Soren Sondergaard

Estimation of functional residual capacity at the bedside using standard monitoring equipment: a modified nitrogen washout/washin technique requiring a small change of the inspired oxygen fraction.

We developed a modified nitrogen washin/washout technique based on standard monitors using inspiratory and end-tidal gas concentration values for functional residual capacity (FRC) measurements in patients with acute respiratory failure (ARF). For validation we used an oxygen-consuming lung model ventilated with an inspiratory oxygen fraction (Fio2) between 0.3 and 1.0. The respiratory quotient of the lung model was varied between 0.7 and 1.0. Measurements were performed changing Fio2 with fractions of 0.1, 0.2, and 0.3. In 28 patients with ARF, duplicate measurements were performed. In the lung model, an Fio2 change of 0.1 resulted in a value of 103 ± 5% of the reference FRC value of the lung model, and the precision was equally good up to an Fio2 of 1.0 with a value of 103 ± 7%. In the patients, duplicate measurements showed a bias of −5 mL with a 95% confidence interval [−38; 29 mL ]. A comparison of a change in Fio2 of 0.1 with 0.3 showed a bias of −9 mL and limits of agreement of [−365; 347 mL]. This study shows good precision of FRC measurements with standard monitors using a change in Fio2 of only 0.1. Measurements can be performed with equal precision up to an Fio2 of 1.0.

Warning! Suctioning. A lung model evaluation of closed suctioning systems

Background: Closed system suctioning, CSS, has been advocated to avoid alveolar collapse. However, ventilator manufacturers indicate that extreme negative pressure levels can be obtained during closed system suctioning, impeding the performance of the ventilator.

Direct Measurement of Intratracheal Pressure in Pediatric Respiratory Monitoring

We describe a method based on a Fabry-Perot interferometer at the tip of an optic fiber with a diameter of 0.25 mm for direct measurement of tracheal pressure in pediatric respiratory monitoring. The response time of the pressure transducer and its influence on the resistance of pediatric endotracheal tubes (internal diameter, 2.5 to 5 mm) during constant and dynamic flow at different ventilator settings in a lung model were measured. The transducer was positioned at −1.5 (inside), 0, and +1.5 cm (outside) relative to the tip of the endotracheal tube and compared with a reference pressure inside the trachea. The clinical application of the transducer was tested in five pediatric patients. The response time of the transducer was 1.3 ms. The influence of the fiberoptic transducer on tube resistance was negligible during constant flow in inspiratory and expiratory directions for all endotracheal tubes tested. There was no difference in pressure measurements with the transducer positioned at or 1.5 cm below or above the tip of the endotracheal tube during dynamic measurements. During clinical circumstances insertion of the fiberoptic transducer was easy, recordings were stable, and the safety of the patient was not jeopardized. The fiberoptic transducer provided a reliable and promising way of monitoring tracheal pressure in intubated pediatric patients. The presence of the probe did not interfere with either pressure-flow relationship or patient care and safety. The technique is proposed for monitoring of respiratory mechanics and calculation of changes in tube resistance caused by kinking and secretions.

Evaluation of pressure/volume loops based on intratracheal pressure measurements during dynamic conditions

Background: The aim of this study was to evaluate and compare information about lung mechanics obtained by dynamic pressure/volume loops based on Y‐piece and intratracheal airway pressure.

A new method for non-invasive, manoeuvre-free determination of “static” pressure–volume curves during dynamic/therapeutic mechanical ventilation

Background: Lung mechanics are usually measured using static or quasistatic methods, abandoning normal ventilatory treatment. We have developed a method to calculate the alveolar pressure during dynamic/therapeutic conditions, “the dynostatic pressure” (Pdyn), using airway pressure (P) measured in the trachea and volume (V) and flow (V˙) at the Y‐piece.

Pavane for a pulse pressure variation defunct.

Hemodynamic management of critically ill patients in the ICU or high-risk patients in the operating room has paradoxically shown progress in terms of outcome after the systematic application of volume responsiveness/flow optimization based on pulse pressure variation and/or stroke volume variation during controlled, positive-pressure ventilation in patients without spontaneous respiratory efforts. This assessment of circulatory optimization should ideally be based on an exhaustive, predictive and coherent physiological understanding of the cardiovascular system model. This paper sketches the extremely complex physiological background of the concept of volume responsiveness, concluding that it is not a reliable means of guiding hemodynamic optimization because it is based on a nonexhaustive, nonpredictive and incoherent physiological model.

Central venous pressure: we need to bring clinical use into physiological context.

The place of central venous pressure (CVP) measurement in acute care has been questioned during the past decade. We reviewed its physiological importance, utility and clinical use among anaesthetists and intensivists.

Effect of PEEP, blood volume, and inspiratory hold maneuvers on venous return.

According to Guytons model of circulation, mean systemic filling pressure (MSFP), right atrial pressure (RAP), and resistance to venous return (RVR) determine venous return. MSFP has been estimated from inspiratory hold-induced changes in RAP and blood flow. We studied the effect of positive end-expiratory pressure (PEEP) and blood volume on venous return and MSFP in pigs. MSFP was measured by balloon occlusion of the right atrium (MSFPRAO), and the MSFP obtained via extrapolation of pressure-flow relationships with airway occlusion (MSFPinsp_hold) was extrapolated from RAP/pulmonary artery flow (QPA) relationships during inspiratory holds at PEEP 5 and 10 cmH2O, after bleeding, and in hypervolemia. MSFPRAO increased with PEEP [PEEP 5, 12.9 (SD 2.5) mmHg; PEEP 10, 14.0 (SD 2.6) mmHg, P = 0.002] without change in QPA [2.75 (SD 0.43) vs. 2.56 (SD 0.45) l/min, P = 0.094]. MSFPRAO decreased after bleeding and increased in hypervolemia [10.8 (SD 2.2) and 16.4 (SD 3.0) mmHg, respectively, P < 0.001], with parallel changes in QPA Neither PEEP nor volume state altered RVR (P = 0.489). MSFPinsp_hold overestimated MSFPRAO [16.5 (SD 5.8) vs. 13.6 (SD 3.2) mmHg, P = 0.001; mean difference 3.0 (SD 5.1) mmHg]. Inspiratory holds shifted the RAP/QPA relationship rightward in euvolemia because inferior vena cava flow (QIVC) recovered early after an inspiratory hold nadir. The QIVC nadir was lowest after bleeding [36% (SD 24%) of preinspiratory hold at 15 cmH2O inspiratory pressure], and the QIVC recovery was most complete at the lowest inspiratory pressures independent of volume state [range from 80% (SD 7%) after bleeding to 103% (SD 8%) at PEEP 10 cmH2O of QIVC before inspiratory hold]. The QIVC recovery thus defends venous return, possibly via hepatosplanchnic vascular waterfall.

Electrical impedence tomography and heterogeneity of pulmonary perfusion and ventilation in porcine acute lung injury

Background: The heterogeneity of pulmonary ventilation (V), perfusion (Q) and V/Q matching impairs gas exchange in an acute lung injury (ALI). This study investigated the feasibility of electrical impedance tomography (EIT) to assess the V/Q distribution and matching during an endotoxinaemic ALI in pigs.

Can baroreflex sensitivity and heart rate variability predict late neurological outcome in patients with traumatic brain injury

Background: Previous studies have suggested that depressed heart rate variability (HRV) and baroreflex sensitivity (BRS) are associated with early mortality and morbidity in patients with acute brain injuries of various etiologies. The aim of the present study was to assess changes in HRV and BRS in isolated traumatic brain injury (TBI), with the hypothesis that measurement of autonomic nervous system dysfunction can provide prognostic information on late neurological outcome. Materials and Methods: Nineteen patients with TBI, requiring mechanical ventilation, sedation and analgesia, and with arterial and intracranial pressure monitoring for at least 1 week, were included. Physiological and treatment variables were collected and power spectral analyses of HRV and BRS analyses in time domain were performed daily. HRV in the high-frequency (HF) and low-frequency (LF) domains, as well as LF/HF ratio and total power, were investigated. The power of these variables to predict poor (Glasgow Outcome Scale Extended [GOSE] score <5), late (1 y) neurological outcome was assessed. Results: Total power, LF, HF, and BRS were all significantly depressed in patients with GOSE score <5. This difference could not be explained by a more severe brain injury at admission or more extensive use of sedative or analgesic drugs. The autonomic variables predicted the late neurological outcome with areas under the receiver-operating curves between 0.78 and 0.83 (sensitivity: 0.63 to 0.88 and specificity: 0.73 to 0.82). Conclusions: HRV and BRS measures in TBI patients during intensive care treatment, including sedative, analgesic, and vasoactive drugs, may identify patients with poor late neurological outcome.