Luc Eberhard

Continuous calculation of intratracheal pressure in tracheally intubated patients

Background:Intratracheal pressure (Ptrach) should be the basis for analysis of lung mechanics. If measured at all, Ptrach is usually assessed by introducing a catheter into the trachea via the lumen of the endotracheal tube (ETT). The authors propose a computer-assisted method for calculating Ptrach on a point-by-point basis by subtracting the flow-dependent pressure drop ΔPETT(&OV0312;) across the ETT from the airway pressure (Paw), continuously measured at the proximal end of the ETT. Methods:The authors measured the pressure-flow relationship of adult endotracheal tubes with different diameters (ID, 7–9 mm) at different lengths and of tracheostomy tubes (ID, 8–10 mm) in the laboratory. The coefficients of an approximation equation were fitted to the measured pressure-flow curves separately for inspiration and expiration. In 15 tracheally intubated patients under volume-controlled ventilation and spontaneous breathing, the calculated Ptrach was compared with the measured Ptrach. Results:The authors present the coefficients of the “nonlinear approximation”: ΔPETT = K1 · &OV0312;K2, with ΔPETT being the pressure drop across the ETT and K1 and K2 being the coefficients relating &OV0312; to ΔPETT. An important result was an inspiration/expiration asymmetry: the pressure drop caused by the inspiratory flow exceeds that of the expiratory flow. A complete description of the pressure-flow relationship of an ETT, therefore, requires a set of four coefficients: K1I, K2I, K1E, and K2E. The reason for this asymmetry is the abrupt sectional change between ETT and trachea and the asymmetric shape of the swivel connector. Comparison of calculated and measured Ptrach in patients gives a correspondence within ± 1 cmH2O (mean limits of agreement). The mean root-mean-square (rms) deviation is 0.55 cmH2O. Conclusions:Ptrach can be monitored by combining our ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.

Determination of volume-dependent respiratory system mechanics in mechanically ventilated patients using the new SLICE method.

In patients mechanically ventilated for severe respiratory failure, respiratory system mechanics are non-linear, i.e., volume-dependent. We present a new computer-based multipoint method for simultaneously determining volume-dependent dynamic compliance and resistance. Our method is based on continuously determined tracheal pressure (Ptrach). Tidal volume is subdivided into six volume slices of equal size. One compliance value (intrinsic PEEP considered) and one resistance value are determined for each volume slice by applying of the least-squares-fit (LSF) analysis based on the linear RC-model; we therefore call this the SLICE method. The method gives the course of dynamic compliance and resistance within the tidal volume. The method was evaluated using physical models of the respiratory system with linear and non-linear passive mechanical properties. The relative error of the method is smaller than ±5%. The method needs no special ventilatory pattern. Using data from 14 patients mechanically ventilated for adult respiratory distress syndrome (ARDS) we found a very good correspondence between the measured end-inspiratory airway pressure (Paw,Ie) and the end-inspiratory alveolar pressure (Palv,Ie) calculated from the dynamic compliance values determined with the SLICE method (Palv,Ie = 1.02 * Paw,Ie + 0.097; r2 = 0.977). The SLICE method allows continuous monitoring of non-linear pulmonary mechanics on a breath-by-breath basis at the bedside.

Automatic compensation of endotracheal tube resistance in spontaneously breathing patients

The considerable additional ventilatory work needed to overcome the resistance of the endotracheal tube (ETT) is flow-dependent. In spontaneously breathing intubated patients this additional ventilatory work is therefore dependent on the flow pattern and cannot be adequately compensated for by support with a constant pressure. We propose a method to fully compensate for the ETT resistance during inspiration and expiration by regulating tracheal pressure (Ptrach),Ptrach is calculated at a rate of 500 Hz by measurement of flow and pressure at the outer end of the ETT and from coefficients describing the flow-dependent ETT resistance. The calculated tracheal pressure is fed into a modified demand-flow ventilator which can then control tracheal pressure to a target value (Ptrach,targ). Tracheal pressure can either be kept constant (automatic tube compensation, ATC), or changed in any chosen fashion. We tested our system on a laboratory lung model simulating a spontaneously breathing patient. Even under the simulation of extreme conditions the maximum deviation of Ptrach from Ptrach,targ was smaller than 2.5 mbar. We evaluated our system in 10 spontaneously breathing intubated patients breathing at ATC with or without volume proportional pressure support (VPPS) by measuring Ptrach. The mean maximum deviation of Ptrach from Ptrach,targ was 2.9 mbar. The rms-deviation was 1.1 mbar (inspiration and expiration considered) and 1.7 mbar (inspiration alone). The accuracy of the control of Ptrach is thus comparable to the control of airway pressure afforded by the unmodified demand-flow ventilator.

Detection of endotracheal tube obstruction by analysis of the expiratory flow signal.

Objective: Acute obstruction of endotracheal tubes (ETT) increases airway pressure, decreases tidal volume, increases the risk of dynamic hyperinflation by prolonging the duration of passive expiration, and prevents reliable calculation of tracheal pressure. We propose a computer-assisted method for detecting ETT obstruction during controlled mechanical ventilation. The method only requires measurement of the expiratory flow. Design: Computer simulation; prospective study in two cases; retrospective study in one case and in seven patients with the adult respiratory distress syndrome (ARDS). Setting: Laboratory of the Section of Experimental Anaesthesiology (University of Freiburg); surgical adult intensive care units in a university hospital (University of Basel) and in a university affiliated hospital (Zentralklinikum Augsburg). Patients: 3 patients with partial ETT or bronchial obstructions and 7 ARDS patients. Measurements and results: Expiratory flow was measured using a pneumotachograph and integrated to obtain expiratory volume. The time-constant of passive expiration (τE) as a function of expired volume [τE(VE) function] was calculated from the expiratory volume/flow curve. We investigated the τE(VE) function of data obtained from: (1) computer simulation of mechanically ventilated homogeneous and inhomogeneous lungs intubated with ETTs of different sizes; (2) one patient with an artificial ETT obstruction of 7.5 and 25 % of the cross-sectional area of the ETT (case 1); (3) one patient with ETT obstruction due to secretions (case 2); (4) one patient with acute bronchial constriction (case 3); (5) seven ARDS patients who showed an increase in airway resistance of more than 2 cm H2O · s/l. It was found that an ETT obstruction caused an increase in τE in early expiration (at high flow), whereas τE in late expiration was virtually unchanged. The reason for this is the flow dependency of the increase in ETT resistance produced by ETT obstruction. Unlike ETT obstruction, an increase in pure airway resistance produced an increase in τE throughout expiration. Conclusions: An ETT obstruction can be reliably distinguished from an increase in pure airway resistance by a characteristic pattern change in the τE(VE) function, which can be detected easily even by an automated pattern recognition system.

Delayed derecruitment after removal of PEEP in patients with acute lung injury.

Background:A step decrease in positive end‐expiratory airway pressure (PEEP) is not followed by an instantaneous loss of the PEEP‐induced increase in end‐expiratory lung volume (EELV). Rather, the reduction of EELV is delayed, while adverse PEEP effects on hemodynamics are immediately attenuated upon the drop in airway pressure. Step PEEP increments were applied to the lungs of patients with acute lung injury. It was investigated retrospectively whether enlargement of end‐expiratory lung volume and changes in lung mechanics persist 45 min after removal of the PEEP increment.

Polymorphous Ventilation: A New Ventilation Concept for Distributed Time Constants

After a laparotomy the arterial PO2 is reduced and only comes back to preoperative values after about 5 days. This was already interpreted in the 1930s as being connected with another postoperative finding, the reduction in pulmonary gas volume (Beecher 1933 a, b; Beecher et al. 1933; Bendixen 1964; Bendixen and Laver 1965; Berggren 1942). In the following years many aspects of the anesthesia, surgical procedure, and mechanical ventilation were discussed as possible causes of this hypoxemia, including among others the altered pattern of diaphragmatic movements (Westbrook et al. 1973; Froese and Bryan 1974; Rehder et al. 1975), a change in the surfactant factor (Clemens et al. 1961), and the shift of blood volume. It is now generally accepted that the above mentioned reduction in pulmonary gas volume has something to do with the cause of hypoxemia. This connection is the rational basis of volume controlled ventilation with PEEP, which in the last two decades has been successfully employed with respect to oxygenation. However, it is only in recent years that an understanding of the chain of events has been conclusively elaborated and the speed at which these events develop worked out (Hedenstierna et al. 1985a, b, 1989; Rehder et al. 1985; Tokics et al. 1987): After intubation and mechanical ventilation a sizable volume of dependent lung regions collapses within 3 min. This occurs even in patients who originally had normal lungs.

Respiratory mechanics II

To determine whether i.v.NAC has beneficial effects in patients with mild-to-moderate ALI in terms of ventilatory support(VS),FIO2 requirement-,evolution of the lung injury score(LIS),development of severe lung injury(ARDS)and mortality rate,we prospectively enrolled 61 adult patients with ALI to receive either NAC 40 mg/kg/day or Placebo(PL)during 3 days.Respiratory dysfunction was assessed daily considering the need of VS,the F102 necessary to achieve a Pa02 of 70 to 80 mmHg and the evolution of 3 components of the LIS (chest X-ray,Pa02-FIO2 ratio and respiratory system compliance).Data were collected at baseline (day 0),on the first 3 days after admission to the ICU and on discharge.NAC and PL groups(32 vs 29 patients)were comparable at entry in terms of SAPS and values of the LIS.At day 0, 69% of the patients were ventilated in the NAC group versus 76% in the PL group;at day 3, 83% of the NAC treated patients did not require any further VS, versus 52% in the PL group(p=0.01).Pa02/FIO2 improved significantly(p=0.05)from day 0 to day 3 only in the NAC group.The LIS showed a signifi cant improvement(p=0.003)in the NAC treated group within the first 10 days of treatment;no change was observed in the PL group.3 patients in each group progressed to ARDS.The one-month mortality rate was 22% for the NAC and 35% for the PL group In conclusion,early treatment with NAC seems to affect favourably pulmonary gas exchange and decrease the need for prolonged VS in patients with mild-to-moderate ALI.