George T. Ozaki

Characterization of variables defining hindpaw withdrawal latency evoked by radiant thermal stimuli

We have examined the stability and sources of variation within the nociceptive model of rat hind paw withdrawal from an under-glass radiant stimulus (Hargreaves et al., 1988) using a system where stimulus intensity and floor temperature can be controlled and reproducibly changed. The current study demonstrates that: (i) increased stimulus intensity with a fixed surface temperature is associated with a monotonic decrease in mean response latency and its variance; (ii) for a fixed stimulus intensity, the mean paw withdrawal latency and variance increased as the glass floor temperature is lowered from 30 degrees C to room temperature (25 degrees C). Using subcutaneously-implanted thermocouples and a 30 degrees C glass surface, the subcutaneous paw temperature observed at an interval corresponding to the time at which the animal displayed a paw withdrawal did not differ across multiple heating rates (41-42.5 degrees C). This finding is in agreement with human studies of pain thresholds and C-fiber activity. These studies emphasize the importance of maintaining a fixed surface temperature to reduce experimental variability and the utility of this apparatus across multiple stimulus intensities to define agonist efficacy.

Spinal and Systemic Action of the α2 Receptor Agonist Dexmedetomidine in DogsAntinociception and Carbon Dioxide Response

Background:4aL2 Agonists are powerful analgesics after spinal delivery. The current work characterizes the dose-dependent antinociception and effects upon respiratory function of dexmedetomidine after intrathecal, epidural, intravenous, and intracisternal delivery in chronically prepared dogs. Methods:Dogs were prepared with chronic tracheostomies and trained to perform rebreathing studies. These animals were then prepared with chronic lumbar intrathecal, epidural, or intracisternal catheters. Results:A rapid dose-dependent increase in the thermal skin twitch response latency and paw withdrawal to mechanical pinch was observed after intrathecal, epidural, and intravenous dexmedetomidine (dose required to reach 50% of maximal effect for skin twitch = 1.8, 10, and 15 μg, respectively) but not after intracisternal dexmedetomidine (>15 μg), with the maximally effective dose lasting approximately 90 min. The spinal effect was unaccompanied by effects upon behavioral alertness, motor function, or changes in CO2 response. In contrast, intravenous dexmedetomidine (1–10 μg/kg) resulted in a dose-dependent sedation and a significant reduction in heart rate and respiratory rate and a diminished response to increased CO2, these effects lasting approximately 2 h. Intracisternal administration of up to 15 μg had no effect upon the nociceptive threshold, and CO2 response, and failed to result in a significant reduction in alertness. All of the effects of dexmedetomidine were antagonized by the α2-antagonist atipamezole (30–300 μg/kg, intravenous), but not by the opioid antagonist naloxone (30 μg/kg, intravenous), while atipamezole did not reverse the antinociceptive or respiratory depressant actions of intravenous sufentanil (50 μg), effects which were reversible by naloxone. Conclusions:Dexmedetomidine, acting through an α2-receptor, produces a powerful antinociceptive effect, mediated at the spinal level, while systemic redistribution of the drug leads to a hypnotic state with significant cardiorespiratory effects.

Depth of Placement of Left Double-lumen Endobronchial Tubes

Data on the normal depth of insertion of double-lumen tubes have not been published. We studied 101 adult patients undergoing thoracic operations whose tracheas were intubated with a left double-lumen tube. A fiberoptic bronchoscope was introduced into the tracheal lumen, and tube position was adjusted until the cephalad surface of the bronchial cuff was immediately below the carinal bifurcation. The average depth of insertion for both male and female patients 170 cm tall was 29 cm, and for each 10-cm increase or decrease in height, average placement depth was increased or decreased 1 cm. The correlation between depth of insertion and height was highly significant (P < 0.0001) for both male and female patients. As depth of DLT insertion at any given height was normally distributed, a technique to confirm correct double-lumen tube position always should be used after initial placement.

Operative lung constant positive airway pressure with the univent bronchial blocker tube

Constant positive airway pressure (CPAP) to the operative lung during one-lung ventilation (1-LV) with a double-lumen tube increases Pao2; there have been no reports of application of CPAP to the operative lung during 1-LV with the Univent bronchial blocker (BB) tube. This study determined the method of administration and the effect on Pao2 of 10 cm H2O of CPAP to the operative lung during 1-LV (l-LV+10 CPAP) produced by the Univent BB system. We designed our CPAP system for the Univent BB using an in vitro lung model so that low O2 flow rates (2–4 L/min) yielded clinically relevant levels of CPAP (5–20 cm H2O) over a wide range of lung compliance. The CPAP system simply consisted of placing a resistance to a variable oxygen flow distal to the operative lung. Seven consenting patients who required thoracotomy and 1-LV were anesthetized and their tracheas were intubated with the Univent BB tube; the BB was inserted into the appropriate mainstem bronchus until the proximal surface of the BB cuff was just distal to the tracheal carina. Pao2 was measured in the seven patients during 12 sequences of two-lung ventilation (2-LV), one-lung ventilation (1-LV), and 1-LV with 10 cm H2O CPAP (1-LV+10 CPAP). 1-LV+10 CPAP was always instituted on the deflation phase of a previous single tidal inhalation. We found in our patients with a lung compliance of 32 ± 4 mL/cm H2O that 2.4 ± 0.2 L/min of oxygen flow produced 1-LV + 10 CPAP. The mean ± SD Pao2 during 2-LV, 1-LV, and 1-LV+10 CPAP was 347 ± 159, 127 ± 93, and 312 ± 122 mm Hg, respectively. The Pao2 during 1-LV was significantly decreased compared with both 2-LV and 1-LV+10 CPAP (both P < 0.001), and there was no significant difference in Pao2 during 2-LV compared with 1-LV+10 CPAP (P > 0.2). One-lung ventilation + 10 CPAP did not have any statistically significant hemodynamic effect or interfere with surgical exposure. We conclude that 1-LV+10 CPAP through the Univent BB tube is technically simple to administer and is potentially an efficacious treatment of hypoxemia during 1-LV.

Can an anesthesia machine flush valve provide for effective jet ventilation

Transtracheal jet ventilation (TTJV) using a percutane-ously inserted intravenous (IV) catheter for the patient who cannot be ventilated or tracheally intubated or, using a jet stylet for changing endotracheal tubes (Em) in patients for whom subsequent ventilation and/or tracheal reintubation may be difficult, are extremely valuable therapeutic options. The jet ventilation system must have a sufficiently high pressure-oxygen source to drive oxygen through noncompliant tubing and through relatively small IV catheters and/or jet stylets in order to achieve adequate ventilation and oxygenation. There is no evidence that using the common gas outlet of an anesthesia machine by. activating the flush valve can provide enough flow (&OV0312;) and tidal volume (VT) for effective jet ventilation. This in vitro study utilized a mechanical lung model that had a varying lung compliance [Cset (10–100 mL/cm H20)] to determine the VT (measured by integrating a pneumotachograph flow signal) and corresponding minute ventilation (VE) through 14-, 16-, and 18-gauge IV catheters and small, medium, and large jet stylets. The flow of O2 was generated by activating the flush valve of Dráger Narkomed 2 and 2A and Ohmeda Modulus II and II Plus anesthesia machines at an inspiratory: expiratory (1:E) ratio = 1:1 (unit of time = 1 s). We found that the largest VT and resultant &OV0312;E were consistently obtained by activating the flush valve of the Ohmeda Modulus II and Dráger Narkomed 2 anesthesia machines. The smallest VT and &OV0312;E were produced using the Ohmeda Modulus II Plus anesthesia machine. The large differences in V, VT, and &OV0312;E generated by each of the anesthesia machines are due to, and are explained by, the internal construction of the flush valve unique to each of these machines. A one-way outlet check valve is positioned between the vaporizers and flush valve in the Ohmeda Modulus II and Dráger Narkomed 2 anesthesia machines which directs the entire flow of oxygen (45–75 L/min at approximately 50 psi) out the common gas outlet when the flush valve is activated. There is no such valve in the Ohmeda Modulus II Plus or Dráger Narkomed 2A anesthesia machines, so that activating the flush valves directs oxygen toward both the vaporizers and the common gas outlet. Each of these anesthesia machines has a pressure-limiting mechanism so that the pressure out of the common gas outlet of both anesthesia machines is only 7 and 18 psi, respectively, resulting in dramatically lower VT and subsequent &OV0312;E. In conclusion, the Ohmeda Modulus II Plus anesthesia machine is not an acceptable power source for providing total ventilatory support by activating the flush valve for jet ventilation, although the Dráger Narkomed 2, 2A, and Ohmeda Modulus II anesthesia machines are all acceptable power sources for jet ventilation for providing partial, if not total, ventilatory support in most clinical situations. (Anesth Analg 1993;76:800–808)

A comparison in a lung model of low- and high-flow regulators for transtracheal jet ventilation

There is widespread agreement that transtracheal jet ventilation (TTJV) using a percutaneously inserted intravenous (iv) catheter through the cricothyroid membrane is a simple, quick, relatively safe, and extremely effective treatment for the situation in which neither ventilation nor intubation can be achieved. No study has reported whether a low-flow pressure-reducing regulator (LFR) can provide enough driving pressure and flow under a variety of clinical circumstances for adequate TTJV. We determined, using a high-flow regulator (HFR) as our control, the tidal volume (VT) (measured by integrating a pneumotachograph signal) that a LFR could deliver via a Carden jet injector through 14- and 20-G iv catheters initially at an inspiratory:expiratory ratio (I:E) = 1:1 (unit of time = 1 s) in a mechanical model that had varying lung compliance (Cset, 10-100 ml/cmH2O) and airway diameters (proximal trachea 15.0, 4.5, or 3.0 mm ID and distal mainstem bronchi 9.0 or 4.5 mm ID). The lowest Cset (10 ml/cmH2O) and smallest airway diameter (tracheal diameter = 3.0 mm, bronchial diameter = 9.0 mm) resulted in the lowest VT (220 and 320 ml for the 20- and 14-G iv catheters, respectively, with the LFR), and the highest Cset (100 ml/cmH2O) and largest airway diameter (tracheal diameter = 15 mm, bronchial diameter = 9.0 mm) resulted in the highest VT (780 and 1040 ml for the 20- and 14-G iv catheters, respectively, with the LFR). The VT produced during TTJV was greatly dependent on air entrainment (measured by a second pneumotachograph), with the contribution to total VT ranging from 15 to 74%; the amount of air entrainment was independently confirmed by excellent agreement between measured and calculated alveolar oxygen concentrations. Decreasing Cset (with the largest airway diameter) and decreasing airway diameter (at Cset = 50 ml/cmH2O) over the full range studied resulted in approximately a 45-80% decrease in VT for all iv catheter/regulator combinations. Increasing Cset and narrowing airway diameter over the full range studied resulted in a progressive increase in end-expiratory volume (EEV) for all iv catheter/regulator combinations. The I:E ratio was also varied from 1:3 to 3:1 (unit of time = 1 s) using the 14-G catheter at Cset = 50 ml/cmH2O with both regulators at the extremes of the proximal tracheal diameters (15.0 and 3.0 mm ID), and we found that jet ventilation through a proximal tracheal diameter of 3.0 mm with the HFR at I:E ratios = 1:1 and 3:1, EEV exceeded the capacity of the mechanical lung (4,000 ml).(ABSTRACT TRUNCATED AT 400 WORDS)

Quantification of the jet function of a jet stylet.

The concept and use of a jet stylet as an additional safety measure during tracheal extubation of patients in whom subsequent ventilation and/or reintubation of the trachea may be difficult has recently been described. If jet ventilation through a jet stylet could provide for effective gas exchange, it would allow additional time to assess the need for reintubation of the trachea. We determined the tidal volumes (measured by integrating a pneumotachograph flow signal) that 50-psi jet ventilation, at an inspiratory to expiratory time ratio of 1:1 (unit of time = 1 s), could deliver through small, medium, and large Sheridan tube exchangers into an in vitro lung model that had lung compliances of 50 and 30 mL/cm H2O (six experimental permutations). The tidal volume (VT) produced during jet ventilation was moderately dependent on air entrainment (measured by a volume spirometer), with the contribution to total VT ranging from 0% to 31%; the amount of air entrainment was confirmed by excellent correlation between the alveolar oxygen concentration (FAO2) measured by an oxygen analyzer and the FAO2 calculated from entrained and total VT. Decreased lung compliance caused decreased VT and end-expiratory volume for all six experimental conditions. The largest VT and minute ventilation (VE) generated were 1680 mL and 51.6 L/min (large tube exchanger, high lung compliance) and the lowest VT and VE were 440 mL and 13.2 L/min (small tube exchanger, low lung compliance), respectively. These findings validate the term “jet stylet” for all three tube exchangers as even the smallest tube exchanger, coupled with a low lung compliance, can provide a VE consistent with total ventilatorv support for most clinical situations.

The relationship among bronchial blocker cuff inflation volume, proximal airway pressure, and seal of the bronchial blocker cuff

The resting volume and diameter of the bronchial blocker cuff (defined as inflation of the cuff to just its natural shape) of the Univent (Fuji Systems Corp, Tokyo, Japan) tube are 2 mL and 5 mm. However, much larger inflation volumes may be required to seal an adult mainstem bronchus and the surface area of contact between the resultant spherical or ellipsoid-shaped cuff and the wall of the mainstem bronchus may be small and susceptible to leak with the application of high proximal airway pressures. This experiment determined the relationship among airway diameter, proximal airway pressure, inflation volume of the bronchial blocker cuff, and leakage of air around the bronchial blocker cuff in an in vitro model. The experimental model consisted of silicon tubing of 12.8-, 16.0-, and 19.2-mm ID as the mainstem bronchus. The main tracheal cuff sealed the Univent tube into the proximal end of the mainstem bronchus and the bronchial blocker cuff was inflated with various volumes near the distal end of the mainstem bronchus. The space between the tracheal cuff and the bronchial blocker cuff was then progressively pressurized in either a static or pulsed manner. The very distal end of the bronchus was functionally submerged under a beaker of water so that a bronchial blocker cuff leak would be indicated by bubbling. It was found that the Univent bronchial blocker cuff sealed the 12.8- and 16.0-mm ID mainstem bronchi against airway pressures as great as 100 cmH2O, with inflation volumes that were within the manufacturers recommendation of 6 to 7 mL.(ABSTRACT TRUNCATED AT 250 WORDS)

Confirmation of endotracheal intubation over a jet stylet : in vitro studies

An accepted method of tracheal reintubation is to pass an endotracheal tube (ETT) over a jet stylet (JS).It is desirable to confirm tracheal reintubation prior to removing the JS from its known intratracheal location. The purpose of this study was to determine the functional size equivalent of the annular space between the JS and ETT for all combinations of variously sized ETTs and JSs and to determine whether this annular space will permit detection of exhaled carbon dioxide (CO2). Our experiment consisted of two parts. One model measured the airflow resistance of variously sized test catheters (14- to 18-gauge intravenous catheters and ETT sizes 2.5-9.0 mm inside diameter (ID)) and all of the possible combinations of small, medium, and large Sheridan JSs within 4.9-9.0-mm ID ETTs (ETT/JS) by determining pressure versus annular space flow curves. The other model measured the times to first detection of CO2, to 70% maximum (max) [CO2] detection, and from first detection to 70% max [CO2] through empty 4.0- to 9.0-mm ID ETTs and through the annular space between all possible ETT/JS combinations at lung driving pressures of 5-10 mm Hg. The resistance of the catheters and ETT/JS combinations increased as the flow rate increased and/or the net conducting area of the conduit decreased. Some ETT/JS had an annular space < a 4.0 mm ID ETT. All three CO2 detection times increased with decreasing size of the net conducting area and with decreasing driving pressure. There was a logarithmic relationship between the net conducting area and time to first detection of CO2, to 70% max [CO2], and from initial detection to 70% max [CO2], regardless of which ETT or JS was used. Most ETT/JS detected CO2 satisfactorily. We conclude that CO2 detection should be the standard procedure for assessing ETT placement since it requires a conducting area that is less than 4 mm ID. (Anesth Analg 1995;80:800-5)

Auscultation cannot distinguish esophageal from tracheal passage of tube

We quantitatively compared the acoustic characteristics of passage of an endotracheal tube into the trachea with those of passage into the esophagus by analyzing the loudness and frequency (90% spectral edge frequency) of the sounds when auscultated at the suprasternal notch. We found that there was a significant difference (P<0.01) in maximum loudness between esophageal and tracheal intubations (0.15±0.05 and 0.25±0.06 V, respectively). However, there were no significant differences between the 90% spectral edge frequencies. We conclude that, without directly comparing the maximal acoustic amplitude of tracheal intubation with that of esophageal in each patient, one cannot distinguish between the two types of intubation by means of auscultation.