Andreas Bloch

Effect of Very-High-Flow Nasal Therapy on Airway Pressure and End-Expiratory Lung Impedance in Healthy Volunteers

BACKGROUND: Previous research has demonstrated a positive linear correlation between flow delivered and airway pressure generated by high-flow nasal therapy. Current practice is to use flows over a range of 30–60 L/min; however, it is technically possible to apply higher flows. In this study, airway pressure measurements and electrical impedance tomography were used to assess the relationship between flows of up to 100 L/min and changes in lung physiology. METHODS: Fifteen healthy volunteers were enrolled into this study. A high-flow nasal system capable of delivering a flow of 100 L/min was purpose-built using 2 Optiflow systems. Airway pressure was measured via the nasopharynx, and cumulative changes in end-expiratory lung impedance were recorded using the PulmoVista 500 system at gas flows of 30–100 L/min in increments of 10 L/min. RESULTS: The mean age of study participants was 31 (range 22–44) y, the mean ± SD height was 171.8 ± 7.5 cm, the mean ± SD weight was 69.7 ± 10 kg, and 47% were males. Flows ranged from 30 to 100 L/min with resulting mean ± SD airway pressures of 2.7 ± 0.7 to 11.9 ± 2.7 cm H2O. A cumulative and linear increase in end-expiratory lung impedance was observed with increasing flows, as well as a decrease in breathing frequency. CONCLUSIONS: Measured airway pressure and lung impedance increased linearly with increased gas flow. Observed airway pressures were in the range used clinically with face-mask noninvasive ventilation. Developments in delivery systems may result in this therapy being an acceptable alternative to face-mask noninvasive ventilation.

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.

Understanding circulatory failure in sepsis.

Introduction Septic shock or acute circulatory failure in sepsis causes a mismatch between tissue perfusion and metabolic demands. The heart, the vasculature and alterations in various tissue and cellular functions are involved in the pathophysiology. The clinical presentation can be highly variable, changes over time and is modified by preceding and concomitant treatment and comorbidities. The clinical hallmarks of septic shock are signs of tissue hypoperfusion, hypotension or need for vasopressors to prevent hypotension, despite adequate fluid resuscitation. Signs of tissue hypoperfusion vary and can include impaired capillary perfusion, oliguria, elevated blood lactate and altered mentation. The blood pressure level that is clinically relevant varies between patients, and “adequate” fluid resuscitation is highly subjective. Therefore, septic shock defies explicit, objective definitions, as shown by the current debate around attempts to define it [1, 2]. Nevertheless, increasing severity of circulatory failure is associated with increasing mortality [3]. Delayed treatment increases the severity of circulatory failure in sepsis, necessitates more support with fluids and vasoactive drugs, and increases mortality [4].

Impact of Simulator-Based Training in Focused Transesophageal Echocardiography: A Randomized Controlled Trial.

BACKGROUND: The aim of the study was to determine if training in transesophageal echocardiography (TEE) using a TEE simulator improves the ability of novice operators to perform and interpret a focused critical care TEE. METHODS: In this prospective, randomized, controlled study with blinded outcome assessment, 44 intensive care unit trainees were randomly assigned to a control group receiving 4 hours of lecture-based training only, or an intervention group which was additionally trained for 4 hours using a TEE simulator. After the training intervention, each participant performed 2 TEEs in intensive care unit patients which were evaluated by blinded assessors. The imaging quality of TEEs was measured using a predefined examination quality score ranging from 0 to 100 points. The correct quantification of pathologies and the interpretation of the TEEs were evaluated by blinded assessors using focused and comprehensive expert TEEs as comparators. RESULTS: A total of 114 TEEs were assessed. The mean examination quality score was 55.9 (95% confidence interval [CI], 50.3–61.5) for TEEs of the control group, 75.6 (95% CI, 70.1–81.0) for TEEs of the intervention group, and 88.5 (95% CI, 79.3–97.7) for TEEs in the expert group. The multiple comparisons revealed significant differences between all groups (19.7 [95% CI, 12.8–26.6], P < .001 for intervention versus control; 32.6 [95% CI, 23.0–42.3], P < .001 for expert versus control; 12.9 [95% CI, 3.4–22.5], P = .008 for expert versus intervention). Substantial agreement of the quantification and interpretation ratings of basic TEEs by the intervention (86.7% for quantification and 97.1% for interpretation) or expert group (93.2% for quantification and 98.4% for interpretation) with blinded assessors was detected. The control groups TEEs agreed less (75.6% for quantification and 91.8% for interpretation). CONCLUSIONS: Simulation-based TEE training improves the ability of novice operators to perform a focused critical care TEE in comparison to lecture-based education only. After 8 hours of simulator and lecture-based training, the majority of TEEs of novices are of sufficient quality for clinical use. Furthermore, a substantial skill level in correct quantification and interpretation of imaging is achieved.

Right atrial pressure and venous return during cardiopulmonary bypass

The relevance of right atrial pressure (RAP) as the backpressure for venous return (QVR) and mean systemic filling pressure as upstream pressure is controversial during dynamic changes of circulation. To examine the immediate response of QVR (sum of caval vein flows) to changes in RAP and pump function, we used a closed-chest, central cannulation, heart bypass porcine preparation (n = 10) with venoarterial extracorporeal membrane oxygenation. Mean systemic filling pressure was determined by clamping extracorporeal membrane oxygenation tubing with open or closed arteriovenous shunt at euvolemia, volume expansion (9.75 ml/kg hydroxyethyl starch), and hypovolemia (bleeding 19.5 ml/kg after volume expansion). The responses of RAP and QVR were studied using variable pump speed at constant airway pressure (PAW) and constant pump speed at variable PAW Within each volume state, the immediate changes in QVR and RAP could be described with a single linear regression, regardless of whether RAP was altered by pump speed or PAW (r2 = 0.586-0.984). RAP was inversely proportional to pump speed from zero to maximum flow (r2 = 0.859-0.999). Changing PAW caused immediate, transient, directionally opposite changes in RAP and QVR (RAP: P ≤ 0.002 and QVR: P ≤ 0.001), where the initial response was proportional to the change in QVR driving pressure. Changes in PAW generated volume shifts into and out of the right atrium, but their effect on upstream pressure was negligible. Our findings support the concept that RAP acts as backpressure to QVR and that Guytons model of circulatory equilibrium qualitatively predicts the dynamic response from changing RAP.NEW & NOTEWORTHY Venous return responds immediately to changes in right atrial pressure. Concomitant volume shifts within the systemic circulation due to an imbalance between cardiac output and venous return have negligible effects on mean systemic filling pressure. Guytons model of circulatory equilibrium can qualitatively predict the resulting changes in dynamic conditions with right atrial pressure as backpressure to venous return.

Gastrointestinal Impedance Spectroscopy to Detect Hypoperfusion During Hemorrhage.

Background: Changes in tissue impedance (&OHgr;) have been proposed as early signs of impaired tissue perfusion. We hypothesized that hemorrhage may induce early changes in alimentary tract tissue impedance and that these can be detected by impedance spectroscopy. We evaluated impedance spectroscopy in an acute hemorrhage model in pigs. Methods: Twenty anesthetized pigs were randomized to stepwise hemorrhage to mean arterial blood pressure (MAP) targets of 60 mm Hg, 50 mm Hg, 45 mm Hg, and 40 mm Hg, followed by retransfusion in two steps, or control (n = 10 each). In the end, 500 mL of enteral nutrition was administered in both groups. &OHgr; in four sites (sublingually, esophagus, stomach, proximal jejunum) and cardiac output (Qtot thermodilution), superior mesenteric artery blood flow (QSMA; Doppler ultrasound), and jejunal mucosal blood flow (LDF; laser Doppler) were measured. Findings: The bleeding (total volume 838 ± 185 mL; mean ± SD) resulted in progressive hypotension (actual MAP 65 ± 3 mm Hg, 59 ± 4 mm Hg, 55 ± 5 mm Hg, and 46 ± 6 mm Hg) and decrease in Qtot, QSMA, and mucosal LDF. Bleeding did not change &OHgr; in any of the monitoring sites. Retransfusion restored the blood flows to at least baseline levels, again without change in &OHgr;. Enteral nutrition did not alter &OHgr; or any of the blood flows. Five animals (three in the hemorrhage group, two in the control group) had histologically proven acute gastric focal necrosis at the site of It transducer. Conclusions: Gastrointestinal impedance spectroscopy does not detect early changes in tissue perfusion during progressive hemorrhage or retransfusion. &OHgr; spectroscopy is unlikely to provide any additional information of hypovolemia-induced early changes in gastrointestinal perfusion.

Non-Invasive Assessment of Intra-Abdominal Pressure Using Ultrasound Guided Tonometry - a Proof-of-Concept Study.

Background: Intra-abdominal hypertension jeopardizes abdominal organ perfusion and venous return. Contemporary recognition of elevated intra-abdominal pressure (IAP) plays a crucial role in reducing mortality and morbidity. We evaluated ultrasound-guided tonometry in this context hypothesizing that the vertical chamber diameter of this device inversely correlates with IAP. Methods: IAP was increased in six 5 mmHg steps to 40 mmHg by instillation of normal saline into the peritoneal cavity of eight anesthetized pigs. Liver and renal blood flows (ultrasound transit time), intravesical, intraperitoneal, and end-inspiratory plateau pressures were recorded. For ultrasound-based assessment of IAP (ultrasound-guided tonometry), a pressure-transducing, compressible chamber was fixed at the tip of a linear ultrasound probe, and the system was applied on the abdominal wall using different predetermined levels of external pressure. At each IAP level (reference: intravesical pressure), two investigators measured the vertical diameter of this chamber. Results: All abdominal flows decreased (by 39%–58%), and end-inspiratory plateau pressure increased from 15 mbar (14–17 mbar) to 38 mbar (33–42 mbar) (median, range) with increasing IAP (all P < 0.01). Vertical chamber diameter decreased from 14.9 (14.6–15.2) mm to12.8 (12.4–13.4) mm with increasing IAP. Coefficients of variations between and within observers regarding change of the vertical tonometry chamber diameter were small (all <4%), and the results were independent of the externally applied pressure level on the ultrasound probe. Correlation of IAP and vertical pressure chamber distance was highly significant (r = −1, P = 0.0004). Ultrasound-guided tonometry could discriminate between normal (baseline) pressure and 15 mmHg, between 15 and 25 mmHg) and between 25 and 40 mmHg IAP (all P ⩽ 0.18). Similar results were obtained for end-inspiratory plateau pressures. Conclusions: In our model, values obtained by ultrasound-guided tonometry correlated significantly with IAPs. The method was able to discriminate between normal, moderately, and markedly increased IAP values.

Compression sonography for non-invasive measurement of lower leg compartment pressure in an animal model.

INTRODUCTION Compression ultrasound is a non-invasive technique allowing for qualitative visualization and quantitative measurements of mechanical tissue properties. In acute compartment syndrome (ACS), cadaver studies have proven that the intra-compartmental pressure (ICP) measured by compression sonography correlates with the ICP measured invasively. This study aimed to evaluate compression sonography for compartment pressure measurements in an animal model. MATERIAL AND METHODS The pressure in the anterior tibial compartment of 6 domestic pig legs was increased from baseline to 40mmHg in 5mmHg steps. Using compression sonography, the compartment diameter was measured without external pressure and during manual application of five levels of external pressure. The elasticity ratio (ER) was computed as the ratio of the compartment diameter with and without external pressure. At 40mmHg of external pressure the ERs at different ICP levels were compared using repeated ANOVA measurements. Post-hoc comparisons evaluated the lowest detectable ICP fulfilling the definition of ACS (ICP≥30mmHg) by starting from each pressure below 30mmHg (baseline, 20mmHg and 25mmHg, respectively). Receiver operator characteristic analyses defined ER limits with appropriate sensitivity and specificity to detect ACS. RESULTS The ER increased from 79.0% at baseline ICP to 89.3% at 40mmHg ICP. The ER at baseline and at 20mmHg ICP significantly differed from the ER at 30mmHg ICP (p=0.007 and 0.002, respectively); the ER at 25mmHg ICP significantly differed from the ER at 40mmHg ICP (p=0.001). An ER less than 87.1% had a sensitivity of 94.4% and a specificity of 88.9% to proper diagnosis of ACS. CONCLUSION Compression sonography might offer a non-invasive technique to guide treatment in cases of uncertain acute compartment syndrome. Further studies are needed to collect elasticity ratio data in humans and to clinically validate compression sonography for compartment pressure measurements.