Mathematical modeling of the impact of recirculation on exchange kinetics in tandem extracorporeal membrane oxygenation and therapeutic plasma exchange.

Vascular access connection configurations during tandem extracorporeal membrane oxygenation (ECMO) and therapeutic plasma exchange (TPE) may impact exchange kinetics. In these tandem procedures, typically the TPE inlet line is proximal to the TPE return line with respect to blood flow in the ECMO device, maximizing the opportunity for replacement fluid homogenization within the ECMO circuit. However, if TPE inlet and return line connections are switched, recirculation-a phenomenon in which replacement fluid leaving the TPE return line is prematurely drawn into the TPE inlet line prior to satisfactory homogenization within the ECMO circuit-will occur. Such recirculation could diminish TPE efficacy in patients on ECMO and mitigate therapeutic benefits. Using a mathematical model of recirculation in tandem ECMO and TPE, we demonstrate that the predicted impact of recirculation is negligible and vascular access connection positioning does not appear to be a point of clinical concern with regard to TPE kinetics.

Accuracy and Reliability of a Disposable Vascular Pressure Device for Arterial Pressure Monitoring in Critical Care Transport.

OBJECTIVE: Arterial catheterization is a commonly performed procedure in intensive care units to guide the management of critically ill patients who require precise hemodynamic monitoring; however, this technology is not always available in the transport setting because of cumbersome and expensive equipment requirements. We compared the accuracy and reliability of a disposable vascular pressure device (DVPD) with the gold standard (ie, the transducer pressure bag invasive arterial monitoring system) used in intensive care units to determine if the DVPD can be reliably used in place of the traditional pressure transducer setup. METHODS: This study was a single-center, prospective, observational study performed in the adult intensive care unit of a large academic university hospital. A convenience cohort of hemodynamically stable, adult critically ill patients with femoral, brachial, or radial arterial catheters was recruited for this study. The Compass pressure device (Centurion Medical Products, Williamston, MI) is a disposable vascular pressure-sensing device used to assure venous access versus inadvertent arterial access during central line placement. The DVPD was attached to an in situ arterial catheter and measures the mean intravascular pressure via an embedded sensor and displays the pressure via the integrated LCD screen. Using a 3-way stopcock, the DVPD was compared with the standard arterial setup. We compared the mean arterial pressure (MAP) in the standard setup with the DVPD using Bland-Altman plots and methods that accounted for repeated measures in the same subject. RESULTS: Data were collected on 14 of the 15 subjects enrolled. Five measurements were obtained on each patient comparing the DVPD with the standard arterial setup at 1-minute intervals over the course of 5 minutes. A total of 70 observations were made. Among the 15 subjects, most (10 [67%]) were radial or brachial sites. The average MAP scores and standard deviation values obtained by the standard setup were 83.5 mm Hg (14.8) and 81.1 mm Hg (19.3) using the DVPD. Just over half (51.4%) of the measurements were within a ± 5-mm Hg difference. Using Bland-Altman plotting methods, standard arterial measurements were 2.4 mm Hg higher (95% confidence interval, 0.60-4.1) than with the DVPD. Differences between the 2 devices varied significantly across MAP values. The standard arterial line measurements were significantly higher than the DVPD at low MAP values, whereas the DVPD measurements were significantly higher than the standard arterial line at high MAP values. CONCLUSION: The DVPD provides a reasonable estimate of MAP and may be suitable for arterial pressure monitoring in settings where standard monitoring setups are not available. The DVPD appears to provide "worst-case" values because it underestimates low arterial blood pressure and overestimates high arterial blood pressure. Future trials should investigate the DVPD under different physiological conditions (eg, hypotensive patients, patients with ventricular assist devices, and patients on extracorporeal membrane oxygenation), different patient populations (such as pediatric patients), and in different environments (prehospital, air medical transport, and austere locations).

Evaluation of a Disposable Vascular Pressure Device for Pre- and Postmembrane Pressure Monitoring During Venovenous Extracorporeal Membrane Oxygenation.

Membrane pressure monitoring during extracorporeal membrane oxygenation (ECMO) is integral to monitoring circuit health. We compared a disposable vascular pressure device (DVPD) to the transducer pressure bag arterial line (TPBAL) monitoring system to determine whether the DVPD can reliably and accurately monitor membrane pressures during venovenous extracorporeal membrane oxygenation (VV ECMO). We analyzed existing quality assurance data collected at a single center as part of routine circuit performance monitoring and process improvement on a convenience sample of four VV ECMO circuits. We placed and zeroed a DVPD in line with the pre- and postmembrane TPBAL setups in coordination with a standard transducer setup. We recorded DVPD and TPBAL pressure measurements every 4 hours for 2.5 days on the four separate VV ECMO circuits. We compared the standard and DVPD pressures using Bland-Altman plots and methods that accounted for repeated measures in the same subject. We recorded 58 pre/postmembrane pressures. Mean membrane pressure values were similar in the DVPD (pre: 208 mmHg [SD, 50.8]; post: 175 mmHg [46.3]) compared to the standard TPBAL setup (pre: 205 mmHg [52.0]; post: 177 mmHg [46.3]). Using Bland-Altman methods, premembrane pressures were found to be 2.2 mmHg higher (95% confidence interval [CI]: -5.3 to 9.7) in the standard TPBAL setup compared to DVPD and 1.8 mmHg higher (95% CI: -5.3 to 8.9) than the postmembrane pressures. The DVPD provided an accurate measurement of circuit pressure as compared to the TPBAL setup. Across the range of pre- and postmembrane pressures, both methods reliably agreed. Future trials should investigate DVPD accuracy in different environments such as prehospital field cannulation or critical care transport of ECMO patients.

Ultrasound determination of chest wall thickness: implications for needle thoracostomy.

OBJECTIVE: Computed tomography measurements of chest wall thickness (CWT) suggest that standard-length angiocatheters (4.5 cm) may fail to decompress tension pneumothoraces. We used an alternative modality, ultrasound, to measure CWT. We correlated CWT with body mass index (BMI) and used national data to estimate the percentage of patients with CWT greater than 4.5 cm. METHODS: This was an observational, cross-sectional study of a convenience sample. We recorded standing height, weight, and sex. We measured CWT with ultrasound at the second intercostal space, midclavicular line and at the fourth intercostal space, midaxillary line on supine subjects. We correlated BMI (weight [in kilograms]/height(2) [in square meters]) with CWT using linear regression. 95% Confidence intervals (CIs) assessed statistical significance. National Health and Nutrition Examination Survey results for 2007-2008 were combined to estimate national BMI adult measurements. RESULTS: Of 51 subjects, 33 (65%) were male and 18 (35%) were female. Mean anterior CWT (male, 2.1 cm; CI, 1.9-2.3; female, 2.3 cm; CI, 1.7-2.7), lateral CWT (male, 2.4 cm; CI, 2.1-2.6; female, 2.5 cm; CI 2.0-2.9), and BMI (male, 27.7; CI, 26.1-29.3; female, 30.0; CI, 25.8-34.2) did not differ by sex. Lateral CWT was greater than anterior CWT (0.2 cm; CI, 0.1-0.4; P < .01). Only one subject with a BMI of 48.2 had a CWT that exceeded 4.5 cm. Using national BMI estimates, less than 1% of the US population would be expected to have CWT greater than 4.5 cm. CONCLUSIONS: Ultrasound measurements suggest that most patients will have CWT less than 4.5 cm and that CWT may not be the source of the high failure rate of needle decompression in tension pneumothorax.

Models for plasma kinetics during simultaneous therapeutic plasma exchange and extracorporeal membrane oxygenation.

This paper focuses on the derivation and simulation of mathematical models describing new plasma fraction in blood for patients undergoing simultaneous extracorporeal membrane oxygenation and therapeutic plasma exchange. Models for plasma exchange with either veno-arterial or veno-venous extracorporeal membrane oxygenation are considered. Two classes of models are derived for each case, one in the form of an algebraic delay equation and another in the form of a system of delay differential equations. In special cases, our models reduce to single compartment ones for plasma exchange that have been validated with experimental data (Randerson et al., 1982, Artif. Organs, 6, 43-49). We also show that the algebraic differential equations are forward Euler discretizations of the delay differential equations, with timesteps equal to transit times through model compartments. Numerical simulations are performed to compare different model types, to investigate the impact of plasma device port switching on the efficiency of the exchange process, and to study the sensitivity of the models to their parameters.

Impact of positive pressure ventilation on thoracostomy tube removal.

BACKGROUND: Little data exist examining the impact of positive pressure ventilation on safe thoracostomy tube removal. We sought to evaluate the impact of positive-pressure ventilation (PPV) on recurrent pneumothoraces (PTX) after removal of thoracostomy tubes (TT). METHODS: A retrospective cohort analysis was performed evaluating all trauma patients requiring TT drainage of PTX or hemothoraces during a 3-year period. All chest radiographs before and after TT removal were reviewed to identify PTX recurrence. The principle outcome was recurrent PTX after TT removal. The 95% confidence intervals were calculated to assess for significance. RESULTS: We studied 234 TT removals in 190 patients. One hundred thirty-six (58%) TTs were removed under PPV. PTX recurred in 15 (11%) and 6 (4%) required reinsertion. In 10 patients (7.4%), there was a radiographically stable small PTX before and after removal not requiring TT reinsertion. In comparison, 98 (42%) TTs were removed under spontaneous ventilation. PTX recurred in 16 (16%) and 3 (3%) required reinsertion. There were 25 (25.5%) stable small PTXs before and after removal. The overall recurrence rate difference was -5.3% (confidence interval: -14.8 to 3.5) and reinsertion rate difference was 1.35% (confidence interval: -4.7 to 6.6). CONCLUSIONS: The rate of recurrent PTX or TT replacement after removal is not associated with PPV status. The slightly lower recurrence rate on PPV combined with the smaller proportion of patients with stable small PTX before removal may reflect more careful clinician selection of ideal patients or technique of TT removal among patients on PPV. Prospective data are needed to clarify these associations.