Ben Fabry

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.

Breathing pattern and additional work of breathing in spontaneously breathing patients with different ventilatory demands during inspiratory pressure support and automatic tube compensation

Objective: We designed a new ventilatory mode to support spontaneously breathing, intubated patients and to improve weaning from mechanical ventilation. This mode, named Automatic Tube Compensation (ATC), compensates for the flow-dependent pressure drop across the endotracheal tube (ETT) and controls tracheal pressure to a constant value. In this study, we compared ATC with conventional patient-triggered inspiratory pressure support (IPS). Design: A prospective, interventional study. Setting: A medical intensive care unit (ICU) and an ICU for heart and thoracic surgery in a university hospital. Patients: We investigated two groups of intubated, spontaneously breathing patients: ten postoperative patients without lung injury, who had a normal minute ventilation (VE) of 7.6 ± 1.7 l/min, and six critically ill patients who showed increased ventilatory demand (VE = 16.8 ± 3.0 l/min). Interventions: We measured the breathing pattern [VE, tidal volume (VT), and respiratory rate (RR)] and additional work of breathing (WOBadd) due to ETT resistance and demand valve resistance. Measurements were performed under IPS of 5, 10, and 15 mbar and under ATC. Results: The response of VT, RR, and WOBadd to different ventilatory modes was different in both patient groups, whereas VE remained unchanged. In postoperative patients, ATC, IPS of 10 mbar, and IPS of 15 mbar were sufficient to compensate for WOBadd. In contrast, WOBadd under IPS was greatly increased in patients with increased ventilatory demand, and only ATC was able to compensate for WOBadd. Conclusions: The breathing pattern response to IPS and ATC is different in patients with differing ventilatory demand. ATC, in contrast to IPS, is a suitable mode to compensate for WOBadd in patients with increased ventilatory demand. When WOBadd was avoided using ATC, the patients did not need additional pressure support.

Respiratory comfort of automatic tube compensation and inspiratory pressure support in conscious humans

ObjectiveTo compare the new mode of ventilatory Support, which we call automatic tube compensation (ATC), with inspiratory pressure support (IPS) with respect to pereeption of respiratory comfort. ATC unloads the resistance of the endotracheal tube (ETT) in inspiration by increasing the airway pressure, and in expiration by decreasing the airway pressure aecording to the non-linear pressure-flow relationship of the ETT.DesignProspective randomized single blind cross-over study.SettingLaboratory of the Section of Experimental Anaesthesiology (Clinic of Anaesthesiology; University of Freiburg).SubjectsTen healthy volunteers.InterventionsThe subjects breathed spontaneously through an ETT of 7.5 mm i. d. Three different ventilatory modes, each with a PEEP of 5 cmH2O, were presented in random order using the Dräger Evita 2 ventilator with prototype software:(1)IPS (10 cmH2O,1 s ramp),(2)inspiratory ATC (ATC-in),(3)inspiratory and expiratory ATC (ATC-in-ex).Measurements and main resultsImmediately following a mode transition, the volunteers answered with a hand sign to show how they perceived the new mode compared with the preceding mode in terms of gain or loss in subjective respiratory comfort: “better”, “unchanged” or “worse”. Inspiration and expiration were investigated separately analyzing 60 mode transitions each. Flow rates were continuously measured. The transition from IPS to either type of ATC was perceived positively, i.e. as increased comfort, whereas the opposite transition from ATC to IPS was perceived negatively, i. e. as decreased comfort. The transition from ATC-in to ATC-in-ex was perceived positively whereas the opposite mode transition was perceived negatively in expiration only. Tidal volume was 1220 ± 404 ml during IPS and 1017 ± 362 ml during ATC. The inspiratory peak flow rate was 959 ± 78 ml/s during IPS and 1048 ± 197 ml/s during ATC.ConclusionsATC provides an increase in respiratory comfort compared with IPS. The predominant cause for respiratory discomfort in the IPS mode seems to be lung over-inflation.

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.

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.

Effects of mechanical unloading/loading on respiratory loop gain and periodic breathing in man

We investigated the effect of mechanical unloading and loading on Cheyne-Stokes respiration (CSR) in seven intubated patients with preexisting CSR. For mechanical loading patients had to breathe against the resistance of the endotracheal tube. For mechanical unloading patients were supported with a volume-proportional pressure support in the proportional assist ventilation (PAV) mode whilst the flow-dependent (nonlinear) endotracheal tube resistance was continuously compensated for by means of the automatic tube compensation (ATC) mode. Mechanical unloading aggravated CSR as revealed by a prolongation of apnea time and by an increase in the so-called strength index whereas mechanical loading shortened apnea time and decreased strength index. To test whether the observed changes are caused by the effect of mechanical unloading/loading on respiratory loop gain (relationship between minute ventilation and arterial CO2 tension), the response of respiratory loop gain on mechanical unloading/loading was determined in five healthy subjects (without CSR). In each subject, mechanical unloading increased respiratory loop gain whereas mechanical loading decreased it.

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.

Influence of mechanical loading and unloading on respiratory controller gain in healthy subjects

Conclusions(1) Mechanical unloading (using PAV) increases respiratory controller gain, and mechanical loading decreases it. (2) The observed amplification of periodic breathing under PAV can thus be explained by an increase in respiratory controller gain.