A Hand-Held Device to Measure Oxygen Uptake

A Hand-Held Device to Measure Oxygen Uptake: Performance Characteristics, Patient Selection and the Propagation of its Measureme | Journal of Invasive Cardiology invasivecardiology.com /article/6849

A Hand-Held Device t o Measure Oxygen Upt ake: Perf ormance Charact erist ics, Pat ient Select ion and t he Propagat ion of it s Measureme Volume 19 - Issue 3 - March, 2007 Posted on: 8/1/08 0 Comments 7005 reads Author(s): Albert P. Shepherd, PhD, Beth M. Terpolilli, BA, *John M. Steinke, PhD The classic oxygen- based Fick Principle has long been considered the gold standard for measuring cardiac output, but its rigorous implementation is being used less and less frequently during cardiac catheteriz ation and in intensive care because, in our opinion, the instrumentation necessary to measure oxygen consumption from a patient’s expired gases is cumbersome, expensive, time- consuming and difficult for unskilled operators to use effectively. Thus, instead of actually measuring the patient’s rate of oxygen consumption, many cardiac catheteriz ation laboratories simply estimate it from regression equations based on the patient’s height, weight, age, sex, heart rate or other anthropomorphic data.1– 4 Unfortunately, studies 5–11 have shown consistently and convincingly that using an assumed or estimated rate of

oxygen consumption, instead of a measured value, “frequently results in large errors in the determination of cardiac output”.5 Even worse, inaccurate cardiac output determinations subsequently affect clinical decisions regarding ventricular function, valve areas and systemic and pulmonary vascular resistances. An inexpensive, fist- siz e device called MedGem® (Microlife USA, Inc., Dunedin, Florida) recently became available for measuring oxygen consumption. A series of studies has compared the new device’s measurements of oxygen uptake with those made by conventional metabolic carts, and the results appear promising.12–20 However, little is known about the characteristics of the device that would affect its utility during cardiac catheteriz ation and in intensive care. Therefore, the purposes of this report are: (1) to delineate the advantages and limitations that would affect its use in these settings; (2) to define the patient population in which its use would be safe and effective; and (3) to estimate the accuracy of cardiac output determinations based on its measurements of oxygen consumption. All of the measured variables in the Fick equation, the arterial and mixed venous oxygen concentrations and the oxygen consumption rate, contain random measurement errors that are propagated into the calculated cardiac output.21,22 Therefore, the accuracy of cardiac output determinations based on measurements of oxygen consumption by the MedGem or conventional metabolic carts cannot be assessed from the inaccuracy of those instruments alone, but must take into account the measurement errors of the oximeters and co- oximeters used to analyz e blood oxygen. Thus, an additional purpose of this report is to present a statistical model that calculates cardiac output error as a function of the random errors in the measurements of arterial and venous oxygen concentrations and the oxygen consumption rate. Using the methods presented in the Appendix, interested readers can compute the cumulative measurement error in cardiac output determinations made with the particular set of instruments in their institutions. Our results support the conclusions that the MedGem is suitable for a wide range of adults and children at rest (but not during exercise), and that cardiac output determinations made with the MedGem’s measurements of oxygen uptake are clinically acceptable if the oximeter used to measure oxygen in arterial and mixed venous blood is also sufficiently accurate.

Materials and Methods Descript ion of t he t est inst rument . The MedGem can be connected to the patient with a disposable mouthpiece that is used with a nose clip. A hard- shell facemask is also available that is used with a soft, disposable insert. Both the

mouthpieces and facemask inserts are equipped with a microbial filter. A 12 VDC wall transformer powers the MedGem. The small power jack also serves as a serial data port (RS- 232). The device weighs only 110 grams, and has dimensions of 5.5 x 5.5 x 11.5 cm. The MedGem starts recording data as soon as the patient’s first breath is detected. At the completion of the measurement, the device beeps, the indicator light changes to amber and the VO 2 reading is displayed on the LCD. Dat a collect ion. In order to obtain information regarding the new device’s performance characteristics, we reviewed the previous publications that compared it with reference methods, reviewed the manufacturer’s in- house studies and interviewed the manufacturer’s engineering staff to obtain information that was not in the published specifications. To assess the precision (repeatability) of the new device, we made paired measurements on healthy volunteers. To explore the effects of a range of tidal volumes and breathing frequencies, we performed experiments on anesthetiz ed dogs, pigs and goats. We also timed each measurement to determine the average measurement time. These experiments were conducted under protocols approved, respectively, by our university’s Institutional Review Board and the Institutional Animal Care and Use Committee. To measure the dead space that the device adds to a patient’s airways, we used a self- sealing film (Parafilm, Sargent Welch, Buffalo, New York) to block the openings of the air tubes, facemask and mouthpiece, and weighed them before and after filling them with water. In the experiments on healthy volunteers, we attempted to make all measurements in a basal metabolic state so that the repeated measurements would be as consistent as possible. Each volunteer had had no food or caffeine- containing beverages for 12 hours, had not exercised in 24 hours, had not smoked for 12 hours and had rested for 10 minutes before the first measurement. After the first measurement, there was a 10- minute rest period followed by the second measurement. All measurements were made in the supine position with the subject using one hand to hold the MedGem in place. On one volunteer, one measurement per day was made on 10 different days. To check the maximal oxygen uptake rate the device can measure, the same subject walked on a treadmill at gradually increasing speeds until an error code was displayed. To assess the repeatability of cardiac output determinations made with the MedGem, we performed an experiment on a mongrel dog that weighed 31.8 kg. The animal was anesthetiz ed with sodium pentobarbital (30 mg/kg), and an endotracheal tube was inserted. Cutdowns were performed, and the right jugular vein and the femoral arteries and veins were isolated. After the surgical procedures were complete, sodium heparin (200 U/kg) was administered intravenously. Arterial pressure was measured with a transducer connected to a catheter inserted into one of the femoral arteries and advanced into the abdominal aorta; the same catheter was used intermittently to take samples of arterial blood. A catheter in a femoral vein was used to administer additional anesthetic and return sampled blood to the animal. For access to mixed venous blood, a catheter was inserted into the right external jugular vein and was advanced until the tip of the catheter was in the pulmonary artery. The catheter was then connected to a pressure transducer so that the position of the tip of the catheter could be verified by the morphology of the pressure waveform. A Grass multichannel chart recorder was used to record the aortic and pulmonary arterial pressures. In order to connect the dog to the MedGem, we cemented a standard 15 mm endotracheal tube connector into a MedGem mouthpiece. The MedGem was interfaced to a personal computer with a serial cable and software available from Microlife. During each cardiac output determination, we started the MedGem and then began to withdraw 10- mL samples of blood from the aorta and the pulmonary artery. We withdrew these samples slowly so that each syringe contained blood from multiple breathing cycles. Then two 50- µL cuvettes were filled from each syringe and analyz ed on an AVOXimeter 1000E (Avox Systems, Inc., San Antonio, Texas). The rest of the blood in the syringe was returned to the animal. After the MedGem completed its measurement, a 5- minute rest period was observed before the next cardiac output determination was made. We also attempted to make noninvasive measurements of oxygen uptake only in pigs and goats. Bookmark/Search this post with