Steven J. Barker

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.

Factors associated with postoperative pulmonary complications in patients with severe chronic obstructive pulmonary disease.

The purpose of this study was to determine the incidence of different postoperative pulmonary complications (PPCs) and their associated risk factors in patients with severe chronic obstructive pulmonary disease (COPD) (forced expiratory volume in 1 s [FEV1] <or=to1.2 L and FEV1/forced vital capacity (FVC) <75%) undergoing noncardiothoracic operations. Thirty-nine of 105 patients (37%) had one or more PPCs (death, pneumonia, prolonged intubation, refractory bronchospasm, or prolonged intensive care unit (ICU) stay). Thirty-eight of 39 patients (97%) with a PPC had an anesthetic duration >2 h. Our study patients had a 47% 2-yr mortality rate. We determined specific risk factors for each PPC by analyzing potential preoperative and intraoperative risk factors. Pulmonary factors alone do not predict the likelihood of PPCs in severe COPD patients. Multiple logistic regression identified composite scoring systems, such as the ASA physical status, as the best preoperative predictors of PPCs, probably because they include both pulmonary and nonpulmonary factors. During the intraoperative period, avoiding general anesthesia with tracheal intubation may decrease the risk of postoperative bronchospasm. Shortening the duration of surgery and anesthesia may decrease the risk of prolonged ICU stay. (Anesth Analg 1995;80:276-84)

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.

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.

Case 6-5-1992 Anesthetic considerations for thoracoscopic procedures

Cm! 1 A 77-year-old white man was admitted with a left-sided pleura1 effusion. His past history included prolonged asbestos exposure and heavy smoking. He presented with dyspnea and wcight 10s~. Physical examination revealed a thin individual with a large barrel-shaped chest. Preoperative pulmonary function tests showed an FEVt of 1.2 L. A computerized tomography scan showed multiple pleura1 plaques and areas of thickening, especially along the mediastinal pleura. Because of the patient’s poor pulmonary status, flexible bronchoscopy and thoracoscopy were scheduled. The planned perioperative monitoring included indwelling arterial pressure, end-tidal CO1 (ETCO& and peripheral oxygen saturation (SpO

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

Anesthesia was induced with thiamylal, fentanyl, and vecuronium. The trachea was intubated initially with a single-lumen endotracheal tube for flexible fiberoptic bronchoscopy. Following this, a double-lumen right-sided endobronchial tube was inserted. After confirmatory chest auscultation and flexible bronchoscopy, the patient was positioned in the right lateral decubitus position. Anesthetic maintenance included fentanyl, vecuronium, and isoflurane/air/oxygen. One-lung ventilation (OLV) was maintained during the thoracoscopic procedure. During diagnostic thoracoscopy, the stopcock of the cannula was left open to room air to maintain an open pneumothorax. Air insufflation pressures of 10 to 11 mmHg were used to attain better visibility. Hemodynamic monitoring and management included special attention to changes in blood pressure, heart rate and rhythm, end-tidal CO2 because of the potential for arrhythmias, fa11 in venous return and cardiac output, gas embolism, and direct compression of cardiac structures. The surgical procedure involved multiple pleura1 biopsies and

Transcutaneous oxygen tension: A physiological variable for monitoring oxygenation

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

Prevention o f hypoxia is the most important goal of patient monitoring during anesthesia. Unfortunately, the currently monitored variablcs of heart rate, blood pressure, breath sounds, and skin and blood color are not sensitive to the level o f oxygenation or to trends, but detect only tile physiological conscquences of inadequate oxygenation. Intermittent arterial blood gas san> piing establishes the level o f arterial oxygcn tension (PaO2), but does not provide continuous information and, more important, does not assess oxygen delivery to the tissues. The transcutaneous oxygen tension (PtcO2) monitor and tile transcutaneous oximeter are two noninvasive monitors of oxygenation that have become available to anesthesiologists. Although these devices employ entirely differcnt principlcs to measure different but related variables, they are often compared for the purpose o f determining which is a bet ter monitor [1]. This essay will briefly review the development of PtcO2 measurement, describe what the technique actually measures, and compare PtcOe measurement and pulse oximetry as monitors of patient oxygenation during anesthesia.

Measurement of pulmonary CO2 elimination must exclude inspired CO2 measured at the capnometer sampling site

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.