Alex Vesely

MRI mapping of cerebrovascular reactivity using square wave changes in end-tidal PCO2.

Cerebrovascular reactivity can be quantified by correlating blood oxygen level dependent (BOLD) signal intensity with changes in end‐tidal partial pressure of carbon dioxide (PCO2). Four 3‐min cycles of high and low PCO2 were induced in three subjects, each cycle containing a steady PCO2 level lasting at least 60 sec. The BOLD signal closely followed the end‐tidal PCO2. The mean MRI signal intensity difference between high and low PCO2 (i.e., cerebrovascular reactivity) was 4.0 ± 3.4% for gray matter and 0.0 ± 2.0% for white matter. This is the first demonstration of the application of a controlled reproducible physiologic stimulus, i.e., alternating steady state levels of PCO2, to the quantification of cerebrovascular reactivity. Magn Reson Med 45:1011–1013, 2001.

Dispersal of Respiratory Droplets With Open vs Closed Oxygen Delivery Masks: Implications for the Transmission of Severe Acute Respiratory Syndrome

Nosocomial transmission of droplet-borne respiratory infections such as severe acute respiratory syndrome (SARS) may be influenced by the choice of oxygen face mask. A subject inhaled saline mist and exhaled through three oxygen masks to illustrate the pattern of dispersal of pulmonary gas. In two commonly used masks, exhaled gas formed a plume emanating from the side vents, while a third mask with a valved manifold, which was modified by adding a respiratory filter, retained the droplets. Maintaining respiratory isolation during the administration of oxygen may reduce the risk of the nosocomial transmission of respiratory infections such as SARS.

Changes in respiratory control after 5 days at altitude.

These experiments examined changes in the chemoreflex control of breathing and acid-base balance after 5 days at altitude (3480 m) in six healthy males. The partial pressures of carbon dioxide (P(CO2)) at which ventilation increased during isoxic hypoxic and hyperoxic modified rebreathing tests at sea level fell significantly at altitude by mean+/-S.E.M. of 12.8+/-2.51 mmHg and 9.5+/-1.77 mmHg, respectively, but response slopes above threshold were unchanged. Altitude exposure produced a respiratory alkalosis evidenced by a decrease in mean resting end-tidal P(CO2) from 41+/-0.84 mmHg at sea level to 32+/-2.04 mmHg at altitude, but pH did not increase significantly from its sea level value. Blood samples were analyzed to discover acid-base changes, using a modification of the equations for acid-base balance proposed by [Stewart, P.A., 1983. Modern quantitative acid-base chemistry. Can. J. Physiol. Pharmacol. 61, 1444-1461]. While strong ion difference at altitude was not significantly different from its sea level value, albumin concentration was increased significantly from 38.6+/-0.30 g L(-1) to 49.8+/-0.76 g L(-1). We suggest that the respiratory alkalosis was produced by a fall in the chemoreflex threshold and pH was corrected by an elevation in the concentration of weakly dissociated protein anions.

A simple apparatus for accelerating recovery from inhaled volatile anesthetics

Hyperpnea increases anesthetic elimination but is difficult to implement with current anesthetic circuits without decreasing arterial Pco2. To circumvent this, we modified a standard resuscitation bag to maintain isocapnia during hyperpnea without rebreathing by passively matching inspired Pco2 to minute ventilation. We evaluated the feasibility of using this apparatus to accelerate recovery from anesthesia in a pilot study in four isoflurane-anesthetized dogs. The apparatus was easy to use, and all dogs tolerated being ventilated with it. Under our experimental conditions, isocapnic hyperpnea reduced the time to extubation by 62%, from an average of 17.5 to 6.6 min (P = 0.012), but not time from extubation to standing unaided. This apparatus may provide a practical means of applying isocapnic hyperpnea to shorten recovery time from volatile anesthetics.

Differences in the control of breathing between Andean highlanders and lowlanders after 10 days acclimatization at 3850 m

We used Duffins isoxic hyperoxic ( mmHg) and hypoxic ( mmHg) rebreathing tests to compare the control of breathing in eight (7 male) Andean highlanders and six (4 male) acclimatizing Caucasian lowlanders after 10 days at 3850 m. Compared to lowlanders, highlanders had an increased non‐chemoreflex drive to breathe, characterized by higher basal ventilation at both hyperoxia (10.5 ± 0.7 vs. 4.9 ± 0.5 l min−1, P= 0.002) and hypoxia (13.8 ± 1.4 vs. 5.7 ± 0.9 l min−1, P < 0.001). Highlanders had a single ventilatory sensitivity to CO2 that was lower than that of the lowlanders (P < 0.001), whose response was characterized by two ventilatory sensitivities (VeS1 and VeS2) separated by a patterning threshold. There was no difference in ventilatory recruitment thresholds (VRTs) between populations (P= 0.209). Hypoxia decreased VRT within both populations (highlanders: 36.4 ± 1.3 to 31.7 ± 0.7 mmHg, P < 0.001; lowlanders: 35.3 ± 1.3 to 28.8 ± 0.9 mmHg, P < 0.001), but it had no effect on basal ventilation (P= 0.12) or on ventilatory sensitivities in either population (P= 0.684). Within lowlanders, VeS2 was substantially greater than VeS1 at both isoxic tensions (hyperoxic: 9.9 ± 1.7 vs. 2.8 ± 0.2, P= 0.005; hypoxic: 13.2 ± 1.9 vs. 2.8 ± 0.5, P < 0.001), although hypoxia had no effect on either of the sensitivities (P= 0.192). We conclude that the control of breathing in Andean highlanders is different from that in acclimatizing lowlanders, although there are some similarities. Specifically, acclimatizing lowlanders have relatively lower non‐chemoreflex drives to breathe, increased ventilatory sensitivities to CO2, and an altered pattern of ventilatory response to CO2 with two ventilatory sensitivities separated by a patterning threshold. Similar to highlanders and unlike lowlanders at sea‐level, acclimatizing lowlanders respond to hypobaric hypoxia by decreasing their VRT instead of changing their ventilatory sensitivity to CO2.

Efficiency of oxygen administration: sequential gas delivery versus "flow into a cone" methods.

Objective:Fio2 values of a new oxygen mask that exploits efficiencies afforded by sequential gas delivery (SGD) were compared to those of a nonrebreathing mask (NRM) and a Venturi oxygen mask. Design:Prospective, single-blinded, randomized study. Setting:Laboratory study. Subjects:Eight healthy male volunteers. Interventions:Volunteers breathed through each of the masks at various minute ventilations (&OV0312;e). Oxygen flows were 2, 4, and 8 L/min to the SGD mask but only 8 L/min to the other masks. Measurements and Main Results:Net Fio2 was calculated from end-tidal fractional concentrations of oxygen and CO2 with the alveolar gas equation. Only the SGD mask at an oxygen flow of 8 L/min consistently provided both Fio2 >0.95 (at resting &OV0312;e) and higher Fio2 than the other masks at all &OV0312;e. The SGD mask delivered Fio2 comparable to other masks at only a fraction of the oxygen flow and was characterized by a consistent relation between Fio2 and oxygen flow for a given &OV0312;e. Conclusion:We conclude that SGD can be exploited to provide Fio2 >0.95 with oxygen flows as low as 8 L/min, as well as accurate and efficient dosing of oxygen even in the presence of hyperpnea.

The in-vivo oxyhaemoglobin dissociation curve at sea level and high altitude.

Animals native to hypoxic environments have adapted by increasing their haemoglobin oxygen affinity, but in-vitro studies of the oxyhaemoglobin dissociation curve (ODC) in humans show no changes in affinity under physiological conditions at altitudes up to 4000m. We conducted the first in-vivo measurement of the ODC; inducing progressive isocapnic hypoxia in lowlanders at sea level, acutely acclimatized lowlanders at 3600m, and native Andeans at that altitude. ODC curves were determined by administering isocapnic steps of increasing hypoxia, and measuring blood oxygen partial pressure and saturation. The ODC data were fitted using the Hill equation and extrapolated to predict the oxygen partial pressure at which haemoglobin was 50% saturated (P50). In contrast to findings from in-vitro studies, we found a pH-related reduction in P50 in subjects at altitude, compared to sea-level subjects. We conclude that a pH-mediated increase in haemoglobin oxygen affinity in-vivo may be part of the acclimatization process in humans at altitude.

Rescuer position for tracheal intubation on the ground

BACKGROUND Emergency oral tracheal intubations in the pre-hospital setting can be more difficult because the rescuers position with respect to a patient lying on the ground may not provide optimal conditions for intubation. Since optimal visualisation of the larynx often depends on the force generated during laryngoscopy, we measured the pressure required for intubation (P(i)) as well as the maximum pressure (P(max)) that can be generated with the laryngoscopy blade in seven intubator positions. METHODS Nineteen hospital personnel with intubation experience participated in this study. A modified #3 Macintosh laryngoscope blade was used to measure the pressure exerted on the tongue of a manikin placed on the ground during intubation. The following positions were studied: standard, sitting, prone, kneeling, left and right lateral decubitus and straddling. RESULTS Intubating in the straddling position required the lowest P(i), as a percent of P(max) (68+/-14%). This was significantly less than the prone, right lateral decubitus and sitting positions. (Tukeys W procedure, P<0.05) CONCLUSION The straddling position affords the intubator significantly more reserve force than the prone, right lateral decubitus or sitting position. We suggest that the straddling position may be an advantageous position for pre-hospital intubations especially when visualisation of the glottis is difficult.

A simple breathing circuit to maintain isocapnia during measurements of the hypoxic ventilatory response.

We report the development and testing of a simple breathing circuit that maintains isocapnia in human subjects during hypoxic hyperpnea. In addition, the circuit permits rapid switching between two gas mixtures with different partial pressures of oxygen. Eleven volunteers breathed repeated cycles of exposure to air (2 min of 21% O(2), balance N(2)) and hypoxia (2 min of 8.3+/-0.1% O(2), balance N(2)). Hypoxia induced significant increases in minute ventilation, breathing frequency and tidal volume (P < 0.05) that were consistent over repeated cycles of hypoxia (P > 0.1, one-way ANOVA). The system successfully maintained isocapnia in all subjects, with an average change in end-tidal CO(2) of only -0.2 mmHg during hyperventilation in hypoxia (range 0.4 to -0.8 mmHg). This system may be suitable for repeated tests of the hypoxic ventilatory response (HVR) and may prove useful for exploring intra- and inter-individual variability of HVR in humans.

Calculation Of O2 Consumption During Low-Flow Anesthesia from Tidal Gas Concentrations, Flowmeter, and Minute Ventilation

We present the principles of a new method to calculate O2 consumption (VO2) during low-flow anesthesia with a circle circuit when the source gas flows, end-tidal O2 concentrations and patient inspired minute ventilation are known. This method was tested in a model with simulated O2 uptake and CO2 production. The difference between calculated VO2 and simulated VO2 was 0.01 ± 0.02 L/min. A similar approach can be used to calculate uptake of inhaled anesthetics. At present, with this method, the limiting factor in precision of measurement of VO2 and uptake of anesthetic is the precision of measurement of gas flow and gas concentration (especially O2 concentration in end-tidal gas, FETO2) available in clinical anesthetic units.