Computer-assisted Doppler monitoring to enhance detection of air emboli

Currently, two of the most sensitive clinical approaches commonly used to monitor for venous air embolism, i.e., precordial Doppler audio and capnography, require the attention of the anesthesiologist's eye or ear, which is a distraction from other aspects of care. To assess the feasibility of allowing the computer to relieve the necessity for continuous human monitoring, we developed a computer algorithm for monitoring the precordial Doppler audio. This algorithm extracted (1) the amplitude of certain higher-frequency components of the Doppler audio, (2) a measure of the average value of the envelope of Doppler audio, and (3) the ratio between the average value of the Doppler envelope and the amount of envelope signal variation at heart rate frequency and its multiples. These three features were monitored by an adaptive pattern recognition algorithm that compared each new value for each feature with the previously developed mean and standard deviation for that feature. If the changes in the three features exceeded a detection threshold, an alarm (indicating suspected air embolism) was activated. Implemented as a prototype system, the algorithm was given preliminary testing in 2 dogs and activated alarms at levels of air well below those reported to cause clinically significant hemodynamic changes in dogs. While decreasing the distraction for the anesthesiologist, this early prototype alarm system alerts its user to the need for analysis of the Doppler signals when it senses an air embolus.     

Detection of venous air embolism in dogs by emission spectrometry

Emission spectrometers provide alternative, relatively inexpensive methods for detecting the concentration of respiratory gas nitrogen. Mass spectrometers are accepted as reliable monitors of end-tidal nitrogen for detection of venous air embolisms. We evaluated an inexpensive emission spectrometer for detecting changes in nitrogen levels and compared it with a mass spectrometer for detecting increased endtidal nitrogen levels in dogs with venous air embolisms. During in vitro gas flow studies (helium; oxygen; helium/ oxygen mixtures; or 70% nitrous oxide/30% oxygen with 0, 1, 2, or 3% isoflurane), air boluses (0.01 to 5.0 ml) were injected into a gas flow circuit and outlet nitrogen levels were measured by a Collins 21232 emission spectrometer. Responses were greater after each bolus when helium rather than oxygen was the major diluent gas. During in vivo studies, 5 dogs were anesthetized, ventilated, denitrogenated, and given venous air embolisms (0.1, 0.5, and 1.0 ml. kg^-1) during oxygen and then during Heliox (20% oxygen:80% helium) breathing. End-tidal nitrogen increased approximately two-fold after venous air embolisms given during Heliox as compared with oxygen ventilation. In all 0.1-ml. kg^-1 venous air embolisms end-tidal nitrogen increased when the emission spectrometer was used, but venous air embolisms less than 1.0 ml. kg^-1 were not consistently detected by mass spectrometry. Emission spectrometry can be used to detect increased end-tidal nitrogen levels indicative of venous air embolism and may be a more sensitive detector than mass spectrometry. Its sensitivity and relatively low cost (one-eighth of a magnetic fixed-sector mass spectrometer) make it a great potential monitor for both clinical detection of venous air embolism and air embolism research.     

End-tidal PCO2 monitoring via nasal cannulae in pediatric patients: Accuracy and sources of error

Objective. To assess the correlation and accuracy of end-tidal PCO₂ (PetCO₂) sampled via nasal cannulae in pediatric patients by comparison to the criterion standard PaCO₂, and to identify sources of error during PetCO₂ monitoring via nasal cannulae.Methods. PetCO₂ was monitored continuously by sampling end-tidal gas through nasal cannulae that had been designed and manufactured for this purpose in spontaneously breathing children undergoing conscious or deep sedation during either cardiac catheterization (n = 43) or critical care (n = 54). When both the capnographic wave form and the PetCO₂ value had been stable for at least 10 minutes, the PetCO₂ value was recorded while blood was drawn from an indwelling arterial line for PaCO₂ measurement. The effects of age, weight, respiratory rate, oxygen delivery system, airway obstruction, mouth breathing, and cyanotic heart disease were evaluated by linear regression analysis and calculation of absolute bias (PaCO₂-PetCO₂).Results. Mouth breathing, airway obstruction, oxygen delivery through the ipsilateral nasal cannula, and cyanotic heart disease adversely affected accuracy. In patients without those factors, PetCO₂ correlated well with PaCO₂ (R² = 0.994), and absolute bias was 3.0 ± 1.8 mmHg.Conclusions. Several factors — some controllable and all recognizable — affect the accuracy of PetCO₂ monitored via nasal cannulae in pediatric patients. When these factors are not present, PetCO₂ correlates well with PaCO₂ and appears to be a useful monitor of ventilatory status during conscious or deep sedation.     

End-tidal oxygen concentration and pulse oximetry for monitoring oxygenation during intratracheal jet ventilation

Objective. In this study, we evaluated the usefulness of end-tidal oxygen monitoring during intratracheal jet ventilation (ITJV) for endolaryngeal laser surgery.Methods. A total of 20 consecutive patients of both genders scheduled for endolaryngeal procedures under general anesthesia were studied. Inspiratory oxygen concentration and respiratory rate were varied, with patients serving as their own controls. Readings of pulse oximetry, airway oxygen, and carbon dioxide concentrations were recorded, and arterial blood samples for blood gas analysis were taken.Results. At jet cycle rates of 20 cycles/min, end-tidal oxygen (ETo₂) concentration indicated alveolar hypoxia 30 to 60 sec before hypoxemia was detected by pulse oximetry. Jet mixing of inspiratory and expiratory gas caused a larger difference between end-tidal and arterial gas concentrations than normally seen with conventional ventilation. Correlations between ETo₂ concentrations, oxygen saturations, and arterial oxygen levels depended on respiratory rate and inspiratory oxygen concentration; correlations were stronger at low than at high inspiratory oxygen concentrations and stronger at low than at high respiratory rates.Conclusions. ETo₂ concentration should be maintained well over 21% during ITJV to prevent alveolar and arterial hypoxia. Monitoring of respiratory oxygen concentrations at jet cycle rates of 20 cycles/min and less verifies safe oxygen levels during laser surgery, and confirms adequate alveolar oxygenation.We are grateful to Anneli Innanmaa, RN, for her skillful help with this study, and to Hannu Laine, fine mechanic, for the construction of the copper tubes.Grants were received from Instrumentarium Scientific Foundation, Helsinki, for construction of the special jet ventilator and the pressure curve monitor. Datex, Helsinki, provided the OSCAR OXY monitor.Part of the results of this study were presented at the 10th World Congress of Anaesthesiologists, The Hague, 1992 (Abstract A41 [ISBN 90-800899-2-3]).     

运用三维彩色多普勒超声心动图计算每搏动能进行 心功能评价的实验研究

目的探讨三维彩色多普勒超声心动图在评价心脏每搏动能方面的应用价值.方法实验动物选用杂种犬10只,体重10.5～18.5kg.利用TomTec三维彩色多普勒成像工作站进行三维图像获取和重建,采用心导管测量左室压力.以三维彩色多普勒重建图像测量收缩期通过主动脉瓣口的血流速度和血流量;在此基础上计算出心脏每搏动能(Ek).分别在静息、静注多巴酚丁胺和结扎左冠状动脉前降支三种不同的心功能状态下进行实验.结果静息状态下心功能正常时,Ek为0.30g-m±0.09g-m;静注多巴酚丁胺后心功能增强,Ek测值为0.58g-m±0.13g-m,较静息心功能正常状态明显升高(P＜0.05);结扎左冠状动脉前降支后心功能减弱,Ek测值为0.15g-m±0.04g-m,较静息心功能正常状态明显降低(P＜0.05).结论利用三维彩色多普勒超声心动图的血流测值可以计算出心脏每搏动能(Ek),并能够可靠地反映心脏收缩功能的变化,为全面准确地评价心脏泵血功能提供了新的无创性手段.     

Monitoring pulmonary function with superimposed pulmonary gas exchange curves from standard analyzers

Objective.A repetitive graphic display of the single breath pulmonary function can indicate changes in cardiac and pulmonary physiology brought on by clinical events. Parallel advances in computer technology and monitoring make real-time, single breath pulmonary function clinically practicable. We describe a system built from a commercially available airway gas monitor and off the shelf computer and data-acquisition hardware. Methods.Analog data for gas flow rate, O₂, and CO₂ concentrations are introduced into a computer through an analog-to-digital conversion board. Oxygen uptake (VO₂) and carbon dioxide output (VCO₂) are calculated for each breath. Inspired minus expired concentrations for O₂ and CO₂ are displayed simultaneously with the expired gas flow rate curve for each breath. Dead-space and alveolar ventilation are calculated for each breath and readily appreciated from the display. Results.Graphs illustrating the function of the system are presented for the following clinical scenarios; upper airway obstruction, bronchospasm, bronchopleural fistula, pulmonary perfusion changes and inadequate oxygen delivery. Conclusions.This paper describes a real-time, single breath pulmonary monitoring system that displays three parameters graphed against time: expired flow rate, oxygen uptake and carbon dioxide production. This system allows for early and rapid recognition of treatable conditions that may lead to adverse events without any additional patient measurements or invasive procedures. Monitoring systems similar to the one described in this paper may lead to a higher level of patient safety without any additional patient risk.     

Microstream capnograpy technology: a new approach to an old problem

Significant technical limitations inherent in blackbody infrared technology used in conventional sidestream and mainstream capnography have hindered the acceptance and growth of capnography as a monitoring tool outside the operating room environment. We describe a new technology (Microstream) for CO₂ monitoring, based on molecular correlation spectroscopy, which results in a highly efficient and selective emission of a spectrum of discrete wavelengths exactly matching those for CO₂ absorption. The CO₂ specific emissions allow for an extremely small sample cell (15 µl), which in turn, permits the use of a very low sample flow rate (50 ml/min) without compromising waveform integrity or end-tidal CO₂ accuracy. Design and technology features of the CO₂ emission source, sample cell, and breath sampling circuits are described.     

Continuous monitoring of ventilatory mechanics during one-lung ventilation

Objective. The Ultima SV respiratory monitor can be used to monitor the intraoperative effects of the lateral decubitus position and one-lung ventilation on ventilatory mechanics.Methods. Eight patients with esophageal cancer who required one-lung ventilation for esophagectomy and reconstruction were enrolled in the study. We monitored pressure-volume or flow-rate-volume loops continuously throughout the operation. Respiratory parameters were evaluated closely during five conditions of ventilation: two-lung ventilation in the supine position, two-lung ventilation in the lateral decubitus position, dependent one-lung ventilation in the lateral decubitus position, nondependent one-lung ventilation in the lateral decubitus position, and dependent one-lung ventilation in the lateral decubitus position with the chest opened. Respiratory rate was controlled at 10 breaths/min, and tidal volume was kept constant (10 ml/kg) during surgery.Results. Peak inspiratory pressure increased to 29.0 ± 9.0 (mean ± SD) cm H₂O in the dependent one-lung in the lateral decubitus position with the chest opened (p < 0.01). Dynamic compliance decreased to 29.4 ± 4.9 ml/cm H₂O in the dependent one-lung in the lateral decubitus position with the chest opened (p < 0.01). The changing configuration of the loops also offered additional and instantaneous information during one-lung ventilation.Conclusions. One-lung ventilation caused several changes in the whole respiratory system (lung, thorax, and endotracheal tube). Continuous monitoring of flow-rate-volume or pressure-volume loops with in-line spirometry provided comprehensive information regarding parameters in one-lung ventilation.     

Sampled gas need not be returned during low-flow anesthesia

Objective. The purpose of this investigation was to study the N₂ flux between the patient and the breathing circuit, and the excess gas during N₂O anesthesia with the low, fresh gas flow technique.Methods. Forty patients were studied. After a 6-minute high, fresh gas flow denitrogenation period, the O₂ fresh gas flow was set at about 4 ml/kg/min and the N₂O fresh gas flow was set to maintain an inspired O₂ fraction of 0.30. The excess gas flow and N₂ excretion were measured by a variant of the Douglas bag method.Results. The mean inspired N₂ concentration reached a peak of 5.9% at 40 minutes. The estimated mean N₂ excretion was 39 ml/min at 10 minutes, declining to 18 ml/min at 60 minutes. A calculation of N₂ homeostasis during closed-circuit anesthesia based on the results of the patient study indicated that sampling for gas analysis actually reduces the gas costs if the sampled gas is scavenged instead of returned to the circle system, since intermittent flushing with high, fresh gas flow for denitrogenation is unnecessary in the former situation.Conclusions. Regardless of the fresh gas flow used, sampled gas need not be returned during N₂O anesthesia.Financial support was provided by AGA AB Medical Research Fund (supplier of O₂ and N₂O).     

End-tidal carbon dioxide as a measure of arterial carbon dioxide during intermittent mandatory ventilation

To determine if end-tidal carbon dioxide tension (PetCO₂) is a clinically reliable indicator of arterial carbon dioxide tension (PaCO₂) under conditions of heterogeneous tidal volumes and ventilation-perfusion inequality, we examined the expiratory gases of 25 postcardiotomy patients being weaned from ventilator support with intermittent mandatory ventilation. Using a computerized system that automatically sampled airway flow, pressure, and expired carbon dioxide tension, we were able to distinguish spontaneous ventilatory efforts from mechanical ventilatory efforts. ThePetCO₂ values varied widely from breath to breath, and the arterial to end-tidal carbon dioxide tension gradient was appreciably altered during the course of several hours. About two-thirds of the time, thePetCO₂ of spontaneous breaths was greater than that of ventilator breaths during the same 70-second sample period. The most accurate indicator of PaCO₂ was the maximalPetCO₂ value in each sample period, the correlation coefficient being 0.768 (P < 0.001) and the arterial to end-tidal gradient being 4.24 ± 4.42 mm Hg (P < 0.01 compared with all other measures). When all values from an 8-minute period were averaged, stability was significantly improved without sacrificing accuracy. We conclude that monitoring the maximalPetCO₂, independent of breathing pattern, provides a clinically useful indicator of PaCO₂ in postcardiotomy patients receiving intermittent mandatory ventilation.