Pressure-time curve predicts minimally injurious ventilatory strategy in an isolated rat lung model.

BACKGROUND: We tested the hypothesis that the pressure-time (P-t) curve during constant flow ventilation can be used to set a noninjurious ventilatory strategy. METHODS: In an isolated, nonperfused, lavaged model of acute lung injury, tidal volume and positive end-expiratory pressure were set to obtain: (1) a straight P-t curve (constant compliance, minimal stress); (2) a downward concavity in the P-t curve (increasing compliance, low volume stress); and (3) an upward concavity in the P-t curve (decreasing compliance, high volume stress). The P-t curve was fitted to: P = a. tb +c, where b describes the shape of the curve, b = 1 describes a straight P-t curve, b < 1 describes a downward concavity, and b > 1 describes an upward concavity. After 3 h, lungs were analyzed for histologic evidence of pulmonary damage and lavage concentration of inflammatory mediators. Ventilator-induced lung injury occurred when injury score and cytokine concentrations in the ventilated lungs were higher than those in 10 isolated lavaged rats kept statically inflated for 3 h with an airway pressure of 4 cm H2O. RESULTS: The threshold value for coefficient b that discriminated best between lungs with and without histologic and inflammatory evidence of ventilator-induced lung injury (receiver-operating characteristic curve) ranged between 0.90-1.10. For such threshold values, the sensitivity of coefficient b to identify noninjurious ventilatory strategy was 1.00. A significant relation (P < 0.001) between values of coefficient b and injury score, interleukin-6, and macrophage inflammatory protein-2 was found. CONCLUSIONS: The predictive power of coefficient b to predict noninjurious ventilatory strategy in a model of acute lung injury is high.

A new ventilator for monitoring lung mechanics in small animals.

Researchers investigating the genetic component of various disease states rely increasingly on murine models. We have developed a ventilator to simplify respiratory research in small animals down to murine size. The new ventilator provides constant-flow inflation and tidal volume delivery independent of respiratory parameter changes. The inclusion of end-inspiratory and end-expiratory pauses simplifies the measurement of airway resistance and compliance and allows the detection of dynamic hyperinflation (auto-positive end-expiratory pressure). After bench testing, we performed intravenous methacholine challenge on two strains of mice (A/J and C57bl/bj) known to differ in their responses by using the new ventilator. Dynamic hyperinflation and a decrease in compliance developed during methacholine challenge whenever respiratory rates of 60-120 breaths/min were employed. In contrast, if dynamic hyperinflation was prevented by lengthening expiratory time, (respiratory rate = 20 breaths/min), static compliance remained constant. More importantly, the coefficient of variation of the results decreased when lung volume shifts were prevented. In conclusion, airway challenge studies have greater precision when dynamic hyperinflation is prevented.

A simple breathing circuit minimizing changes in alveolar ventilation during hyperpnoea.

Many clinical and research situations require maintenance of isocapnia, which occurs when alveolar ventilation (V'A) is matched to CO2 production. A simple, passive circuit that minimizes changes in V'A during hyperpnoea was devised. It is comprised of a manifold, with two gas inlets, attached to the intake port of a nonrebreathing circuit or ventilator. The first inlet receives a flow of fresh gas (CO2=0%) equal to the subject's minute ventilation (V'E). During hyperpnoea, the balance of V'E is drawn (inlet 2) from a reservoir containing gas, the carbon dioxide tension (PCO2) approximates that of mixed venous blood and therefore contributes minimally to V'A. Nine normal subjects breathed through the circuit for 4 min at 15-31 times resting levels. End-tidal PCO2 (Pet,CO2) at rest, 0, 1.5 and 3.0 min were (mean+/-SE) 5.1+/-0.1 kPa (38.1+/-1.1 mmHg), 4.9+/-0.1 kPa (36.4+/-1.1 mmHg), 5.0+/-0.2 kPa (37.8+/-1.6 mmHg) and 5.0+/-0.2 kPa (37.6+/-1.4 mmHg) (p=0.53, analysis of variance (ANOVA)), respectively; without the circuit, Pet,CO2 would be expected to have decreased by at least 2.7 kPa (20 mmHg). Six anaesthetized, intubated dogs were first ventilated at control levels and then hyperventilated by stepwise increases in either respiratory frequency (fR) from 10 to 24 min(-1) or tidal volume (VT) from 400 to 1,200 mL. Increases in fR did not significantly affect arterial CO2 tension (Pa,CO2) (p=0.28, ANOVA). Only the highest VT decreased Pa,CO2 from control (-0.5 +/- 0.3 kPa (-3.4 +/- 2.3 mmHg), p<0.05). In conclusion, this circuit effectively minimizes changes in alveolar ventilation and therefore arterial carbon dioxide tension during hyperpnoea.

PCO2 affects tracheal tone during apnea in anesthetized dogs.

We hypothesized that CO2, like hypoxia and withdrawal of pulmonary slowly adapting receptor input, would cause tracheal constriction during neural apnea (absence of phrenic activity). In seven anesthetized paralyzed dogs ventilated to neural apnea, we increased arterial PCO2 (PaCO2) in steps by adding CO2 to the inspirate while keeping ventilation constant. Increases in PaCO2 caused tracheal constriction during neural apnea in all dogs; 69 +/- 26 (SD)% of the change in tracheal diameter occurred during neural apnea. Average sensitivity of tracheal diameter to CO2 was 0.44 mm/Torr PaCO2. Our data suggest that central chemoreceptor inputs to brain stem neurons controlling smooth muscle of the extrathoracic airway bypass central mechanisms generating inspiration.

Evaluation of a new pulse oximeter testing device.

BACKGROUND: Valid routine testing of pulse oximeters and their sensors is problematic. A suitable testing device must not only generate the pulsatile signal the pulse oximeter requires for its operation, but must possess light absorption characteristics similar to those of living tissue. A new device called Pulse Oximeter Tester (POT) has recently become available which, it is claimed, addresses these problems. PURPOSE: To evaluate the POT as a suitable stimulus for pulse oximeters. METHOD: We tested all the pulse oximeters and their sensors with a set of POTs simulating blood oxygen saturation of 80%, 90% and 100%. The tests were performed at simulated heart rates of 30, 75 and 110 bpm. RESULTS: The SpO2 readings (mean +/- SD) obtained with the 80%, 90% and 100% POTs were 80.7 +/- 1.3%, 90.3 +/- 0.9% and 100 +/- 0.0% respectively. There were no significant differences in readings obtained at the different simulated heart rates. Two pulse oximeters gave readings that deviated more than 2 SD from the mean. Their sensors were subsequently found to be defective. CONCLUSION: POTs provide suitable stimuli for testing pulse oximeters. In our study sample they were found to be highly specific, but of unknown sensitivity.

Systolic arterial pressure determination by a new pulse monitor technique.

The Doppler ultrasound (DUS) technique is a widely accepted non-invasive technique to estimate systolic blood pressure (SBP) accurately in paediatric patients. The DUS has a number of limitations. A new pulse monitor, Mr Pulse (MP), operating on the principle of a finger plethysmograph, was developed to offer an alternative technique to estimate SBP. From 104 paired SBP measurements taken in 16 paediatric patients undergoing general anaesthesia, SBP determined by the MP technique correlated closely with that by the standard DUS technique (r2 = 0.98). Analysis of degree of agreement performed indicated that there was good agreement between SBP obtained by the MP and the DUS techniques. The mean +/- standard deviation of differences in paired SBP values between the two measurement techniques was 0.55 +/- 3.59 mmHg. Mr Pulse is as accurate as the DUS technique in estimating SBP and has the advantage of less critical sensor positioning as it is not subject to electrical interference. It has no electrical hazard.

Design and evaluation of a pneumatic pulse monitor for use during magnetic resonance imaging.

We describe a portable, battery-operated instrument designed to monitor pulsations in the finger or toe. The monitor is based on pneumatic principles and is suitable for use in the electromagnetically harsh environment of magnetic resonance imaging. A compliant Silastic chamber is deformed with each cardiac pulsation; the resulting pressure wave is then transmitted along a 25-ft (7.5-m) microbore tube to a pressure sensor in a remote electronic monitor.

The minimum alveolar concentration (MAC) and hemodynamic effects of halothane, isoflurane, and sevoflurane in newborn swine.

To determine the minimum alveolar concentration (MAC) and hemodynamic responses to halothane, isoflurane, and sevoflurane in newborn swine, 36 fasting swine 4-10 days of age were anesthetized with one of the three volatile anesthetics in 100% oxygen. MAC was determined for each swine. Carotid artery and internal jugular catheters were inserted and each swine was allowed to recover for 48 h. After recovery, heart rate (HR), systemic systolic arterial pressure (SAP), and cardiac index (CI) were measured awake and then at 0.5, 1.0, and 1.5 MAC of the designated anesthetic in random sequence. The (mean +/- SD) MAC for halothane was 0.90 +/- 0.12%; the MAC for isoflurane was 1.48 +/- 0.21%; and the MAC for sevoflurane was 2.12 +/- 0.39%. Awake (mean +/- SD) measurements of HR, SAP, and CI did not differ significantly among the three groups. Compared to the awake HR, the mean HR decreased 35% at 1.5 MAC halothane (P less than 0.001), 19% at 1.5 MAC isoflurane (P less than 0.005), and 31% at 1.5 MAC sevoflurane (P less than 0.005). Compared to awake SAP, mean SAP measurements decreased 46% at 1.5 MAC halothane (P less than 0.001), 43% at 1.5 MAC isoflurane (P less than 0.001), and 36% at 1.5 MAC sevoflurane (P less than 0.005). Mean SAP at 1.0 and 1.5 MAC halothane and isoflurane were significantly less than those measured at equipotent concentrations of sevoflurane (P less than 0.005). Compared to awake CI, mean CI measurements decreased 53% at 1.5 MAC halothane (P less than 0.001) and 43% at 1.5 MAC isoflurane (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

End-tidal carbon dioxide measurements in critically ill neonates: a comparison of side-stream and mainstream capnometers.

To determine whether end-tidal PCO2 (PETCO2) measurements obtained with two infrared capnometers accurately approximates the arterial PCO2 (PaCO2) in critically ill neonates, simultaneous measurements of PETCO2 were obtained from the distal and proximal ends of the tracheal tube with a sidestream capnometer (Puritan Bennett/Datex--BP/D) and from the proximal end with a mainstream capnometer (Hewlett-Packard-HP) in 20 intubated neonates. Distal sidestream PETCO2 and mainstream PETCO2 correlated with the PaCO2 (r2 = 0.66 and 0.61, respectively) within the range of 26-57 mmHg PaCO2. However, proximal PETCO2 with the sidestream capnometer correlated very poorly (r2 = 0.09) with PaCO2. The slope of the least square regression line for the distal sidestream capnometer, 0.67, was significantly less than that for the mainstream capnometer, 0.78 but both were significantly greater than that for the proximal sidestream capnometer, 0.39 (P less than 0.05). The slope of the regression for the proximal sidestream capnometer did not differ significantly from horizontal. Insertion of the mainstream sensor for the HP capnometer significantly increased the transcutaneous CO2 when compared with preinsertion values. We conclude that both distal sidestream and mainstream capnometry provide accurate estimates of the PaCO2 in critically ill neonates.

Clinical evaluation of the augmented delta quotient monitor for intraoperative electroencephalographic monitoring of children during surgery and cardiopulmonary bypass for repair of congenital cardiac defects.

To assess the augmented delta quotient (ADQ) monitor as a monitor of cerebral function during cardiac surgery, we monitored during operation the electroencephalograms of 48 young subjects (aged 2 weeks to 19 yr). We found ADQ patterns produced by cardiopulmonary bypass, hypothermia and general anaesthetic agents correlated with those obtained from a compressed spectral array (CSA) monitor and could be differentiated from changes caused by cerebral ischaemia, except in the youngest group of patients (less than or equal to 18 months) undergoing deep hypothermia (19.4 (SD 0.8) degrees C nasopharyngeal). In all other age groups the ADQ proved to be a simple monitor of the adequacy of cerebral perfusion. Neurological deficit occurred only if the ADQ was abnormal during hypotension for a period exceeding 7 min. ADQ evaluation of cerebral function was limited by events which produced artificially normal ADQ readings such as low amplitude EEG activity and the described isoflurane effect that was demonstrated to occur in some cardiac patients. The results obtained by the ADQ were comparable to those obtained by compressed spectral array and the ADQ was easier to use and interpret.

Cardiopulmonary effects of the volume recruitment manoeuvre in infant swine.

The volume recruitment manoeuvre is a non-invasive technique used to measure respiratory mechanics in infants. Because airway pressure increases during this manoeuvre, lung volume, compliance and cardiac output may change. In order to assess possible changes in cardiopulmonary function caused by the volume recruitment manoeuvre, we applied this technique to seven intubated infant swine breathing spontaneously during anaesthesia with halothane and N2O. Tidal volume (VT), respiratory frequency, arterial blood gases, cardiac output (CO) and total respiratory compliance were measured before and after the manoeuvre. In three swine functional residual capacity (FRC) was measured by helium dilution before the manoeuvre, and in four swine diaphragmatic EMG was recorded continuously. Finally, all swine were paralysed during volume recruitment to assess the contribution of the respiratory muscles to post-manoeuvre respiratory mechanics. VT and f increased immediately after the manoeuvre but there were no significant changes in PaCO2, or alveolar to arterial oxygen gradient. There was a small but statistically significant decrease in CO. Compliance increased by 17.8 +/- 3.6 per cent and FRC increased by a mean of 41.1 ml (or 51.9 per cent increase above the baseline FRC). The increase in FRC could not be explained by active mechanisms since the diaphragmatic EMG showed no post-inspiratory activity and neuromuscular paralysis did not decrease FRC. We conclude that the volume recruitment manoeuvre increases FRC and compliance by recruiting collapsed alveoli, and this effect must be taken into consideration when applying this test to infants.

Expiratory muscle activity in anesthetized children: effect on the single breath technique.

Phasic expiratory activity of the abdominal muscles occurs in adults during halothane anesthesia, but has not been demonstrated in children. If present, abdominal muscle activity would preclude the use of recently developed tests of respiratory mechanics in children during anesthesia. We therefore measured abdominal muscle activity throughout induction of anesthesia with halothane in 10 patients between 1.5 and 9.5 years of age, seven with normal respiratory function and three with chronic airway obstruction. During induction of anesthesia with halothane in N2O and oxygen, the abdominal wall electromyograph (a-EMG) was continuously recorded from surface electrodes. At the same time, the expiratory time constant (tau a) was measured using the single breath test (SBT). The patients were then paralyzed with succinyl choline, and the a-EMG signal and expiratory time constant during paralysis (tau p) were recorded. The raw a-EMG signal and its moving time average were compared with the phase of respiration and with the end-tidal fraction of halothane (Fehalo), and the effect of abdominal muscle activity on tau a was noted. Of the 10 patients, 2 had no abdominal muscle activity at any time during induction. Of the remaining 8 patients, 3 had continuous abdominal muscle activity throughout induction, including one patient with asthma. In the remaining five patients, abdominal muscle activity was present during light halothane anesthesia and disappeared at increased Fehalo. When abdominal muscle activity was present, tau a was significantly less than tau p. It is concluded that abdominal muscle activity in expiration is undetectable during deep halothane anesthesia in most children.(ABSTRACT TRUNCATED AT 250 WORDS)

A modified Pitot tube for the accurate measurement of tidal volume in children.

A device using modification of a Pitot tube has been designed for measurement of tidal volume in infants and small children. Its accuracy was compared both n vitro and n vivo to that of a calibrated pneumotachograph (Fleish #1) designed for a similar flow range. In vitro measurement of air flow with the modified Pitot tube (MPT) was within 5% of the pneumotachograph readings over a range of 1-60 l/min. Similar accuracy was found with measurement of tidal volumes from 20 ml to 1 l, delivered by a calibrated volume-cycled ventilator using a variety of inspiratory flow rates. Tidal volume measurements with the MPT were compared to the pneumotachograph using helium, oxygen, carbon dioxide, and a range of nitrous oxide/oxygen mixtures. A manual control was incorporated into the MPT electronics to allow direct measurements of tidal volume with different nitrous oxide/oxygen concentrations. In vivo, the insertion of the MPT into the patient circuit caused no apparent changes in ventilatory parameters in children under 20 kg. Measurement of tidal volumes with the MPT agreed to within 8% of pneumotachograph readings. The low dead space (1.5 cc) and light weight (12 gm) of the MPT confer advantages over the pneumotachograph (15 ml dead space and a weight of 90 gm) for routine use in pediatric anesthesia.

A brain function monitor for use during anaesthesia. Preliminary report.

This simple on-line monitor provides simultaneous noninvasive quantification of continuous processes within the brain, by measuring the augmented delta quotient (ADQ; the proportion of low frequency components in the electroencephalogram). The response of the ADQ, both during experiments in animals and in children undergoing surgery, during cardiopulmonary bypass with and without hypothermia, demonstrates that this concept may prove valuable both to monitor anaesthetic depth and to warn of impending cerebral impairment.

The mechanism of anaesthesia: a new hypothesis based on the effects on electrical properties of a model membrane: preliminary studies.

Changes induced by inhalational anaesthetic agents were investigated in a simple membrane model. All the agents tested increased the membrane's electrical capacitance and conductance linearly, proportional to concentration and potency, and decreased its dialectric strength independently of concentration. When protein was substituted for lipid, the same agents had no significant effect on capacitance, conductance, or dielectric strength. Based on these findings, the following hypothesis is presented. Anaesthetic agents exert their influence on the lipid portion of nerve membranes. They enhance excitability by reducing the dielectric strength of the membrane and depress excitability by increasing the membrane's dielectric constant and passive conductance. The degree of excitability or narcosis at any instant depends upon the relative magnitude of these opposing effects.

The effect of splenectomy on circulatory adjustments to hypoxaemia in the anaesthetized dog.

Six intact and seven splenectomized dogs were subjected to a similar degree of hypoxaemia while under light anaesthesia and artificial ventilation. In the intact animals, heart rate, cardiac output, myocardial contractility and oxygen consumption were increased as a result of hypoxaemia; oxygen transport was not affected. In the splenectomized animals the changes in cardiac output and myocardial contractility were small, and oxygen availability was decreased.