Positive-pressure ventilation during transport: a randomized crossover study of self-inflating and flow-inflating resuscitators in a simulation model.

BACKGROUND: Positive-pressure ventilation during transport of intubated patients is generally delivered via a hand-pressurized device. Of these devices, self-inflating resuscitators (SIR) and flow-inflating resuscitators (FIR) constitute the two major types used. Selection of a particular device for transport, however, remains largely an institutional practice. OBJECTIVE: To evaluate the hypothesis that transport ventilation goals of intubated pediatric patients are better achieved using an FIR compared to an SIR. METHODS: This randomized crossover simulation study compared the performance of SIR and FIR among anesthesia providers in a pediatric transport scenario. Subjects hand-ventilated a test lung while simultaneously maneuvering a stretcher bed to simulate patient transport. Hand ventilation was carried out using a Jackson-Rees circuit (FIR) and a Laerdal pediatric silicone resuscitator (SIR). The primary outcome was the proportion of total breaths delivered within the predefined target PIP/PEEP range (30+/- 3, 10+/- 3 cm H2O). Secondary outcomes included proportion of total breaths delivered with operationally defined unacceptable breath variables (PIP > 35 cm H2O or PEEP < 5 cm H2O). RESULTS: Overall, participants were four times more likely to deliver target breaths and one-third less likely to deliver unacceptable breaths using the FIR compared to the SIR. When comparing device performance, a 44% increase in the proportions of target breaths and a 40.4% decrease in unacceptable breaths using the FIR were observed (P < 0.0001 for both). CONCLUSIONS: Hand ventilation during patient transport is superior using the FIR compared to the SIR to achieve target ventilatory goals and avoid unacceptable ventilatory cycles.

Quantifying the mechanical work of resting quadriceps muscle tone.

Our laboratory has shown that resting muscle, commonly thought to be mechanically inert, is actually mechanically active. We report a study of the mechanics of resting quadriceps muscle in adult surgical patients that determines how much metabolic activity can be attributed to quadriceps resting-muscle mechanical work. This was calculated by studying the motion of relaxed supine subjects' instrumented legs dropped onto a pillow before and after anesthesia with muscle paralysis. By subtracting the acceleration of the dropping leg of the conscious subject before the quadriceps is paralysed from that found after paralysis, resting muscle tensile force and power of the quadriceps muscles can be calculated. Mechanomyography was also recorded, using an accelerometer. Paralysis produced an increase in acceleration in all cases (pre-paralysis 6.99 +/- 1.51 m s(-2); post-paralysis 7.65 +/- 1.51 m s(-2); P = 0.00007) and a decrease in mechanomyographic mean absolute amplitude (pre-paralysis 10.6 +/- 3.7 mm s(-2); post-paralysis 4.5 +/- 2.6 mm s(-2); P = 0.00003). Calculated force exerted by resting quadriceps was 22.6 +/- 16.8 N; power 0.34 +/- 0.17 W, corresponding to a daily caloric expenditure of 7.0 +/- 3.6 kcal day(-1). This corresponds to approximately 205 kcal day(-1) for all skeletal muscle. Knowledge of the phenomenon of resting muscle mechanical activity may be of clinically importance in the study and treatment of obesity and of disorders of muscle tone.

Effects of spinal anesthesia on resting metabolic rate and quadriceps mechanomyography.

Previous work by our group has shown by mechanomyography (MMG) that resting muscle is mechanically active. Ten patients having spinal anesthesia for surgery, which paralyses muscle below the waist, were studied to help determine whether resting-muscle mechanical activity plays a significant role in resting metabolism, and to further determine if the phenomenon is neurally mediated. Resting metabolic rate (RMR) by indirect calorimetry, and mid-anterior thigh MMG by accelerometer, were recorded before and during spinal anesthesia. Spinal anesthesia produced a 25% decrease in oxygen uptake (mean +/- standard deviation: pre-spinal 228 +/- 76; during spinal 171 +/- 67 ml min(-1); P < 0.001) and 37% decrease in mean absolute MMG signal amplitude (pre-spinal-anesthetic 10.6 +/- 3.9; during spinal: 6.7 +/- 3.5 mm s(-2); P < 0.001). Decreased oxygen uptake in individuals correlated with decreased resting-muscle mechanical activity (R = 0.624; P = 0.05). Paralysis of muscle below the waist reduced RMR and resting-muscle mechanical activity.

Resting mechanomyography before and after resistance exercise.

A number of mechanisms have been proposed to explain the elevation in oxygen consumption following exercise. Biochemical processes that return muscle to its pre-exercise state do not account for all of the extra oxygen consumed after exercise (excess post-exercise oxygen consumption, EPOC). Muscle at rest after aerobic exercise produces mechanomyographic (MMG) activity of increased amplitude, compared to the pre-exercise state, which declines exponentially with the same time constant as EPOC. The purpose of this study was to determine how the resting MMG is affected by resistance exercise, and whether any change is related to oxygen consumption (VO(2)). Ten young male subjects (22.9 years) performed 30 min of resistance exercise consisting of one set of 10 repetitions at 50% 1-repetition maximum (1-RM) followed by five sets of eight repetitions at 75% of 1-RM for leg press and leg (knee) extension, with 1 min rest between sets. Oxygen consumption was measured by indirect calorimetry, MMG by an accelerometer placed over the rectus femoris, and surface electromyogram (EMG) with electrodes placed distal to the accelerometer. Recordings were made before exercise and for 5.5 h after exercise. MMG activity, expressed as mean absolute acceleration, was significantly elevated after exercise (P = 0.0006), as was EMG activity expressed as root-mean-square voltage (P = 0.03). MMG and VO(2) demonstrated exponential decay after exercise with similar time constants of 7.5 +/- 2.2 and 7.2 +/- 1.0 min, respectively. We conclude that resting muscle is more mechanically active following resistance exercise and that this may contribute to an elevated VO(2).

A PVDF transducer for low-frequency acceleration measurements.

A unique acceleration transducer, using piezoelectric PVDF, has been developed for low-frequency vibration monitoring. The paper develops the theoretical model for this low-cost, robust sensor. The theoretical model is validated using experimental results from laboratory tests. The sensor was also installed in an underground potash mine alongside a commercial geophone for a three-month in-mine test producing results that show a close correspondence between the two transducers.

Effects of ambient temperature on mechanomyography of resting quadriceps muscle.

It has been speculated that resting muscle mechanical activity, also known as minor tremor, microvibration, and thermoregulatory tonus, has evolved to maintain core temperature in homeotherms, and may play a role in nonshivering thermogenesis. This experiment was done to determine whether resting muscle mechanical activity increases with decreasing ambient temperature. We cooled 20 healthy, human, resting, supine subjects from an ambient temperature of 40° to 12 °C over 65 min. Core temperature, midquadriceps mechanomyography, surface electromyography, and oxygen consumption (VO2) were recorded. Resting muscle mechanical and electrical activity in the absence of shivering increased significantly at temperatures below 21.5 °C. Women defended core temperature more effectively than men, and showed increased resting muscle activity earlier than men. Metabolism measured by VO2 correlated with resting muscle mechanical activity (R = 0.65; p = 0.01). Resting muscle mechanical activity may have evolved, in part, to maintain core temperature in the face of mild cooling.

Effects of graded levels of exercise on ipsilateral and contralateral post-exercise resting rectus femoris mechanomyography.

Mechanomyography has shown that "resting" muscle is mechanically active, with greater activity after vigorous exercise. This experiment studied the post-exercise resting mechanomyography activity that results from different levels of exercise; the effects of exercise levels on the contralateral non-exercised limb; and the effects of resting muscle length on post-exercise resting mechanomyographic activity. Ten healthy volunteers had mechanomyography recordings over both mid-rectus femoris, at rest, before and after sets (1, 5, 10, 20, and 30 repetitions) of right leg extensions on an isokinetic dynamometer at 60 s(-1). Sets were performed a week apart, after only sedentary activity during the previous two hours. No definite threshold effect was shown. There was a linear correlation between mechanomyography and work done (R = 0.61, P < 0.01). There was a positive correlation of change of activity between the two thighs (R = 0.62, P < 0.01), with the non-exercised thigh demonstrating about half the activity of the exercised thigh. Finally, we observed that mechanomyographic activity was greater when rectus femoris muscle length was shorter (i.e. when the leg was extended versus flexed). We conclude that resting mechanomyography increases with increasing work and that there is a cross-over for increase in mechanomyography in the non-exercised leg, suggesting a neural mechanism. The greater mechanomyographic activity at shorter muscle lengths suggests that muscle that is less stretched could more freely oscillate, producing higher MMG amplitudes. Altered activity of the muscle spindle gamma loop or Golgi tendon apparatus may also play a role in altered activity with different muscle length.

Resting mechanomyography after aerobic exercise.

A number of mechanisms have been proposed for the elevation in oxygen consumption following exercise. Biochemical processes that return muscle to its preexercise state do not account for all the oxygen consumed after exercise. It is possible that mechanical activity in resting muscle, which produces low frequency vibrations (i.e., muscle sounds: mechano-myographic [MMG] activity), could contribute to the excess postexercise oxygen consumption. Therefore the purpose of this study was to determine whether the resting MMG amplitude changes after exercise, and whether the change is related to the elevation in oxygen consumption (VO2). Ten young male subjects (22.9 yrs) performed 30 minutes of exercise on a cycle ergometer at an intensity corresponding to 70% peak VO2. Oxygen consumption was measured by indirect calorimetry, and MMG by an accelerometer placed over the mid-quadriceps before exercise and for 5.5 hours after exercise. MMG activity, expressed as mean absolute acceleration, was significantly elevated for the 5.5 hours of measurement after exercise (p < 0.05). MMG and VO2 decayed exponentially after exercise with time constants of 7.2 minutes and 7.4 minutes, respectively. We conclude that muscle is mechanically active following exercise and that this may contribute to an elevated VO2.