Kai Kuck

Theoretical analysis of non-invasive oscillometric maximum amplitude algorithm for estimating mean blood pressure

A theoretical analysis is performed to evaluate the effect of arterial mechanical and blood pressure pulse properties on the accuracy of non-invasive oscillometric maximum amplitude algorithm (MAA) estimates of the mean blood pressure obtained using air-filled occlusive cuffs. Invasively recorded blood pressure pulses, selected for their varied shapes, are scaled to simulate a wide range of blood pulse pressures (diastolic blood pressure minus systolic blood pressure). Each scaled blood pressure pulse is transformed through an exponential model of an artery to create a series of blood volume pulses from which a simulated oscillometric waveform is created and the corresponding MAA estimate of the mean blood pressure and error (mean blood pressure minus MAA estimate) are determined. The MAA estimates are found to depend on the arterial blood pressure. The errors are found to depend on the arterial mechanical properties, blood pressure pulse shape and blood pulse pressure. These results suggest that there is no direct relationship between the mean blood presure and MAA estimate, and that multiple variables may affect the accuracy of MAA estimates of the mean blood pressure obtained using air-filled occlusive cuffs.

Noninvasive cardiac output monitor algorithms are more sophisticated and perform better than indicated in modeling paper

To the Editor:—A 77-yr-old man undergoing insertion of a J-splint for renal obstruction received general anesthesia delivered with an ADU anesthesia machine (Anesthesia Delivery Unit; Datex-Ohmeda, Stockholm, Sweden). A 5% desflurane vaporizer concentration setting with an O2/N2O mixture (2 and 3 l/min, respectively) resulted in stable inspired and expired desflurane concentrations (fig. 1). Immediately after lowering the fresh gas flow (FGF) to 0.35 l/min O2 and 0.35 l/min N2O, and while maintaining the same vaporizer concentration setting, a dramatic increase in inspired and expired desflurane concentrations to about 14% (15:45) was noticed, as shown in figure 1. The duration of this high concentration was short-lived ( 2 min) and did not trigger an alarm; the vaporizer concentration setting was decreased to 4.5% and was left unchanged throughout the remainder of the procedure (until 16:13). After a rapid decrease of the inspiratory and end-expiratory desflurane concentrations to about 7–8.5%, the concentrations started to increase again, leading to a gradual decrease in blood pressure. Because vaporizer malfunction was suspected, the FGF was increased to 5 l/min O2/N2O and was decreased again (to the previous settings) within a period of 1 min (15:56). Inspired and expired concentrations were noticed to decrease and increase again. This maneuver was repeated at 16:03, confirming that indeed something was wrong with the vaporizer output with the use of lower FGF (0.7 l/min). At 16:05, the FGF was therefore increased to 5 l/min. Vaporizer output itself was then checked at low FGF (0.7 l/min) by interrupting ventilation and having the sampling line of the multigas analyzer (Compact Airway Module M-CAiOV, Datex-Ohmeda, Helsinki, Finland) sample gases leaving the common gas outlet (16:10). Desflurane output read 14.5% (at 16:10) during the use of low flows, but matched the dialed 4.5% (16:12) when the FGF was increased again to its previous settings (O2/N2O mixture, 2 and 3 l/min, respectively). An alarm message appeared (“Service fresh gas unit.”). Anesthesia was continued for a few more minutes for the remainder of the surgical procedure with desflurane and high FGF (5 l/min), and the patient was allowed to awaken without further incident. On the same day of our observation, a similar case was reported by the Anesthesiology Discussion Group on GASNet.† With FGF of 0.6 l/min O2 and 0.6 l/min air and a desflurane dial setting at 8%, the desflurane concentration on the agent analyzer display slowly approached 4.5–5.5%. Then, without warning, the desflurane concentration suddenly increased to 15%. It is unclear whether the events were the same as in our case. The ADU vaporizing unit is an electronically controlled, flow-over, variable bypass, and measured flow vaporizer, and its mechanism of action and performance have been described recently. Vaporizer output increased with lower FGF, with the largest error with FGF of 0.2 l/min (4.3 and 7.3% absolute output measured with 3% and 6% dialed, respectively, in a single instance). In the current case, however, substantially higher total FGFs (0.7 l/min) were used. Very preliminary testing by Datex-Ohmeda indicates that the one-way valve that prevents backflow of saturated vapor from the cassette via inspiratory channel toward the bypass channel may have failed to close after lowering the FGF (fig. 2). This problem may be more significant when desflurane is used because the pressure in the desflurane Aladin cassette (Datex-Ohmeda, Stockholm, Sweden) may exceed 1 atm because of its high vapor pressure when the temperature is greater than 22.8°C (boiling point of desflurane at 1 atm pressure). A similar problem in 1999 prompted a redesign of this one-way valve and an upgrading of all ADU anesthesia machines in service worldwide (Mr. Ola Lassborn, Quality Manager, Datex-Ohmeda, Stockholm, Sweden, verbal personal communication, March 2003). Despite the new design, this report suggests a continued problem with this valve with the possible delivery of unintended high concentrations of inhaled anesthetics. It is unclear whether the valve still has a design problem or whether only a few defective valves exist (a manufacturing issue). This safety issue is being addressed by Datex-Ohmeda. For now, it is advisable to monitor carefully for excessive agent concentrations when using the ADU Datex-Ohmeda anesthesia machines, especially if desflurane is administered.

Dual-wavelength reflectance spectroscopy of the superior vena cava: A method for placing central venous catheters at the cavoatrial junction

There are a limited number of methods to guide and confirm the placement of a peripherally inserted central catheter (PICC) at the cavoatrial junction. The aim of this study was to design, test and validate a dual-wavelength, diode laser-based, single optical fiber instrument that would accurately confirm PICC tip location at the cavoatrial junction of an animal heart, in vivo. This was accomplished by inserting the optical fiber into a PICC and ratiometrically comparing simultaneous visible and near-infrared reflection intensities of venous and atrial tissues found near the cavoatrial junction. The system was successful in placing the PICC line tip within 5 mm of the cavoatrial junction.

Anesthesia and critical care ventilation - The tools of 2020

Healthcare costs are rising and have reached levels that are impacting the global competitiveness of national economies - without delivering concomitantly rising levels of output, e.g., in terms of quality or outcome. We must find ways of using our increasingly limited healthcare resources much more efficiently. Technology is one of the key levers for achieving this and for transforming healthcare.

Expressions of accuracy in the evaluation of cardiac output monitoring devices

evaluation study in patients undergoing cardiac surgery (155 patients, 30-85 yrs, 46-141 kg). TDco was measured using 10ml bolus of room temperature 5% dextrose. Three sequential bolus TDco measurements, phased randomly with respect to respiration, were recorded as TD1, TD2 and TD3. Regression analysis, correlation coefficients and Bland-Altman statistics were used to compare the degree of agreement between the three bolus TDco measurements. To evaluate the reproducibility of TDco, a difference of ±3 x SEM% (% standard error of mean = standard error of the mean/average CO) between two bolus TDco determinations was considered to be sufficient to be confident that they were different. 3 Bolus thermodilution cardiac output (TDco) measurements are routinely used as the gold standard when evaluating newer methods of cardiac output (CO) monitoring. The range of accuracy that is acceptable varies widely in the literature. We sought to address this issue by looking into the accuracy of TDco, since newer CO monitoring devices need to equal, if not better, the performance of the gold standard.