Positive Pressure Inflation Research Articles

PATIENT TRIGGERED VENTILATION IN NEONATES

Patient triggered ventilation (PTV), the delivery of positive pressure inflation initiated only by the infant's respiratory efforts, has been assessed in 32 infants of gestational age range of 24 to 36 weeks. A conventional newborn ventilator has been modified, such that in CPAP mode the manual breath control can be triggered by the infant's respiratory efforts to deliver a single positive pressure inflation. The triggering sensor is a pneumotachograph placed between the endotracheal tube and the ventilator circuit, this detects changes in flow and triggering occurs if the inspiratory flow exceeds 0.4 1/min. Inflation time during PTV mode was limited to 0.4 sec or less. Initially PTV was associated in the majority of infants with improvements in oxygenation (p<0.01) when contrasted with conventional ventilation. PTV was maintained till final extubation in 20 infants without complications. One infant only developed pneumothorax. In the remaining 11 infants, who tended to be less mature (p<0.01), PTV had to be discontinued after only a few hours. Predictors of failure of PTV at 1 hour were both a lack of improvement in oxygenation and a relatively slow triggering rate when related to gestational age. We conclude that PTV is a useful alternative for preterm neonates, but at present its use should be restricted to those more mature than 28 weeks gestational age.

Unterdruck and Überdruck, 1904

At the beginning of the 20th century, elective surgery within the chest was not possible; to open the pleural cavity under anesthesia meant death to the patient in almost all cases. The few exceptions to this rule—resulting from diffuse pleural adhesions or brief duration of surgical pneumothorax in robustly healthy persons—still did not permit deliberate intrathoracic procedures to be done with any expectation of success. Two papers that appeared consecutively in the same journal in 1904 proposed opposite approaches to respiratory support in the presence of surgical pneumothorax, by means of differential pressure chambers. Both methods were reasonably successful in clinical use. The life span of the static-pressure chambers was brief, however, and within a few years they were being superseded by cyclic positive-pressure inflation via the airway. The opposing methods are remembered under their German names, Unterdruck and Überdruck.

The premature infant's respiratory response to mechanical ventilation

Comparisons of the components of ventilator waveform were made in two groups of preterm infants ventilated for the respiratory distress syndrome. Infants actively expiring against positive pressure inflation were compared to infants not showing this interaction. The actively expiring infants were ventilated with a higher peak inspiratory pressure (P less than 0.01) and a significantly greater duration of positive pressure plateau (P less than 0.05) than the control group. The two groups were well matched for gestational and postnatal age and there were no other significant differences in the components of ventilator waveform. Adoption of a ventilator waveform with a shorter positive pressure plateau and less steep rise in positive pressure might reduce the incidence of active expiration and hence pneumothoraces.

Neonatal patient triggered ventilation.

Patient triggered ventilation was assessed in 14 neonates (gestational age 24-40 weeks). Inspiratory changes in airflow, monitored by a pneumotachograph, were used to trigger the ventilator and this was not associated with complications. Patient triggered ventilation was maintained for up to eight hours (mean duration five hours). In 13 of 14 infants oxygenation improved and this was associated in most with an increase in rate of delivered positive pressure inflations and inflating volumes. A greater improvement in oxygenation was shown when trigger mode was used during the recovery phase of respiratory distress syndrome. Only one infant, who made very little respiratory effort, failed to improve. We conclude that patient triggered ventilation may be used as an effective form of neonatal ventilation.

Neuronal activity recorded extracellularly from in situ canine mediastinal ganglia

Spontaneous activity of 226 neurons was recorded from in situ mediastinal ganglia in 10 dogs. Forty-two percent of these were active during specific phases of the cardiac cycle, primarily during systole. Cardiovascular-related activity occurred when systolic pressure was between approximately 70 and 185 mmHg (1 mmHg = 133.3 Pa) whether the pressure was altered by positive inotropic pharmacological agents or cross clamping of the aorta. Twenty percent of the identified neurons displayed respiratory-related activity which occurred during positive pressure inflation or deflation. Thirty-eight percent of the identified neurons displayed bursts of activity or sporadic activity. The activity of 17% of the identified neurons was altered by gentle mechanical distortion of localized regions of the neck, left elbow, ventral thoracic wall, ventral abdominal wall, superior vena cava, right ventricle, or aorta. In the majority of instances cardiovascular- or respiratory-related activity persisted following acute decentralization, indicating that neurons in mediastinal ganglia can function in the absence of influences of central nervous system neurons. Five percent of the identified neurons were activated by single 1-4 ms, 10-20 V stimuli delivered at 0.5 Hz to the nerves connected with either the cranial or the caudal poles of the mediastinal ganglion or the ansae. These neurons were activated after a fixed latency when 0.5 Hz was used and in most instances when 10 Hz was used. These data indicate that 5% or less of the neurons identified projected axons out of the mediastinal ganglia investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lung inflation on fluid flux in zone 1 lungs.

Because of conflicting data in the literature, we studied the effect of positive-pressure inflation on transvascular fluid filtration in zone 1 lungs. Lungs from New Zealand White rabbits (n = 10) were excised, perfused with saline and autologous whole blood (1:1), ventilated, and continuously weighed. Pulmonary arterial and venous pressures (Pvas) were referenced to the most dependent part of the lung. A change in vascular volume (delta Vvas) and a fluid filtration rate (FFR) were calculated from the change in lung weight that occurred from 0 to 30 s and from 3 to 5 and 5 to 10 min, respectively, after changing alveolar pressure (PA). FFR's and delta Vvas's were measured with Pvas equal to 2 or 10 cmH2O and PA changing from 15 to 30 cmH2O when the lungs were normal and after they were made edematous. When Pvas = 2 cmH2O, increasing PA increased the Vvas and the FFR in both normal and edematous lungs. However, when Pvas = 10 cmH2O, increasing PA only slightly changed the Vvas and reduced the FFR in the normal lungs, and decreased Vvas and markedly decreased the FFR in the presence of edema. Inflating zone 1 lungs by positive pressure has an effect on transvascular fluid flux that depends on the Pvas. The results suggest that the sites of leakage in zone 1 also vary depending on Pvas and PA.

Effect of inflation on microvascular pressures in lungs of young rabbits.

We have examined the effect of positive pressure inflation on the longitudinal distribution of vascular resistance and intravascular pressures in isolated blood-perfused lungs of 3- to 4-wk-old rabbits. Lungs were perfused in zone 3 at airway inflation pressures (P airway) of 6, 14, and 19 cmH2O (pleural pressure, atmospheric) corresponding to 60, 80, and 90% of total lung capacity. We measured microvascular pressures by the micropipette servo-nulling technique in 20- to 50-microns diameter subpleural arterioles and venules. Pulmonary arterial and left atrial pressures were also measured. Lung blood flow was kept constant at 145 +/- 18 ml X kg body wt-1 X min-1. We found that at P airway of 6 cmH2O, approximately 55% of the total pressure drop was in arteries, approximately 23% in microvessels, and approximately 22% in veins. With increasing P airway and lung volume, there was a significant decrease in arterial and venous resistance, but an increase in resistance in microvessels. We conclude that lung inflation significantly alters the distribution of segmental vascular resistance, and therefore lung volume is an important variable that should be considered during estimation of capillary filtration pressure.

Fighting the ventilator — are fast rates an effective alternative to paralysis?

We investigated the effect of increased ventilator rates on the respiratory activity of 17 infants, all actively expiring against the ventilator at conventional rates. Fast rate ventilation was rarely associated with apnoea (3 babies only) and the infant's respiratory efforts even at rates of 120/min had an important effect on tidal exchange. Seven infants altered their respiratory response to breathe in synchrony with the ventilator at 60 breaths/min and 5 maintained this at 120 breaths/min. Nine of the 17 infants continued to actively expire against positive pressure inflation at 60 breaths/min and in two this persisted at 120/min, the remaining 7 infants showed incoordinated breathing at that rate. We conclude that fast rate ventilation appears to have only limited success in suppressing respiratory activity in infants actively expiring against the ventilator.

Manipulation of ventilator settings to prevent active expiration against positive pressure inflation.

Recent publications have suggested that in infants receiving artificial ventilatory support a particular pattern of interaction between spontaneous breaths and ventilator inflations (active expiration against each ventilator inflation) may be important in the production of pneumothoraces. We have looked at patterns of interaction from 47 preterm infants studied on 51 occasions. We found that active expiration against the ventilator occurred on a total of 16 occasions. This pattern was prevented on 14 occasions by altering the ventilator settings. In two other babies, the pattern persisted but neither baby developed a pneumothorax.

ARE FAST RATES AN EFFECTIVE ALTERNATIVE TO PARALYSIS?

We have previously demonstrated that infants who actively expire against positive pressure inflation at “conventional” ventilator rates develop pneumothoraces and amongst such infants pancuronium significantly reduces the incidence of air leaks (1) However pancuronium is not without side-effects, therefore this study investigated if fast rates could, as effectively, suppress spontaneous respiration and so possibly prevent pneumothoraces. 17 infants were demonstrated as actively expiring at ventilator rates of 30-40/minute by recording oesphageal pressure, tidal flow and volume simultaneously with ventilator pressure changes. The respiratory response of these infants to ventilator rates of 60 and 120/minute was then studied for 20 minute periods. The results are shown in the table. The 9 infants who remained actively expiring or had incoordinated respiration at fast rates were subsequently paralysed,only one developed a pneumothorax during ventilation.The 8 infants who breathed in synchrony or were apnoeic at fast rates were not paralysed and remained on increased rates, 4 later developed air leaks.We conclude that fast rate ventilation appears to have only limited success(18%)in suppressing respiratory activity in infants actively expiring against the ventilator. (1) Greenough et al Lancet 1984,i,1-4

Hemodynamic effects of positive-pressure inflation.

In 14 anesthetized mongrel dogs, we studied the factors that influenced aortic systolic pressure (PaO) and peak aortic systolic flow (Qao) during positive-pressure inflations at three respiratory rates: 8, 14, and 24 breaths/min. At all three respiratory rates, pulmonary arterial flow fell by 15% of the preinspiratory value by end inhalation. Qao also fell at all three respiratory rates, reaching a nadir close to end exhalation (EE). Qao fell less (7% of the base line) at the fast respiratory rate than at the medium and slow respiratory rates (15% of the base line). Pao rose more during the early part of inflation at the medium and fast rates (5 +/- 0.9 Torr) than at the slow rate (3 +/- 0.7 Torr); then Pao fell to a nadir near EE at the slow (-5.6 +/- 1.6 Torr) and at the medium (-2.3 +/- 1 Torr) respiratory rates. At the fast rate it fell to a value not different from control (0.1 +/- 0.5 Torr). The ratio of peak arterial pressure to peak arterial flow increased throughout inflation reaching a peak at EE. A model was developed that predicted the major qualitative features of the changes in Qao seen in these studies. Changes in Pao resulted from the interaction between decreases in Qao and transmitted increases in pleural pressure, and are modified by changes in systemic impedance.

The effect of bupivacaine aerosol on the activity of pulmonary stretch and 'irritant' receptors.

1. Experiments have been done on anaesthetized, paralysed and artificially ventilated cats. 2. Vagal single afferent fibres showing discharges in phase with respiration were isolated in the neck. Two types of fibres were studied at different tidal volumes: (i) showing slowly adapting discharges from the pulmonary stretch receptors; (ii) showing rapidly adapting discharges from the lung 'irritant' receptors. The effect of bupivacaine aerosol, administered by positive pressure inflations, was recorded on the pattern of fibre discharge. 3. The pulmonary stretch fibres were further classified into low-threshold and higher-threshold fibres according to the standard criteria. Bupivacaine aerosol blocked activity in all the low-threshold and in the majority of the higher-threshold fibres. 4. Of the rapidly adapting fibres, bupivacaine completely blocked activity at some tidal volumes and markedly reduced it at most others. 5. The fibre activity data are presented. It is concluded that although bupivacaine aerosol markedly reduced impulse activity in all three types of fibres, the data suggest that there is a small difference in the ease with which low-threshold fibres on the one hand and higher-threshold and "irritant" fibres on the other hand are affected. The reasons for this difference in the behaviour are not understood.

The relationship of bronchopulmonary dysplasia to the occurrence of alveolar rupture during positive pressure ventilation

The effect of altered mechanical ventilation on the incidence of alveolar rupture and bronchopulmonary dysplasia (BPD) was reviewed in infants who had neonatal respiratory distress syndrome (RDS) (N = 99). From 1971 to 1974 we attempted to minimize pulmonary oxygen exposure. Accordingly, during the resolution of respiratory distress syndrome, the ventilator pressures were not reduced until the FIO2 had been lowered to 0.4 (n = 61). In 1974 to 1975 earlier reduction of pressure was instituted as the FIO2 was lowered to 0.6 or less (N = 38). Birth weight and gestational age were comparable in the two groups. With earlier reduction of ventilator pressures, there was a significant decrease in the duration of exposure to peak inspiratory pressures greater than or equal to 40 cm H2O (p is less than 0.004) and greater than or equal to 50 cm H2O (p is less than 0.002). The incidence of alveolar rupture during postive pressure ventilation fell from 51 to 24% (p is less than 0.015) and bronchopulmonary dysplasia from 41 to 13% (p is less than 0.003). In addition, there was a decrease in the duration of mechanical ventilation (p is less than 0.02) and exposure to an FIO2 is greater than or equal to 0.6 (p = 0.07). The results confirm the intimate relationship of pulmonary barotrauma, as reflected by the occurrence of alveolar rupture during positive pressure inflation of the lungs, to the subsequent development of bronchopulmonary dysplasia and that prevention of the former is associated with a concomitant fall in incidence of the latter.

INTERMITTENT MANDATORY VENTILATION

The introduction of the concept of intermittent mandatory ventilation (IMV) has meant that for the first time weaning is controlled. The abrupt cessation of ventilatory support, even for short periods, as was obligatory before IMV, is no longer necessary. The frequency of positive pressure inflations can be gradually and progressively decreased as indicated, leading to spontaneous ventilation in a planned, controlled program. The ease with which other artificial ventilation technics may be incorporated into this system adds to its usefulness. IMV should be considered whenever ventilatory support is to be discontinued.

Functional residual capacity and body position in the dog.

Resting lung volumes in the supine position (FRCs) were determined by N2 washout method in 67 dogs under pentobarbital anesthesia and computed in ml/kg body weight (BW). In 21 other dogs, FRCs and the change in FRC from the supine to upright positions (deltaFRC) were determined; these lung volumes were expressed in ml/kg BW and in percentage of TLC40 (lung volume at 40 cmH2O positive-pressure inflation). It was found that a) FRCs averaged 38.6 plus or minus 8.2 and 42 plus or minus 5.9 ml/kg BW in the two groups of dogs; b) deltaFRC averaged 23 plus or minus 4 ml/kg BW resulted in large data dispersion, a large coefficient of variation (CV) and a poor correlation (r) of lung volume to BW; D) on the contrary, marked uniformity of FRCs and FRCu (upright FRC) was obtained by expressing the resting lung volumes in %TLC40, allowing an accurate prediction of FRC from the inspiratory capacity (IC). Relationship of FRCu to TLC was comparable to human data reported in the literature. FRCs (%TLC40) was smaller than values previously reported for awake human subjects, probably due to the FRCs reduction in our dogs by anesthesia.

Intermittent Positive Pressure Inflation during Fiberoptic Bronchoscopy

We describe a new technique for diagnostic fiberoptic bronchoscopic procedures under general anesthesia. The technique of inflation via nasotracheal catheter, which consists of using intermittent high inflating pressure for ventilation by passing a double catheter through the nose to the glottis and into the trachea, gave very satisfactory ventilaton, with high levels of oxygen in the blood. Intratracheal pressure was monitored continuously to guarantee safety, and pulmonary function was assessed before and after the procedure. Continuous electrocardiographic monitoring was used, and blood gas levels were determined at very frequent intervals. The use of infusions of methohexitone and succinylcholine (suxamethonium) provided adequate safe anesthesia and prompt recovery, with absence of recall of the procedure.

Intermittent Mandatory Ventilation

The introduction of the concept of intermittent mandatory ventilation (IMV) has meant that for the first time weaning is controlled. The abrupt cessation of ventilatory support, even for short periods, as was obligatory before IMV, is no longer necessary. The frequency of positive pressure inflations can be gradually and progressively decreased as indicated, leading to spontaneous ventilation in a planned, controlled program. The ease with which other artificial ventilation technics may be incorporated into this system adds to its usefulness. IMV should be considered whenever ventilatory support is to be discontinued.

Circulatory effects of deep inspirations, blocked expirations and positive pressure inflations at equal transpulmonary pressures in conscious dogs.

1. Circulatory effects of deep inspirations, blocked expirations and constant endotracheal positive pressure inflations were studied in six conscious dogs under comparable geometries of the pulmonary vascular bed, i.e. at equal transpulmonary pressures (around 10.2 cm H(2)O) and similar lung volumes.2. In order to characterize these effects, we measured beat-by-beat left and right ventricular ejections, pulmonary arterial, left atrial and aortic mean transmural pressures, and concomitant intrathoracic and tracheal pressures. Changes in pulmonary-left heart blood volume were also computed.3. During inspiration when intrathoracic pressure became more negative, there was a slight increase in right ventricular output (+15%; P < 0.1) and always a net decrease in left ventricular output (-25%; P < 0.01) despite a significant increase in mean transmural left atrial pressure (+3 cm H(2)O, i.e. +40%; P < 0.005). It is concluded that the more negative intrathoracic pressure increases the left ventricular outflow impedance and that an inspiratory increase in pulmonary vascular capacity cannot explain the observed reduction in left ventricular output since this reduction occurs together with an increase in left ventricular filling pressure.4. During blocked expiration when intrathoracic pressure was positive, decreases in right ventricular output (-17%; P < 0.05) and in pulmonary-left heart blood volume (-12 ml.; P < 0.05) were observed while right ventricular outflow impedance increased. After an initial augmentation in left ventricular output (despite a concomitant progressive decrease in mean transmural left atrial pressure), left ventricular output also decreased (-17%; P < 0.05). Such circulatory changes were similar but less marked than those observed under constant positive pressure inflations. These observations suggest that the decrease in venous return (and consequently in right ventricular output) following the increase in intrathoracic pressure is the leading factor which overshadows the augmentation in left ventricular output associated with the simultaneous decrease in left ventricular outflow impedance.5. Similar experiments performed on two additional dogs in acute conditions showed the same circulatory effects before and after pharmacological blockade. These observations therefore confirm that mechanical factors play a leading part during these respiratory manoeuvres.

Regional lung volumes with positive pressure inflation in erect humans

Four seated subjects attained a preselected lung volume (V O) in two ways: voluntarily, and by relaxing against a positive airway pressure of 14–16 cm H 2O; V O were 1.0–1.5 L below total lung capacity. Using Xe we measured the regional distribution of lung volumes at V O under both conditions. The apex to base differences in regional expansion and, by implication, regional pleural pressure differences were the same in both conditions. All regional volumes were slightly less during positive pressure inflation, implying an increase in regional TLC, presumably due to a shift of blood volume out of the thorax.

The experimental development of modern resuscitation.

Abstract The paper reviews the experimental work which has led to the techniques of resuscitation in use today. Methods of artificial ventilation have changed from those involving chest compression to those employing positive pressure inflation of the lungs from expired air or mechanical means. The evidence is examined that positive pressure inflation is deleterious to the circulation, and also the possibility that some methods of artifical ventilation can induce a circulation of blood. The experimental basis of cardiac resuscitation is reviewed. Evidence is given of the effectiveness of external cardiac massage in producing a circulation of blood, and a review is made of the use of electric shock to defibrillate the heart. The role of drugs in cardiac resuscitation is also reviewed. The little-used technique of centripetal arterial blood transfusion is examined as an example of the difficulties facing resuscitation research in applying laboratory findings to the clinical situation.