Jan Stepanek

Noninvasive Measurement of Central Vascular Pressures With Arterial Tonometry: Clinical Revival of the Pulse Pressure Waveform?

The arterial pulse has historically been an essential source of information in the clinical assessment of health. With current sphygmomanometric and oscillometric devices, only the peak and trough of the peripheral arterial pulse waveform are clinically used. Several limitations exist with peripheral blood pressure. First, central aortic pressure is a better predictor of cardiovascular outcome than peripheral pressure. Second, peripherally obtained blood pressure does not accurately reflect central pressure because of pressure amplification. Lastly, antihypertensive medications have differing effects on central pressures despite similar reductions in brachial blood pressure. Applanation tonometry can overcome the limitations of peripheral pressure by determining the shape of the aortic waveform from the radial artery. Waveform analysis not only indicates central systolic and diastolic pressure but also determines the influence of pulse wave reflection on the central pressure waveform. It can serve as a useful adjunct to brachial blood pressure measurements in initiating and monitoring hypertensive treatment, in observing the hemodynamic effects of atherosclerotic risk factors, and in predicting cardiovascular outcomes and events. Radial artery applanation tonometry is a noninvasive, reproducible, and affordable technology that can be used in conjunction with peripherally obtained blood pressure to guide patient management. Keywords for the PubMed search were applanation tonometry, radial artery, central pressure, cardiovascular risk, blood pressure, and arterial pulse. Articles published from January 1, 1995, to July 1, 2009, were included in the review if they measured central pressure using radial artery applanation tonometry.

Epicardial Fat: An Additional Measurement for Subclinical Atherosclerosis and Cardiovascular Risk Stratification?

BACKGROUND The value of epicardial adipose tissue (EAT) thickness as determined by echocardiography in cardiovascular risk assessment is not well understood. The aim of this study was to determine the associations between EAT thickness and Framingham risk score, carotid intima media thickness, carotid artery plaque, and computed tomographic coronary calcium score in a primary prevention population. METHODS Patients presenting for cardiovascular preventive care (n = 356) who underwent echocardiography as well as carotid artery ultrasound and/or coronary calcium scoring were included. RESULTS EAT thickness was weakly correlated with Framingham risk score. The prevalence of carotid plaque was significantly greater in those with EAT thickness ≥ 5.0 mm who either had low Framingham risk scores or had body mass indexes ≥ 25 kg/m(2), compared with those with EAT thickness <5.0 mm. No significant association between EAT thickness and carotid intima-media thickness or coronary calcium score existed. CONCLUSION EAT thickness ≥ 5.0 mm may identify an individual with a higher likelihood of having detectable carotid atherosclerosis.

Altitude-Related Illnesses

Altitude-related illnesses are a frequent cause of morbidity and occasional mortality in travelers to high altitudes in the United States and throughout the world. The primary altitude illnesses are acute mountain sickness, high-altitude pulmonary edema, and high-altitude cerebral edema. The pathogenesis of these syndromes remains unclear despite considerable research. Altitude also has potential deleterious effects on common medical conditions including coronary artery disease, pulmonary disease, hemoglobinopathies, and pregnancy. Most of these problems are primarily preventable with appropriate information before travel. Education should include information about rate of ascent, diet, alcohol intake, physical activity, and preventive medications, including acetazolamide, nifedipine, and dexamethasone in selected circumstances.

GPS signal reception under snow cover: A pilot study establishing the potential usefulness of GPS in avalanche search and rescue operations

Avalanches are one of the major threats to life in high-mountain terrain and account every year for approximately 150 accidents causing injury or death in the United States alone. Every year avalanches cause significant property damages and a death toll of approximately 15 people in the United States. The specific characteristic of the avalanche accident is the extreme importance of getting to the buried victim as soon as, possible to improve survival. Approximately 40% of all buried victims survive 1 hour, and only about 20% survive 2 hours. Newer studies from Europe indicate that the initial survival probability is 92% at 15 minutes, 30% at 35 minutes, 27% at 90 minutes, and finally drops to 3% at 130 minutes. Unless prompt and efficient search and rescue are ensured, the prospect of buried victims is rather grim. Many tools have been used in the past to aid in retrieving buried victims including the avalanche cord, probing techniques, and in more recent time, the use of electronic beacon devices that allow search teams to locate the buried victim. The advent of satellite navigational aids (GPS, GLONASS) makes it possible to determine ones position with remarkable accuracy. We studied the degree to which the GPS satellite signal could penetrate through snow and be received by a commercially available GPS receiver. This information may lead to the development of an additional tool for precise and quick localization of buried victims in avalanche accidents and thus may substantially improve their survival by shortening the search time at the accident site. In this study we used a Motorola Traxar six-channel GPS receiver with amplifier unit connected to an antenna by means of a shielded coaxial cable. The antenna was buried under incremental covers of compact snow, and the reception of the GPS signal was measured at each burial depth: 5 cm, 15 cm, 25 cm, 35 cm, 45 cm, 55 cm, 1 m, and 1.5 m. The variables that were measured included signal quality, number of satellites received by the receiver, and their respective signal strength. A reference reading was taken from the GPS receiver above the testing site before measurements under snow cover were started. The satellite signals were received with good quality and precise readings up to a burial depth of 1 m. Under 1.5 m of snow the receiver was able to lock on only one satellite, making a positional reading impossible. The reception of the GPS signals under snow cover is possible and warrants further study directed toward the development of a search and rescue device using this technology.

Oculo-vestibular recoupling using galvanic vestibular stimulation to mitigate simulator sickness.

INTRODUCTION Despite improvement in the computational capabilities of visual displays in flight simulators, intersensory visual-vestibular conflict remains the leading cause of simulator sickness (SS). By using galvanic vestibular stimulation (GVS), the vestibular system can be synchronized with a moving visual field in order to lessen the mismatch of sensory inputs thought to result in SS. METHODS A multisite electrode array was used to deliver combinations of GVS in 21 normal subjects. Optimal electrode combinations were identified and used to establish GVS dose-response predictions for the perception of roll, pitch, and yaw. Based on these data, an algorithm was then implemented in flight simulator hardware in order to synchronize visual and GVS-induced vestibular sensations (oculo-vestibular-recoupled or OVR simulation). Subjects were then randomly exposed to flight simulation either with or without OVR simulation. A self-report SS checklist was administered to all subjects after each session. An overall SS score was calculated for each category of symptoms for both groups. RESULTS The analysis of GVS stimulation data yielded six unique combinations of electrode positions inducing motion perceptions in the three rotational axes. This provided the algorithm used for OVR simulation. The overall SS scores for gastrointestinal, central, and peripheral categories were 17%, 22.4%, and 20% for the Control group and 6.3%, 20%, and 8% for the OVR group, respectively. CONCLUSIONS When virtual head signals produced by GVS are synchronized to the speed and direction of a moving visual field, manifestations of induced SS in a cockpit flight simulator are significantly reduced.

Ventilatory Responses to Hypoxia and High Altitude During Sleep in Aconcagua Climbers

Abstract Background/Objective.—We examined the changes in ventilation during sleep at high altitude using the LifeShirt monitoring system on 2 climbers who were attempting to summit Mount Aconcagua (6956 m). Methods.—Prior to the summit attempt, we measured cardiovascular and pulmonary function at 401 m (Rochester, MN) and gathered respiratory and cardiovascular data during sleep using the LifeShirt monitoring system with exposure to normobaric normoxia and normobaric hypoxia (simulated 4300 m). We then monitored the ventilatory response during sleep at 3 altitudes (4100 m, 4900 m, and 5900 m). Results.—During normoxic sleep, subjects had normal oxygen saturation (O2sat), heart rate (HR), respiratory rate (RR), tidal volume (VT) and minute ventilation (VE), and exhibited no periodic breathing (O2sat = 100 ± 2%, HR = 67 ± 1 beats/min, RR = 16 ± 3 breaths/min, VT = 516 ± 49 mL, and VE = 9 ± 1 L/min, mean ± SD). Sleep during simulated 4300 m caused a reduction in O2sat, an increase in HR, RR, VT, and VE, and induced periodic breathing in both climbers (O2sat = 79 ± 4%, HR = 72 ± 14 beats/min, RR = 20 ± 3 breaths/min, VT = 701 ± 180 mL, and VE = 14 ± 3 L/min). All 3 levels of altitude had profound effects on O2sat, HR, and the ventilatory strategy during sleep (O2sat = 79 ± 2, 70 ± 8, 60 ± 2%; HR = 70 ± 12, 76 ± 6, 80 ± 3 beats/min; RR = 17 ± 6, 18 ± 4, 20 ± 6 breaths/min; VT = 763 ± 300, 771 ± 152, 1145 ± 123 mL; and VE = 13 ± 1, 14 ± 0, 22 ± 4 L/min; for 4100 m, 4900 m, and 5900 m, respectively). There were strong negative correlations between O2sat and VE and ventilatory drive (VT/Ti, where Ti is the inspiratory time) throughout the study. Conclusions.—Interestingly, the changes in ventilatory response during simulated altitude and at comparable altitude on Aconcagua during the summit attempt were similar, suggesting reductions in FiO2, rather than in pressure, alter this response.

Clinical Use of Pulse Wave Analysis: Proceedings From a Symposium Sponsored by North American Artery

The use of pulse wave analysis may guide the provider in making choices about blood pressure treatment in prehypertensive or hypertensive patients. However, there is little clinical guidance on how to interpret and use pulse wave analysis data in the management of these patients. A panel of clinical researchers and clinicians who study and clinically use pulse wave analysis was assembled to discuss strategies for using pulse wave analysis in the clinical encounter. This manuscript presents an approach to the clinical application of pulse waveform analysis, how to interpret central pressure waveforms, and how to use existing knowledge about the pharmacodynamic effect of antihypertensive drug classes in combination with brachial and central pressure profiles in clinical practice. The discussion was supplemented by case‐based examples provided by panel members, which the authors hope will provoke discussion on how to understand and incorporate pulse wave analysis into clinical practice.

Early detection of hypoxia-induced cognitive impairment using the king-devick test

INTRODUCTION Hypoxic incapacitation continues to be a significant threat to safety and operations at high altitude. Noninvasive neurocognitive performance testing is desirable to identify presymptomatic cognitive impairment, affording operators at altitude a tool to quantify their performance and safety. METHODS There were 25 subjects enrolled in this study. Cognitive performance was assessed by using the King-Devick (K-D) test. The performance of the subjects on the K-D test was measured in normoxia followed by hypoxia (8% 02 equivalent to 7101 m) and then again in normoxia. RESULTS K-D test completion time in hypoxia for 3 min was significantly longer than the Baseline Test (54.5 +/- 12.4 s hypoxic vs. 46.3 +/- 10.4 s baseline). Upon returning to normoxia the completion time was significantly shorter than in hypoxia (47.6 +/- 10.6 s post test vs. 54.5 +/- 12.4 s hypoxic). There was no statistically significant difference between baseline test and post test times, indicating that all subjects returned to their normoxic baseline levels. SpO2 decreased from 98 +/- 0.9% to 80 +/- 7.8% after 3 min on hypoxic gas. During the hypoxic K-D test, SpO2 decreased further to 75.8 +/- 8.3%. CONCLUSIONS In this study the K-D test has been shown to be an effective neurocognitive test to detect hypoxic impairment at early presymptomatic stages. The K-D test may also be used to afford a reassessment of traditional measures used to determine hypoxic reserve time.

The piglet as an animal model for hypobaric hypoxia

The objective of this study was to evaluate the piglet as a suitable animal model for human diseases of high altitude. We studied 12 piglets, 4-10 weeks old, in a hypobaric chamber under conditions of high altitude at a pressure of 1/2 atmosphere (to approximately 320 Torr) for various periods of time (12, 24, 36, 48, and 72 hours) with continuous monitoring. The animals were decompressed every 24 hours for grooming and feeding. Two animals were studied as nonexposed controls, and one was studied as a control in the chamber without decompression. The animals were euthanized after the exposure, and a complete autopsy was performed. The tissues were then analyzed with light and electron microscopy. The animals all exhibited clinical features of ataxia, tachypnea with Cheyne-Stokes respiration, and lethargy. One animal vomited. The histologic and ultrastructural analysis showed normal organs, particularly lung and brain. The piglet may be a suitable animal model for the study of high altitude-related diseases in humans, but prolonged uninterrupted exposure and a delay in euthanasia after exposure to high altitude may be necessary for the development of reactive pathologic changes.

The effect of galvanic vestibular stimulation on distortion product otoacoustic emissions

Galvanic stimulation has long been used as a nonmechanical means of activating the vestibular apparatus through direct action on the vestibular nerve endings. This stimulation has been reported to be safe, but no studies have examined the potential changes in the corresponding cochlear receptors. The aim of the present study was to evaluate the effect of galvanic vestibular stimulation (GVS) on distortion product otoacoustic emissions (DPOAEs). Fourteen subjects underwent DPOAEs during several conditions of GVS. The DPOAEs ranged from ∼ 1 kHz to ∼ 8 kHz at 65/55 dB for f1/f2 and with an f2/f1 ratio of 1.2. The subjects were evaluated at 10 stimulation conditions that ranged from -2.0 mA to +2.0 mA for each frequency. Statistical analysis showed no significant differences in DPOAE amplitudes for all conditions with and without GVS. Results also showed no significant differences between DPOAE amplitudes before and after GVS. Multivariate analysis found subject variability in DPOAE amplitude, which was not thought to be GVS related. Results indicated that GVS produced neither temporary nor permanent changes in DPOAEs.