 value decreases relative to PaO2 with increasing patient age (Table II). When there is severely reduced cardiac output and peripheral perfusion, the PtcO2 values will deviate from their relationship with the arterial tensions and become blood flow dependent, thus providing quantitative information regarding blood flow. It is likely that the technique of transcutaneous PO2 monitoring will gain wider acceptance because it is a noninvasive and continuous monitor which provides useful information regarding tissue oxygenation.

Continuous fiberoptic arterial oxygen tension measurements in dogs

An experimental study using a new fiberoptic sensor for the continuous intraarterial measurement of oxygen tension is described. This “optode” sensor uses the phenomenon of fluorescence quenching to determine the oxygen tension of the surrounding medium. To assess the accuracy of this device, we anesthetized 4 dogs and monitored them continuously with arterial catheters and an intraarterial optode probe, and intermittently with arterial blood gas analysis. The inspired oxygen fraction was varied from 1.0 to 0.1, and arterial blood gases were measured for comparison with the optode reading. Two hundred ninety data sets yielded a correlation coefficient of 0.96, with a linear regression slope of 0.98 and intercept of 5.1 mm Hg. In the 72 data sets from the last dog, the bias and precision of the optode arterial oxygen tension values were −10.3 mm Hg and 20.0 mm Hg, respectively. The optode probe was easily inserted through a 20-gauge catheter and did not interfere with continuous arterial pressure measurement or blood sampling. This study suggests that the optode has great potential as a continuous, real-time monitor of arterial oxygen tension.

A clinical comparison of transcutaneous PO2 and pulse oximetry in the operating room.

The early detection of hypoxia during anesthesia is one of the most important goals of intraoperative monitoring. Sequential arterial blood gas measurements can detect hypoxemia, but they are invasive, expensive, and do not provide continuous data. Transcutaneous Po2 measurement and pulse oximetry are two noninvasive techniques for assessment of oxygenation of tissue and blood, respectively. Although these two techniques are based upon entirely different principles and measure different variables, they are often compared to determine which is the better monitor of patient oxygenation. Neither technique measures the arterial blood oxygen tension, but both techniques are continuous and noninvasive. In the present study, measurements of transcutaneous oxygen tension (Ptco,) and pulse oximetric oxygen saturation (NSao2) are compared with arterial blood oxygen tension (Pao,) and saturation (Sao,) determined from radial artery blood samples. Data were obtained intraoperatively and postoperatively from patients undergoing various types surgical procedures. A previous study by Knill et al. obtained similar data in anesthetized healthy patients (1). In Knills study, the fraction of inspired oxygen ( F I ~ ~ ) was varied in such a way as to produce end-tidal oxygen concentrations ranging from 6 to 20%. For these hypoxic gas mixtures, the authors found that NSao, had a higher correlation with Sao, than did Ptco2 with Pao2. The goal of the present study is to determine these correlations within the range of FI,, relevant to clinical anesthesia, that is, from 0.21 to 1.0. Our study also includes results for less healthy patients (ASA

Continuous noninvasive estimation of cardiac output by electrical bioimpedance: an experimental study in dogs

A new device has been developed to estimate continuously and noninvasively cardiac output from the thoracic electrical bioimpedance (CObi). CObi was compared to cardiac output by thermodilution (COtd) in five anesthetized dogs. Blood pressure, blood volume, and blood flow were manipulated by hemorrhage and infusions of sodium nitroprusside and phenylephrine. These data were used to determine the correlation between CObi and COtd under conditions of hypotensive normal flow and normotensive low flow, as well as during hemorrhagic shock and resuscitation. The CObi device was calibrated in vivo to COtd for each dog at the beginning of each experiment. CObi had a significant positive correlation with COtd throughout the experiments (r = 0.84, slope = 0.91, intercept = 0.55, p < .001), and CObi predicted COtd with a standard error of the estimate of 0.81 L/min. Neither heart rate nor mean arterial pressure was significantly correlated with COtd or CObi. During severe hemorrhagic shock, CObi could not determine cardiac output in two of the dogs when COtd averaged 1.7 L/min. These data indicate that CObi is a blood-flow related variable that can be monitored continuously.