End-inspiratory Pause Research Articles

Maneuver-Free Determination of Compliance and Resistance in Ventilated ARDS Patients

At present, most methods of lung mechanics analysis do not take nonlinearities of compliance and resistance into account. Nevertheless, nonlinearity of compliance is an inherent property of the respiratory system in ARDS and nonlinearity of resistance is an inherent property of the endotracheal tube. Herein we describe a computer-assisted multipoint method (LOOP) for breath-by-breath calculation of total respiratory system compliance (Ctrs) and total respiratory system resistance (Rtrs). Unlike our previously published method, LOOP excludes nonlinearities of compliance and resistance by confining the data used from the P/V/V loop to sequences with constant flow in inspiration and with steadily decreasing flow in expiration. LOOP was applied to five patients ventilated after open heart surgery (HEART group) and 12 patients ventilated for ARDS (ARDS group). The compliance results from LOOP were compared with the semistatic reference values corrected for intrinsic PEEP (CsST,IP). In the ARDS patients the compliance values from LOOP (46 ml/mbar) corresponded well with the semistatic compliance (CsST,IP = 42 ml/mbar). Despite the fact that there is no reference method for resistance known to date, we also determined the semistatic resistance (RsST) at end-inspiratory pause. The resistance values determined with LOOP were 8.5 mbar/L/s (RsST = 7.3 mbar/L/s) in the HEART group and 11.1 mbar/L/s (RsST = 8.6 mbar/L/s) in the ARDS group. LOOP gives a good correspondence between the linear RC model and the measured data in ARDS patients. In conclusion, LOOP requires neither an end-inspiratory pause (EIP) nor additional determination of intrinsic PEEP and gives Ctrs, automatically corrected for IPEEP, as well as Rtrs breath by breath at the bedside.

Volume-Cycled Decelerating Flow: An Alternative Form of Mechanical Ventilation

The linearly decelerating flow waveform for volume-cycled mechanical ventilation is an option on many modern ventilators. We have developed mathematical models for two available forms of volume-cycled decelerating-flow ventilation (VCDF). These equations use clinician-chosen ventilator settings as inputs (frequency, tidal volume, peak inspiratory flow or inspiratory time fraction, and end-inspiratory pause), and patient-determined inputs which describe the patient's ventilatory impedance (inspiratory [RI] and expiratory [RE] resistance and respiratory system compliance [C]. The equations predict key outcome variables: mean airway pressure; and peak, mean, and end-expiratory alveolar pressures. The mathematical expressions were validated in a mechanical lung analog. Values observed in the test lung were compared to values predicted by the mathematical models for a wide range of ventilator settings and impedance combinations (RI and RE, 5 to 40 cm H2O.s/L; C, 0.02 to 0.10 L/cm H2O). The correspondence between observed and predicted values was generally excellent across the broad range of inputs tested (r greater than or equal to 0.98). Outcome variables were quite sensitive to clinician-chosen inputs over certain critical ranges. Carefully applied, VCDF offers several theoretic advantages for the clinical setting; however, appropriate caution must be exercised to avoid the application of tissue-injuring pressure.

High impedance mechanical ventilator for small animals: use of programmable controller

We built a simple high impedance ventilator, which generates a pattern of flow largely independent of respiratory mechanics, to mechanically ventilate anaesthetized small animals. The system includes a source of compressed gas with an electronic valve and a flow controller on the inspiratory side and a second valve on the expiratory side. The two valves are driven by a programmable controller. To assess the performance of this ventilator we measured the delivered tidal volume while the ventilator was connected to an external, gradually varying resistance. This resistance was progressively increased to simulate bronchoconstriction of the respiratory system. Comparison with a volume-controlled ventilator was made. The use of a programmable controller also allows control of different patterns of mechanical ventilation, such as end-inspiratory pause or the static pressure-volume relationship, which can be used to perform lung function tests. The system is a simple, versatile device allowing both reliable mechanical ventilation and lung function assessment in small rodents and is suitable for routine use in laboratories.

Effect of ventilation with positive end-expiratory pressure on the development of lung damage in experimental acid aspiration pneumonia in the rabbit.

Sixteen anaesthetized rabbits were subjected to tracheostomy and lung damage produced by the instillation of 4.5 ml/kg hydrochloric acid (pH 1.5) into the trachea. Half of the animals were ventilated with a positive end-expiratory pressure (PEEP) of 3 cmH2O and half with a PEEP of 10 cmH2O for 5 h, the mean airway pressure being kept at 12 cmH2O by adjustment of the end-inspiratory pause time. Pressure-volume curves were recorded every hour. Although the arterial PO2 values and compliance above the inflection point on the pressure-volume curve were greater in the group submitted to 10 cmH2O PEEP, there were no significant differences between the groups in terms of survival and histological findings.

Inverse Ratio Ventilation in ARDS: Rationale and Implementation

Conventional ventilatory support of patients with the adult respiratory distress syndrome (ARDS) consists of volume-cycled ventilation with applied positive end-expiratory pressure (PEEP). Unfortunately, recent evidence suggests that this strategy, as currently implemented, may perpetuate lung damage by overinflating and injuring distensible alveolar tissues. An alternative strategy--termed inverse ratio ventilation (IRV)--extends the inspiratory time, and, in concept, maintains or improves gas exchange at lower levels of PEEP and peak distending pressures. There are two methods to administer IRV: (1) volume-cycled ventilation with an end-inspiratory pause, or with a slow or decelerating inspiratory flow rate; or (2) pressure-controlled ventilation applied with a long inspiratory time. There are several real or theoretical problems common to both forms of IRV: excessive gas-trapping; adverse hemodynamic effects; and the need for sedation in most patients. Although there are many anecdotal reports of IRV, there are no controlled studies that compare outcome in ARDS patients treated with IRV as opposed to conventional ventilation. Nonetheless, clinicians are using IRV with increasing frequency. In the absence of well-designed clinical trials, we present interim guidelines for a ventilatory strategy in patients with ARDS based on the literature and our own clinical experience.

Ventilation inhomogeneity during controlled ventilation. Which index should be used?

Six indexes for diagnosing uneven ventilation by tracer gas washout were studied. The indexes were lung clearance index, mixing ratio, Becklake index, multiple-breath alveolar mixing inefficiency, moment ratio, and pulmonary clearance delay, all of which increase with impaired pulmonary gas mixing. In model lung tests, indexes that compared the actual washout curve with a calculated ideal curve (mixing ratio, multiple-breath alveolar mixing inefficiency, and pulmonary clearance delay) were unaffected by changes in tidal volume and series dead space, whereas the others varied markedly. In both spontaneously breathing and mechanically ventilated patients all indexes showed a significant difference between smokers and nonsmokers (P less than 0.002), but the indexes were somewhat different in their assessment of different ventilatory patterns. However, the mean value for all indexes, with the exception of mixing ratio, was smallest with a fast insufflation followed by an end-inspiratory pause. Any of the indexes may be useful if its limitations are recognized, but mixing ratio, multiple-breath alveolar mixing inefficiency, and pulmonary clearance delay seem preferable, because they are not affected by changes in tidal volume and dead space fraction.

Expiratory flow pattern and respiratory mechanics.

During resting breathing, expiration is characterized by the narrowing of the vocal folds which, by increasing the expiratory resistance, raises mean lung volume and airway pressure. This is even more pronounced in the neonatal period, during which expirations with short complete airway closure are commonly occurring. We asked to which extent differences in expiratory flow pattern may modify the inspiratory impedance of the respiratory system. To this aim, newborn puppies, piglets, and adult rats were anesthetized, paralyzed, and ventilated with different expiratory patterns, (a) no expiratory load, (b) expiratory resistive load, and (c) end-inspiratory pause. The stroke volume of the ventilator and inspiratory and expiratory times were maintained constant, and the loads were adjusted in such a way that inflation always started from the resting volume of the respiratory system. After 1 min of each ventilatory pattern, mean inspiratory impedance and compliance of lung and respiratory system were measured. The values were unchanged or minimally altered by changing the type of ventilation. We conclude that the expiratory laryngeal loading is not primarily aimed to decrease the work of breathing. It is conceivable that the expiratory pattern is oriented to increase and control mean airway pressure in the regulation of pulmonary fluid reabsorption, distribution of ventilation, and diffusion of gases.

Gas Exchange during Controlled Ventilation in Children with Normal and Abnormal Pulmonary Circulation

Carbon dioxide single breath tests (SBT-CO2) were obtained during anesthesia and controlled ventilation in 42 children about to undergo thoracic surgery. The tests were obtained with a computerized system based on the Servo ventilator. The system made on-line corrections for compressed volume, apparatus deadspace, and rebreathing. Children with normal pulmonary circulation had excellent gas exchange with high PaO2 values, a mean alveolar deadspace fraction (VDalv/VTalv) of 0.10, and a gently sloping phase III of SBT-CO2. Children with pulmonary hyperperfusion (left to right shunting) due to an atrial septal defect or a ventrical septal defect had significantly lower PaO2 values, steeper phase III slopes, and a greater spread of values for VDalv/VTalv. Children with pulmonary hypoperfusion due to pulmonary stenosis in combination with intracardiac right to left shunting had extremely low PaO2 values, and "adult" values for VDalv/VTalv. They required increased ventilation to maintain CO2 homeostasis. In the pooled material, the airway deadspace was strongly correlated to height, weight, and age. The airway deadspace was unaffected when tidal volume was increased by 37%, and ventilatory frequency simultaneously reduced by 30%, a maneuver that left alveolar ventilation unchanged. This is probably because an end-inspiratory pause was used; when frequency is reduced the length of the end-inspiratory pause increases, allowing proximal diffusion of the alveolar/fresh gas interface.

DEADSPACE AND THE SINGLE BREATH TEST FOR CARBON DIOXIDE DURING ANAESTHESIA AND ARTIFICIAL VENTILATION: Effects of tidal volume and frequency of respiration

Using the single breath test for carbon dioxide (SBT-CO2), the components of physiological deadspace were investigated during anaesthesia with IPPV in 58 patients. A square-wave inspiratory flow and an end-inspiratory pause (25% and 10% of cycle time, respectively) were used. At tidal volumes of 0.45 litre (f = 17 b.p.m.), and 0.75 litre (f = 9 b.p.m.), median values for VDphys/VT were 0.44 and 0.31. Increasing VT and decreasing f did not change airway deadspace (VDaw) so that the fraction VDaw/VT was decreased (P less than 0.001). The alveolar deadspace fraction, VDalv/VTalv, was decreased in 93% of patients (P less than 0.001). These improvements with increasing VT can be attributed to beneficial effects on gas distribution and diffusion time. Patients with large alveolar deadspaces had steeply sloping SBT-CO2 phase III, and increased expiratory time constants of the respiratory system. The median arterial--end-tidal PCO2 difference, (PaCO2-PE'CO2), was 0.6 kPa at small and 0.3 kPa at large tidal volumes (P less than 0.001). Three patients had zero and four had negative (PaCO2-PE'CO2) values at large tidal volumes. When phase III slopes steeply, negative (PaCO2-PE'CO2) values may be observed in the presence of alveolar deadspace.

A Pulse Method of Measuring Respiratory System Compliance in Ventilated Patients

We describe a method of measuring total static respiratory system compliance (Crs) in ventilated patients during inflation, which appears to detect relaxation of respiratory muscles and does not require an end-inspiratory pause or disconnection of a constant-flow intermittent mandatory ventilation (IMV) circuit. Flow is measured with a pneumotachometer attached to the endotracheal tube. Transthoracic pressure is taken as the difference between mouth pressure measured at the proximal pneumotachometer port and body surface (atmospheric) pressure. Flow and transthoracic pressure are displayed on separate channels of a strip chart recorder. The ventilator is adjusted to deliver a constant rule of air flow. When inflation begins, the pressure tracing shows an initial step rise related to the flow resistance of the subject followed by a section with a slower rise and a constant slope. Respiratory system compliance is calculated by dividing the flow rate in L/sec by the slope of the pressure tracing in cm H2O/sec. Pulse Crs was compared with static Crs measured with an end-inspiratory pause in nine subjects receiving mechanical ventilation. Correlation between pulse Crs and static Crs in nine ventilated patients was highly significant (4 = .997, pulse Crs = 1.00 static Crs + 0.001). We conclude that with the pulse method, one can measure static Crs during inflation without an inspiratory pause and without disconnecting an IMV circuit.

Distribution of inspired gas to each lung in anesthetized human subjects.

The distribution of ventilation in man during halothane anesthesia was studied in a two-compartment lung model in which each lung was ventilated separately by means of a double-lumen tracheal tube. Eight subjects were studied prior to scheduled surgery. Tidal volume distribution was even between the lungs in the supine position (horizontal distribution) as was distribution of dynamic lung compliance, resistance and dead space. The vertical distribution was assessed when the patient was in the left lateral position. Dependent dynamic lung compliance and dead space were lower and lung resistance was higher than in the non-dependent lung. These factors favoured a non-dependent lung ventilation and, moreover, caused a re-distribution from dependent to non-dependent lung during an end-inspiratory pause (EIP), thus increasing the inhomogeneity of ventilation. The application of a positive end-expiratory pressure (PEEP) of 10 cmH2O improved dependent ventilation and abolished redistribution between the lungs. In conclusion, uneven distribution of dynamic lung compliance and lung resistance causes inhomogeneous ventilation distribution, favouring the non-dependent lung. An EIP enhances and a PEEP reduces the inhomogeneity of ventilation.

Effects of end-inspiratory pause on respiratory function in the patients undergone open-chest surgery.

Effects of end-inspiratory pause (EIP) on respiratory function were studied in 10 patients operated curatively for esophageal cancer. During intrathoracic manipulation, markedly decreased PaO2, and increased A-aDO2 and Qs/Qt were observed suggesting the occurrence of plenty of atelectatic alveoli, while no significant change in PaCO2 nor VD/VT was recognized. The impaired oxygenation was gradually improved with a stepwise increment of EIP added to the controlled ventilation with 5cmH2O of positive end-expiratory pressure (PEEP) after chest was closed. With 5% of EIP, A-aDO2 and Qs/Qt decreased significantly, but not any change in ventilation was obtained. When EIP was increased to 10%, PaO2 increased remarkably and A-aDO2, Qs/Qt and VD/VT decreased without significant change in circulation. By 15% of EIP, no more improvement in PaO2 nor A-aDO2 was obtained in spite of decrease in Qs/Qt and VD/VT. Arterial blood pressure decreased significantly followed by an increase in mean airway pressure. Considering that essential time for the redistribution of inspired gas between lung compartments is 0.4 sec and that excessive positive airway pressure is harmful to circulation, 10% of EIP with 5cmH2O PEEP was concluded to be the most suitable combination in the controlled ventilation for the patients undergone open-chest surgery.

Influence of an end inspiratory pause on pulmonary ventilation, gas distribution, and lung perfusion during artificial ventilation

Using a constant tidal volume and ventilatory frequency, anesthetized piglets were ventilated with a new tidal volume ventilator. A short inspiratory time without a pause (10% of breathing cycle) was compared with a longer inspiratory time with a pause (33%) both with and without bronchial obstruction. Mechanics of ventilation, pulmonary ventilation, gas exchange, gas distribution, and lung perfusion were measured. The longer inspiratory time with a pause resulted in lower peak airway and end inspiratory pressures and a higher total compliance. Dead space/tidal volume ratio was reduced and the RQ was increased. While the cranial pulmonary fields were less well ventilated, the right caudal field was better ventilated. In the presence of bronchial obstruction, better alveolar ventilation was achieved when an end inspiratory pause was added. The results emphasize the importance of static end inspiratory tracheal conditions although the tidal volumes were kept unchanged.

Optimal Flow Pattern for Mechanical Ventilation of the Lungs. 2. The Effect of a Sine Versus Square Wave Flow Pattern with and without an End-Inspiratory Pause on Patients

Departments of Pediatrics and Biomedical Engineering, University of Virginia Medical Center, and Department of Anesthesia, University of Maryland School of Medicine and Maryland Institute for Emergency Medicine, Charlottesville, Virginia, and Baltimore, Maryland

Optimal flow pattern for mechanical ventilation of the lungs 2. The effect of a sine versus square wave flow pattern with and without an end-inspiratory pause on patients

In four series of patients, the efficiency of ventilation of a sine wave without an end-inspiratory pause was compared to a square wave without a pause, a sine wave with a pause to a square wave with a pause, a sine wave to a sine wave with a pause, and a sine wave with a long pause to one with a short pause. The primary mode of evaluation was through simultaneous airway and arterial argon washout curves. Additional cardiopulmonary measurements were made. Results indicate: (1) a statistically significant improvement in ventilation with a sine wave with a pause; (2) a statistically significant improvement with the longer pause as compared to the short pause.

Optimal flow pattern for mechanical ventilation of the lungs Evaluation with a model lung

The major components entering into the inspiratory pattern of various respirators were tested on a model lung which had an abnormally high airway resistance on one side. The tests consisted of simultaneous nitrogen washout curves from each lung separately utilizing two mass spectrometers. The components tested included a constant versus accelerating wave form and the presence, duration or absence of an end-inspiratory pause. Respirators tested included the Bennett MA1. Engström 300 and the Elema Schonander Servo tventilator 900. The results demonstrated the importance of an end inspiratory pause in improving gas distribution and efficiency of washout. No difference was found between a constant or accelerating air flow. Preliminary results in man appear to confirm the importance of an end inspiratory pause.

Airway Pressure Changes Observable with Constant-Flow Inflation and an End-Inspiratory Pause

Constant-flow inflation and a prolonged end-inspiratory pause were studied, observing the airway pressures in model lungs. The technique demonstrated the changes associated with compliance, resistance and gas redistribution under varying conditions. The studies suggest that when intratracheal pressures indicate a large element of gas redistribution, constant flow inflation and end-inspiratory pause produce pendelluft, and other pressure patterns are preferable for I.P.P.V.

Effect of mechanical ventilation with end-inspiratory pause on blood-gas exchange.

The effects of end-inspiratory pause (EIP) on gas exchange were measured in 10 adult patients with acute respiratory insufficiency while maintained on mechanical ventilation. Four inspiratory patterns were studied with a constant tidal volume (10 to 15 ml/kg body weight), respiratory rate (9 to 12 breaths/min), FIO2 (0.5) and end-expiratory pressure. Inspiratory flow rate (V insp) and EIP time were varied to produce a control pattern (V insp = 60 L/min, EIP = 0), 2 EIP patterns of 0.6 and 1.2 seconds with a similar V insp and a "slow" flow pattern (V insp = 30 L/min) without EIP. The control pattern was applied before and after each study period. Arterial oxygenation was unchanged with both EIP and "slow" flow patterns when compared to control. Dead-space ventilation (VD/VT) and Paco2 were significantly decreased (p less than 0.01) as EIP was increased from 0 to 1.2 seconds, but remained unchanged with slow inspiratory flow. Thus, EIP improved the efficiency of ventilation with no apparent improvement in oxygenation in patients with acute respiratory insufficiency.

The physiologic significance of grunting respiration

The effect of grunting respiration before and after induction of pneumonia was studied in 14 experiments on five dogs artificially ventilated with a modified piston respirator. An end-inspiratory pause was produced which was exactly the same duration as the pause that usually occurs after normal expiration. Only the respiratory pattern was changed; frequency, tidal volume, and flow rates remained constant. Before pneumonia, grunting produced a mean increase in PaO2 of 10.5% (± 7.0%), a mean decrease in PaCO2 of 11.0% (± 5.1%), and a mean increase in VA of 21.8% (± 9.5%). The results after pneumonia were not significantly different. Intrapulmonary gas distribution was somewhat improved by grunting in normal lungs, but the ellect was not as pronounced after pneumonia.

Pulmonary Emphysema, Cardio-Respiratory Disturbance

The time course of inspiration has been shown to have a significant influence on the subsequent maximal expiratory flows and timed forced expiratory volumes in healthy adults and those with COPD. The purpose of this study was to evaluate the effect of two different inspiratory maneuvers on the spirogram in 15 patients with cystic fibrosis, aged 13 to 35 years, who had mild to moderate airway obstruction. Patients performed a forced expiratory maneuver either after a rapid inspiration without an end-inspiratory pause or after a slow inspiration with a 4-s end-inspiratory pause. Flow-time and volume-time curves were measured by a pneumotachograph. The mean values of FVC, FEV1, and peak expiratory flow were significantly larger by 11%, 13%, and 26%, respectively, after the rapid inspiration without an end-inspiratory pause compared to the slow inspiration with the end-inspiratory pause. This discrepancy probably reflects differences in effective elastic recoil pressure between the two maneuvers. Although the nature of this phenomenon is not fully understood, our results show that for spirometry in patients with cystic fibrosis, the preceding inspiratory maneuver influences the results. An important corollary is that this inspiratory maneuver should be standardized.