Ventilator Waveforms Research Articles

Hysteresivity of the lung and tissue strip in the normal rat: effects of heterogeneities

We measured lung impedance in rats in closed chest (CC), open chest (OC), and isolated lungs (IL) at four transpulmonary pressures with a optimal ventilator waveform. Data were analyzed with an homogeneous linear or an inhomogeneous linear model. Both models include tissue damping and elastance and airway inertance. The homogeneous linear model includes airway resistance (Raw), whereas the inhomogeneous linear model has a continuous distribution of Raw characterized by the mean Raw and the standard deviation of Raw (SDR). Lung mechanics were compared with tissue strip mechanics at frequencies and operating stresses comparable to those during lung impedance measurements. The hysteresivity (eta) was calculated as tissue damping/elastance. We found that 1) airway and tissue parameters were different in the IL than in the CC and OC conditions; 2) SDR was lowest in the IL; and 3) eta in IL at low transpulmonary pressure was similar to eta in the tissue strip. We conclude that eta is primarily determined by lung connective tissue, and its elevated estimates from impedance data in the CC and OC conditions are a consequence of compartment-like heterogeneity being greater in CC and OC conditions than in the IL.

Optimizing Patient-Ventilator Synchrony

Mechanical ventilation assumes the work of breathing, improves gas exchange, and unloads the respiratory muscles, all of which require good synchronization between the patient and the ventilator. Causes for patient-ventilator dyssynchrony include both patient factors (abnormalities of respiratory drive and abnormal respiratory mechanics) and ventilator factors (triggering, flow delivery, breath termination criteria, the level and mode of ventilator support, and imposed work of breathing). Although patient-ventilator dyssynchrony can often be detected on physical exam, careful analysis of ventilator waveforms (pressure-time, flow-time) allows for more precise definition of the underlying cause. Patient-ventilator interaction can be improved by reversing patient factors that alter respiratory drive or elevate patient ventilatory requirements and by correcting factors that contribute to dynamic hyperinflation. Proper setting of the ventilator using sensitive triggering mechanisms, satisfactory flow rates, adequate delivered minute ventilation, matching machine T(I) to neural T(I), and applying modes that overcome the imposed work of breathing, further optimize patient-ventilator synchrony.

Technique to determine inspiratory impedance during mechanical ventilation: implications for flow limited patients.

We present the design of an enhanced ventilator waveform (EVW) for routine measurement of inspiratory resistance (R) and elastance (E) spectra in ventilator-dependent and/or severely obstructed flow-limited patients. The EVW delivers an inspiratory tidal volume of fresh gas with a flow pattern consisting of multiple sinusoids from 0.156 to 8.1 Hz and permits a patient-driven exhalation to the atmosphere or positive end-expiratory pressure. Weighted least-squares estimates of the coefficients in a sinusoidal series approximation of the EVW inspirations yielded inspiratory R and E spectra. We first validated the EVW approach using simulated pressure and flow data under different physiological conditions, noise levels, and harmonic distortions. We then applied the EVW in four intubated patients during anesthesia and paralysis: two with mild airway obstruction and two with severe emphysema and flow limitation. While the level of inspiratory R was similar in both groups of patients, the inspiratory E of the emphysematous patients demonstrated a pronounced frequency-dependent increase consistent with severe peripheral airway obstruction. We conclude that the EVW offers a potentially practical and efficient approach to monitor lung function in ventilator-dependent patients, especially those with expiratory flow limitation.

Respiratory monitoring

Monitoring is essential to patient care in the intensive care unit. Although arterial blood gas analysis is often considered the gold standard for evaluation of gas exchange, blood gases may be used excessively in many intensive care units. Pulse oximetry is commonly used to assess arterial oxygenation in critically ill patients. Capnometry is useful to detect esophageal intubation, but use of end-tidal PCO2 as a noninvasive indicator of arterial PCO2 is often unreliable. Transcutaneous monitoring of PO2 and PCO2 is commonly used in the neonatal intensive care unit but not with critically ill adults. A wealth of information is provided by ventilator waveforms such as the risk of alveolar overdistension and the presence of auto-positive end-expiratory pressure. There has been increasing enthusiasm recently for the use of pressure-volume curves to properly set the ventilator.

Partitioning airway and lung tissue resistances in humans: effects of bronchoconstriction.

The contribution of airway resistance (Raw) and tissue resistance (Rti) to total lung resistance (RL) during breathing in humans is poorly understood. We have recently developed a method for separating Raw and Rti from measurements of RL and lung elastance (EL) alone. In nine healthy, awake subjects, we applied a broad-band optimal ventilator waveform (OVW) with energy between 0.156 and 8.1 Hz that simultaneously provides tidal ventilation. In four of the subjects, data were acquired before and during a methacholine (MCh)-bronchoconstricted challenge. The RL and EL data were first analyzed by using a model with a homogeneous airway compartment leading to a viscoelastic tissue compartment consisting of tissue damping and elastance parameters. Our OVW-based estimates of Raw correlated well with estimates obtained by using standard plethysmography and were responsive to MCh-induced bronchoconstriction. Our data suggest that Rti comprises approximately 40% of total RL at typical breathing frequencies, which corresponds to approximately 60% of intrathoracic RL. During mild MCh-induced bronchoconstriction, Raw accounts for most of the increase in RL. At high doses of MCh, there was a substantial increase in RL at all frequencies and in EL at higher frequencies. Our analysis showed that both Raw and Rti increase, but most of the increase is due to Raw. The data also suggest that widespread peripheral constriction causes airway wall shunting to produce additional frequency dependence in EL.

Airway and tissue constrictions are greater in closed than in open-chest conditions

We measured lung impedance (Z L) before and after four doses of methacholine (Mch) infusion in five intact chest (with esophageal balloon) and six open-chest dogs from 0.2 to 8 Hz with an optimal ventilator waveform. From Z l, we estimated airway resistance (R aw) and inertance (I aw) and tissue viscance (G l) and elastance (H l). Two-way analysis of variance revealed that: (1) Mch had a strong influence on all parameters ( p<0.001), but small effect on hysteresivity, η l=G l/H l; (2) closed-chest G l and H l were significantly higher and I aw lower than their open-chest values ( p<0.002, p<0.05 and p<0.0001); and (3) at the highest Mch dose, the relative increase in R aw was six times higher in the closed-chest condition. The reduced impact of Mch on open-chest mechanics may be due to constrictions superimposed on grossly different lung configurations and/or some humoral effects initiated by the thoracotomy. We conclude that Mch doses that elicit mild constriction in open-chest condition can cause a severe constriction in intact animals.

Two-frequency analysis of respiratory mechanics in artificially ventilated rabbits

The frequency dependence of respiratory mechanical properties was studied in 10 paralyzed, artificially ventilated rabbits, by superimposing a single sinusoidal signal with a frequency of 10, 20 or 30 Hz upon the ventilator waveform. The tracheal pressure and flow signals were analyzed both with the usual first order model, which provided total respiratory elastance (Ers) and resistance (Rrs), and by Fourier analysis, which provided respiratory impedance (Zrs) at the breathing frequency (0.85 Hz) and at the superimposed oscillation frequency. The real part of Zrs (Re(Zrs)) decreased by 30% from 0.85 to 10 Hz ( P<0.001), but did not vary significantly from 10 to 30 Hz. This finding is satisfactorily explained by tissue viscoelasticity. Following a histamine aerosol, the frequency dependence of Re(Zrs) changed very little in three out of four rabbit, but increased substantially in the fourth. In that instance, assuming that lung hysteresivity was not markedly modified by histamine, the results suggest inhomogeneous airway obstruction and/or airway wall shunting.

Airway and tissue mechanics during physiological breathing and bronchoconstriction in dogs

In five open-chest dogs and with four to five alveolar capsules we used an optimal ventilator waveform (OVW) to follow frequency and tidal volume (VT) dependence of lung, airway, and tissue resistance (R) and elastance (E) before and during constant infusion of histamine (16 micrograms.kg-1.min-1). OVW contains sufficient flow energy between 0.234 and 4.7 Hz, avoids nonlinear harmonic interactions, and simultaneously ventilates with physiological VT. Each OVW breath permits a smooth estimate of frequency dependence of R and E for the whole lung. A constant-phase model analysis provided estimates of purely viscous resistance (Rvis), which represents the sum of airway resistance (Raw) and any purely newtonian component of tissue resistance (Rti), and parameters G and H, which govern frequency dependence of Rti and tissue elastance (Eti), respectively. Tissue structural damping (eta) is calculated as G/H. This model was applied to the whole lung and tissue impedance as estimated from each capsule. We found a small but inconsequential purely newtonian component of Rti, even during constriction. Four dogs showed a peak response at approximately 4 min in lung Rvis coupled (in time) to initial increases in G, H, eta, and airway inhomogeneities. In two of these dogs the response was severe. Tissue properties estimated from whole lung impedance (G, H, and eta) were nearly identical to values estimated from unobstructed capsules throughout infusion. By using a technique independent of alveolar capsules, our results indicate that a major if not dominant response to a constrictive agonist occurs in lung tissues, resulting in a large increase in Rti and Eti. With severe constriction, significant increases occur in Raw and airway inhomogeneities as well. Finally, separation of airway and tissue properties using input impedance estimated from the frequency-rich OVW avoids use of alveolar capsules and may prove an effective tool for partitioning airway and tissue properties in humans.

Low-frequency respiratory mechanics using ventilator-driven forced oscillations

We evaluated the potential for using a fast Fourier transform (FFT) analysis applied to a standard ventilator waveform to estimate (< 2 Hz) frequency dependence of respiratory or lung resistance (R) and elastance (E). In four healthy humans we measured pressure and flow at the airway opening while applying sine wave forcing from 0.2 to 0.6 Hz at two tidal volumes (VT; 250 and 500 ml). We then applied a step inspiratory ventilator flow wave with relaxed expiration at the same VT and only 0.2 Hz. Step waveform data were also acquired from nine mechanically ventilated patients under intensive care unit conditions. Finally, we simultaneously measured total respiratory (rs), lung (L), and chest wall (cw) impedance data from two dogs (0.156-2 Hz) before and after severe pulmonary edema. Rrs and Ers were estimated by the FFT approach. Humans displayed a small frequency dependence in Rrs and Ers from 0.2 to 0.6 Hz, and both Rrs and Ers decreased at the higher VT. The spectral estimates of Rrs and Ers with the step ventilator wave were often qualitatively comparable to sine wave results below 0.6 Hz but became extremely erratic above the third harmonic. Conversely, in dogs the step wave produced reliable and stable estimates up to 2 Hz in all conditions. Nevertheless, Ecw and Ers still displayed clear and correlated oscillations with increasing frequency, whereas EL showed none. This suggests that nonlinear processes, most likely at the chest wall, contribute to periodic-like fluctuations in respiratory mechanical properties when estimated by applying FFT to a step ventilator wave. Moreover, in humans, but not dogs, a ventilator flow cycle contains insufficient signal energy beyond the third harmonic. We show that the amount of energy available at higher frequencies is largely governed by the mechanical time constant contributing to passive expiratory flow. In dogs the shorter time constant contributes to increased energy. In essence, the frequency content of the flow is subject dependent, and this is not a desirable situation for controlling the quality of the impedance spectra available from a standard ventilator wave.

Optimal ventilation waveforms for estimating low-frequency respiratory impedance.

We present a broad-band optimal ventilator waveform (OVW), the concept of which was to create a computer-driven ventilator waveform containing increased energy at specific frequencies (f). Values of f were chosen such that nonlinear harmonic distortion and intermodulation were minimized. The phases at each f were then optimized such that the resulting flow waveform delivered sufficient volume to maintain gas exchange while minimizing peak-to-peak airway opening pressure. Simulations with a linear anatomically consistent branching airway model and a nonlinear viscoelastic model showed that respiratory resistance (Rrs) and elastance (Ers) estimates at 0.1-2 Hz from the OVW are far superior to those from a standard step ventilator waveform (SVW) during healthy and obstructed conditions and that the OVW reduces the influences of harmonic interactions. Using a servo-controlled oscillator, we applied individual sine waves, an OVW containing energy at 0.15625-2.4 Hz, and an SVW to healthy humans and one symptomatic asthmatic subject before and after bronchodilation. The OVW was markedly superior to the SVW and always provided smooth estimates of Rrs and Ers. Before bronchodilation in the asthmatic subject Rrs was highly elevated and Ers was markedly increased with f; after bronchodilation the level of Rrs and the f dependence of Ers decreased. Although based on results from only one asthmatic subject, these data suggest a dominant influence of airway constriction and lung inhomogeneities during asthmatic bronchoconstriction that is alleviated by bronchodilators. These and other results indicate that the OVW approach has high potential for simultaneously probing f and amplitude dependence in the mechanical properties of clinical subjects during physiological breathing conditions and perhaps during dynamic bronchoconstriction.

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.