Peter Schaller

Measurement of Functional Residual Capacity by Sulfur Hexafluoride in Small-Volume Lungs during Spontaneous Breathing and Mechanical Ventilation

ABSTRACT: We modified a sulfur hexafluoride (SF6) washout technique to allow functional residual capacity (FRC) determinations in small-volume lungs both during spontaneous breathing and controlled mechanical ventilation. This method facilitates measurements in subjects who attempt spontaneous breaths between ventilator-generated breaths. We wished to confirm the accuracy and precision of the measurements and the methods sensitivity to change. The method uses a pneumotach together with a fast, mainstream infrared SF6 sensor mounted between the endotracheal tube and the ventilator circuit. A low flow of pure SF6 is delivered into the constant gas flow of the ventilator circuit to wash in tracer gas at a concentration of less than 2%. The flow signal and the instantaneous SF6 concentration is processed on-line by a computer. The calibration of the SF6 sensors nonlinear signal and the ability of the flow sensor to reflect flow values precisely near zero flow had a major impact on the accuracy of the FRC estimate. This accuracy was tested by comparing measured FRC values with a dummy lungs true FRC that was varied from 7 to 70 mL. The comparison differed by 0.7 ± 3.2% (mean ± SD; range, −5.1 to 7.8%). As a measure of reproducibility (precision) across 20 FRC determinations in five adult rabbits, the average coefficient of variation was 1.7% (range, 0.57 to 4.33%) during continuous positive airway pressure and 1.98% (range, 0.35 to 3.81%) during controlled mechanical ventilation. The method proved sensitive to changes in FRC related to changes in airway pressure. We conclude that SF6 washout allowed unbiased and precise FRC measurements under the circumstances of this study in the range of 5 to 100 mL of FRC.

Effect of positive end expiratory pressure on functional residual capacity and compliance in surfactant-treated preterm infants.

Abstract Positive end expiratory pressure is routinely used when ventilating preterm infants. Elevation of PEEP increases lung volume, as does surfactant treatment. The purpose of this study was to investigate the effect of various levels of PEEP within the range of 0.2 to 0.4 kPa on lung volume, compliance and gas exchange. We measured functional residual capacity, compliance of the respiratory system and arterial blood gases in 20 infants (median birth weight 1240 g, range 660–1690 g; median gestational age 28 weeks, range 24–32 weeks; postnatal age 3–4 days). The infants were studied at 72 hours after their last dose of natural surfactant. At this time the patients were routinely nursed at 0.3 kPa of PEEP, the PEEP level was lowered to 0.2 kPa or raised to 0.4 kPa in random order. The PEEP level was then changed to the third level 0.4 kPa or 0.2 kPa. Each new setting was maintained for 20 min before FRC, compliance and blood gases were measured. FRC was assessed using SF6 washout technique. Increasing PEEP from 0.2 to 0.3 to 0.4 kPa resulted in increases in FRC (p < 0.01) and oxygenation (ns) in all infants. In 16 infants compliance decreased and paCO2 increased with elevation of PEEP. Only in 4 infants compliance increased and CO2 fell. Conclusion: In the majority of our infants reduction of PEEP from 0.4 to 0.2 kPa resulted in increases in compliance and CO2 reduction. Our results might suggest that relatively low levels of PEEP < 0.3 kPa may be appropriate at 72 hours after surfactant replacement. Furthermore, these results underline the importance of PEEP test in clinical practice.

An Infant Ventilator Technique for Resistive Unloading during Spontaneous Breathing. Results in a Rabbit Model of Airway Obstruction

ABSTRACT: The combined system of ventilator circuit, endotracheal tube, and lung commonly imposes a resistive load on spontaneous breathing efforts. It is possible to compensate for this positive resistance by a device generating a “negative ventilator resistance” (NVR), i.e. delivering a positive pressure during inspiration and a negative pressure during expiration in constant proportion to the instantaneous flow of the spontaneous breathing. The concept of NVR implies that there must not be any phase lag between flow and pressure signals. In eight anesthetized, intubated, spontaneously breathing rabbits (mean body wt 3570 g, range 2900-4600 g), challenged either by aerosolized histamine or an extrapulmonary resistive load, lung mechanical data were calculated from esophageal pressure and flow signals. Each animal served as its own control with and without NVR. In a total of 39 experiments, NVR was applied in amounts between 1 and 15 kPa.s/L. During both types of additional resistive load, NVR immediately reduced the resistive work of breathing. There was a strong linear correlation between the amount of NVR applied and the decrease in total resistance, where the total resistance equals the resistive load on the animals respiratory muscles (sum of the resistances of all components of the combined respirator-lung system): r = 0.93, p < 0.001. The relationship between NVR and the drop in resistive work per mL of tidal volume was similar: r = 0.85, p < 0.001. Throughout the experiments, NVR operated in perfect synchronization with the animals spontaneous breathing activity.

A Ventilator Generating a Positive or Negative Internal Compliance

This paper describes technical details of a ventilator for altering the resistive and elastic load placed on respiratory muscles during spontaneous breathing in intubated infants. Positive or negative values for ventilator resistance and/or ventilator compliance can be chosen by superimposing the weighted sum of the flow and/or the volume signal over the input to a pressure controller within the pressure feedback control system of the ventilator. The aim of the study was to compare values of the ventilators compliance (Cv), as measured with a ventilation mechanics calculator, with those Cv values set by the ventilators Cv control knob on the front panel. Another aim was to compare measured values of total compliance of a combined ventilator-lung model system (Ct) with the values expected according to theory where 1/Ct = 1/Cv + 1/C1m (Eq. alpha; C1m is the lung models compliance). The Cv values set on the front panel were nearly identical to those measured (Cvm = 0.97 * Cvs + 0.54) over the whole tested range from -20 to +20 ml/kPa. Similarly, the measured Ct values were almost equal to those expected according to Eq. alpha; the standard deviation of the relative residuals was 2.7% for elastic loading and 12.4% for elastic unloading. We conclude that the ventilator described in this study can effectively provide both elastic loading and elastic unloading of spontaneous breathing, as expected according to theory.

Assisted Mechanical Ventilation Using Elastic Unloading: A Study in Cats with Normal and Injured Lungs

ABSTRACT: Elastic unloading [otherwise known as negative ventilator compliance (Cv) or proportional assist ventilation] is a new mode of assisted mechanical ventilation. The ventilator continuously measures the volume of spontaneous breathing (V) and adjusts the pressure at the airway opening in proportion to V. The quotient of pressure above the baseline end-expiratory level per unit of V (the gain of the assist) is constant at any point in time and can be preset. The apparatus used for this study can also generate clastic loading (positive Cv) by decreasing the pressure at the airway opening in proportion to V. This might be useful during the weaning process. This study compares measured values of total compliance of the combined lung-respirator system (Ctot) with values predicted according to theory, where 1/Ctot = 1/Cv + 1/Cv with C1 being the lung compliance. Respiratory mechanical data were derived from esophagcal pressure and airflow in eight anesthetized, intubatcd, spontaneously breathing cats. Different Cv, levels were set on the ventilator both before and after lung injury with xanthine oxidase. The difference (mean ± SD) between the measured and predicted Ctot was 1.4 ± 21.4% (healthy lungs) and −11.6 ± 14.1% (injured lungs) during unloading and 2.5 ± 7.5% (healthy lungs) during elastic loading. An elevation of Ctot decreased the expiratory airflow. Tidal volume increased slightly in healthy lungs and arterial Pco2 decreased. We conclude that the effects of Cv on the total compliance of the combined lung-respirator system can accurately be predicted.

Effects of different gain settings during assisted mechanical ventilation using respiratory unloading in rabbits

Compared with conventional modes of patient-initiated mechanical ventilation, respiratory mechanical unloading aims at improving the match between ventilator pressure profiles and the specific derangements in lung mechanics. This may reduce lung barotrauma. The ventilator pressure increases either in proportion to the volume or to the flow of spontaneous breathing(elastic or resistive unloading), thereby selectively decreasing elastic or resistive work of breathing. The clinician sets a gain of increase in pressure per unit of volume or flow. In an attempt to develop criteria for selecting an appropriate gain, we investigated the effects of unloading using increasing gains that either partially compensated or overcompensated lung elastance or resistance. We studied spontaneously breathing, anesthetized, and tracheotomized rabbits. Compared with continuous positive airway pressure, respiratory unloading decreased the electromyographic activity of the diaphragm and increased minute ventilation in normal (n = 5) and surfactant-depleted (n = 6) animals when the gain was partially compensating. Fluctuations in systemic blood pressure associated with breathing decreased. The end-expiratory lung volume remained unchanged. Overcompensation of lung elastic recoil during elastic unloading with an excessive gain caused large tidal volumes associated with a cyclic decrease in blood pressure. Overcompensation of resistance induced oscillations. Complete inhibition of spontaneous breathing occurred with a further increase in gain. We conclude that respiratory unloading with an appropriate gain enhances the effect of diaphragmatic muscle activity on ventilation. A stable breathing pattern ensues whenever a regular spontaneous effort is present. However, excessive gain causes large tidal volumes during elastic unloading or oscillations during resistive unloading.

Effects of the inspiratory pressure waveform during patient-triggered ventilation on pulmonary stretch receptor and phrenic nerve activity in cats.

Objective To examine the effects of square wave, sinusoidal, and linear inspiratory pressure waveforms during pressure-controlled assist/control ventilation on the firing pattern of pulmonary stretch receptors and phrenic nerve activity. Design Experimental, comparative study. Setting Research laboratory at a university biomedical center. Subjects Nine anesthetized, endotracheally intubated young cats (2.5–3.4 kg). Intervention With interposed periods of continuous positive airway pressure (0.2 kPa), each cat was exposed to periods of assist/control ventilation with three different pressure waveforms, where the peak inspiratory pressure (0.74 ± 0.13 kPa), end-expiratory pressure (0.2 ± 0.02 kPa), and tidal volume (14.9 ± 5.22 mL/kg) were kept constant. Preset controlled ventilator rate was set below the rate of spontaneous breathing, and the mechanical inflation time equaled the inspiratory time during spontaneous breathing on continuous positive airway pressure. Measurements and Main Results Respiratory rate and arterial blood gases did not change between the three pressure waveforms during assist/control ventilation. Peak pulmonary stretch receptor activity was lower and mean phrenic nerve activity higher during continuous positive airway pressure than during assist/control ventilation (p < .05). Peak inspiratory pulmonary stretch receptor activity was the same with all three pressure waveforms (82 ± 17 impulses·sec−1) but occurred earlier with square wave than with sinusoidal or linear pressure waveforms (p < .05). The total number of impulses in the phrenic nerve activity burst was smaller with square wave than with the other two pressure waveforms (0.21 ± 0.17 vs. 0.33 ± 0.27 and 0.42 ± 0.30 arbitrary units;p < .05), and the phrenic nerve activity burst duration was shorter with square wave (1.10 ± 0.45 vs. 1.54 ± 0.36 and 1.64 ± 0.25 secs;p < .05). Conclusion Square wave pressure waveform during pressure-controlled assist/control ventilation strongly inhibits spontaneous inspiratory activity in cats. One mechanism for this inhibition is earlier and sustained peak pulmonary stretch receptor activity during inspiration. These findings show that differences in inspiratory pressure waveforms influence the spontaneous breathing effort during assist/control ventilation in cats.

A Method of Calculating Total Respiratory System Compliance from Resonant Frequency: Validity in a Rabbit Model

ABSTRACT: Ten anesthetized, tracheotomized, adult rabbits were used to test the validity of a method for calculation of total respiratory system compliance from resonant frequency (Cr). Reference values were obtained during constant flow inflation of the relaxed respiratory system by dividing the volume gain by the related difference in pressure at the airway opening (inflation method compliance, Ci). The animals were connected to a new type of servo-controlled infant ventilator. Besides volume-controlled mechanical ventilation at constant inspiratory flow rate and intermittent mandatory ventilation, there is a negative ventilator resistance mode integrated in this device for resistive unloading (Schulze A, Schaller P, Gehrhardt B, Mädler H-J, Gmyrek D: Pediatr Res 28:79-82,1990). To measure resonant frequency (fr), the respiratory system was totally unloaded for a short period by a negative ventilator resistance exceeding the combined resistances of the endotracheal tube and airways. This evoked a continuous oscillation at fr. By analogy with electrical circuit theory, Cr was calculated according to C=l/(4π2· I · fr 2) where C is compliance and I is inertance. The inertance of the endotracheal tube is given and that of the bronchial tree was ignored assuming a much greater total cross-sectional area and therefore much lower inertance when compared with the endotracheal tube. Three pairs of Ci – Cr values were obtained from each animal: 1) during intact respiratory muscle activity; 2) after pancuronium relaxation, and 3) after surfactant depletion by saline washout. There was a significant linear correlation between Ci and Cr values with the regression line lying close to the identity line (Cr=1.1 Ci – 0.74; r=0.97; p<0.0001). Ci increased from 45.3 ± 9.4 to 47 ± 7.1 mL/kPa after pancuronium relaxation and dropped to 25.5 ± 5.2 after surfactant depletion. The corresponding Cr values were 49.5 ± 11, 51 ± 8.2, and 27 ± 5.9 mL/kPa, respectively. The fr value decreased from 274 ± 33.2 min-1 before to 267 ± 21.4 after relaxation, and was 371 ± 39.5 min-1 after injury.

179 PARTIAL MECHANICAL UNLOADING OF THE ELASTIC WORK OF SPONTANEOUS BREATHING IN CATS WITH NORMAL AND XANTIHINEOXIDASE INJURED LUNGS

A ventilator with an adjustable internal compliance (Cv) has been constructed (Schaller et al 1991). In addition to a baseline “CPAP” it generates a pressure at the endotracheal tube (ETT) proportional to the instantaneous inspired volume. Thus, a decreased compliance of the patients lung (C1) can theoretically be compensated during spontaneous breathing by adjusting Cv to a negative value. To test this hypothesis we assessed how stepwise changes in Cv influenced the compliance of the combined ventilator-ETT-lung system (Ctot) and integrated inspiratory phrenic nerve activity in 13 chloralose anesthetized cats. Ctot improved according to the relation 1/Ctot = 1/C1+1/Cv. With a time lag of no more than 2 or 3 breathing cycles phrenic nerve activity decreased with elastic unloading to a new level. It decreased in a hyperbolic relation to the percentually scaled improvement in Ctot, both before (r=0.85) and after (r=0.69) lung injury. We conclude that elastic unloading effectively decreases inspiratory activity and work of breathing for a given alveolar ventilation i stiff lungs.

EFFECT OF PROPORTIONAL ASSIST VENTILATION ON CHEST WALL DISTORTION IN RABBITS WITH MECONIUM INJURED LUNGS. |[utrif]| 2078

EFFECT OF PROPORTIONAL ASSIST VENTILATION ON CHEST WALL DISTORTION IN RABBITS WITH MECONIUM INJURED LUNGS. ▴ 2078