Hemodynamic monitoring in the critical care environment

Estimation of cardiac stroke volume from radial pulse waveform by artificial neural network

BACKGROUND AND OBJECTIVES

Stroke volume (SV) and cardiac output (CO) are the key indicators for the evaluation of cardiac function and hemodynamic status during the perioperative period, which are very important in the detection and treatment of cardiovascular diseases. Traditional CO and SV measurement methods have problems such as complex operation, low precision and poor generalization ability.

METHODS

In this paper, a method for estimating stroke volume based on cascade artificial neural network (ANN) and time domain features of radial pulse waveform (SV_ANN) was proposed. The simulation datasets of 4000 radial pulse waveforms and stroke volume (SV_meas) were generated by a 55 segment transmission line model of the human systemic vasculature and a recursive algorithm. The ANN was trained and tested by 10-fold cross-validation, and compared with 12 traditional models.

RESULTS

Experimental results showed that the Pearson correlation coefficients and mean difference between SV_ANN and SV_meas (R=0.95, mean standard deviation (SD) = 0.00 ± 6.45) were better than the best results of the 12 traditional models. Moreover, as increasing the number of training samples, the performance improvement of the ANN (R=0.94(Δ + 0.04), mean ± SD = 0.00 ± 6.38(Δ± 2.02)) was better than the other best model, namely, multiple linear regression model (MLR) (R=0.93(Δ + 0.03), mean ± SD = 0.00 ± 6.99(Δ± 1.50)).

CONCLUSIONS

A method is proposed to estimate cardiac stroke volume by the ANN with time domain features of radial pulse wave. It avoids the complicated modeling process based on hemodynamics within traditional models, improves the estimation accuracy of SV, and has a good generalization ability.

Low cost circulatory pressure acquisition and fluid infusion rate measurement system for clinical research

Acquiring patient physiological waveforms is useful for studying hemodynamic management and developing medical monitoring systems. A low cost, Arduino controlled data acquisition system acquires arterial pressure waveforms (Edwards Lifesciences TruWave compatible) and measures fluid infusion rate using hanging scales. This system can be used at the same time as a clinical monitor, enabling recording of patient arterial pressure and fluid delivery for clinical research. The system is powered via a USB connection, which additionally provides serial output, aiding compatibility and customisation. A simple software user interface, developed in Python, shows outputs. Each data acquisition system, including all necessary connection cables costs ~US$90 and is multiple-use.

Preload & Frank-Starling curves, from textbook to bedside: Clinically applicable non-additionally invasive model-based estimation in pigs

BACKGROUND

Determining physiological mechanisms leading to circulatory failure can be challenging, contributing to the difficulties in delivering effective hemodynamic management in critical care. Continuous, non-additionally invasive monitoring of preload changes, and assessment of contractility from Frank-Starling curves could potentially make it much easier to diagnose and manage circulatory failure.

METHOD

This study combines non-additionally invasive model-based methods to estimate left ventricle end-diastolic volume (LEDV) and stroke volume (SV) during hemodynamic interventions in a pig trial (N = 6). Agreement of model-based LEDV and measured admittance catheter LEDV is assessed. Model-based LEDV and SV are used to identify response to hemodynamic interventions and create Frank-Starling curves, from which Frank-Starling contractility (FSC) is identified as the gradient.

RESULTS

Model-based LEDV had good agreement with measured admittance catheter LEDV, with Bland-Altman median bias [limits of agreement (2.5th, 97.5th percentile)] of 2.2 ml [-13.8, 22.5]. Model LEDV and SV were used to identify non-responsive interventions with a good area under the receiver-operating characteristic (ROC) curve of 0.83. FSC was identified using model LEDV and SV with Bland-Altman median bias [limits of agreement (2.5th, 97.5th percentile)] of 0.07 [-0.68, 0.56], with FSC from admittance catheter LEDV and aortic flow probe SV used as a reference method.

CONCLUSIONS

This study provides proof-of-concept preload changes and Frank-Starling curves could be non-additionally invasively estimated for critically ill patients, which could potentially enable much clearer insight into cardiovascular function than is currently possible at the patient bedside.

Tube-load model: A clinically applicable pulse contour analysis method for estimation of cardiac stroke volume

BACKGROUND AND OBJECTIVES

Accurate, reproducible, and reliable real-time clinical measurement of stroke volume (SV) is challenging. To accurately estimate arterial mechanics and SV by pulse contour analysis, accounting for wave reflection, such as by a tube-load model, is potentially important. This study tests for the first time whether a dynamically identified tube-load model, given a single peripheral arterial input signal and pulse transit time (PTT), provides accurate SV estimates during hemodynamic instability.

METHODS

The model is tested for 5 pigs during hemodynamic interventions, using either an aortic flow probe or admittance catheter for a validation SV measure. Performance is assessed using Bland-Altman and polar plot analysis for a series of long-term state-change and short-term dynamic events.

RESULTS

The overall median bias and limits of agreement (2.5th, 97.5th percentile) from Bland-Altman analysis were -10% [-49, 36], and -1% [-28,20] for state-change and dynamic events, respectively. The angular limit of agreement (maximum of 2.5th, 97.5th percentile) from polar-plot analysis for state-change and dynamic interventions was 35.6^∘, and 35.2^∘, respectively.

CONCLUSION

SV estimation agreement and trending performance was reasonable given the severity of the interventions. This simple yet robust method has potential to track SV within acceptable limits during hemodynamic instability in critically ill patients, provided a sufficiently accurate PTT measure.

Incorporating pulse wave velocity into model-based pulse contour analysis method for estimation of cardiac stroke volume

BACKGROUND AND OBJECTIVES

Stroke volume (SV) and cardiac output (CO) are important metrics for hemodynamic management of critically ill patients. Clinically available devices to continuously monitor these metrics are invasive, and less invasive methods perform poorly during hemodynamic instability. Pulse wave velocity (PWV) could potentially improve estimation of SV and CO by providing information on changing vascular tone. This study investigates whether using PWV for parameter identification of a model-based pulse contour analysis method improves SV estimation accuracy.

METHODS

Three implementations of a 3-element windkessel pulse contour analysis model are compared: constant-Z, water hammer, and Bramwell-Hill methods. Each implementation identifies the characteristic impedance parameter (Z) differently. The first method identifies Z statically and does not use PWV, and the latter two methods use PWV to dynamically update Z. Accuracy of SV estimation is tested in an animal trial, where interventions induce severe hemodynamic changes in 5 pigs. Model-predicted SV is compared to SV measured using an aortic flow probe.

RESULTS

SV percentage error had median bias and [(IQR); (2.5th, 97.5th percentiles)] of -0.5% [(-6.1%, 4.7%); (-50.3%, +24.1%)] for the constant-Z method, 0.6% [(-4.9%, 6.2%); (-43.4%, +29.3%)] for the water hammer method, and 0.8% [(-6.5, 8.6); (-37.1%, +47.6%)] for the Bramwell-Hill method.

CONCLUSION

Incorporating PWV for dynamic Z parameter identification through either the Bramwell-Hill equation or the water hammer equation does not appreciably improve the 3-element windkessel pulse contour analysis model's prediction of SV during hemodynamic changes compared to the constant-Z method.

Optical flow sensor for continuous invasive measurement of blood flow velocity

Continuous monitoring of intrapulse measurement of blood flow in humans is currently not achievable with clinically available instruments. In this paper, we demonstrate a method of measuring the instantaneous variations in flow during pulsatile blood flow with an optical flow sensor comprising a fiber Bragg grating sensor and illumination from a 565 nm Light-Emitting-Diode. The LED illumination heats the blood and fluctuations in temperature, due to variations in flow, are detected by the fiber sensor. A set of experiments at different flow rates (20 to 900 mL/min) are performed in a simulated cardiac circulation setup with pulsatile flow. Data are compared with an in-line time of flight ultrasound flow sensor. Our results show that the optical and ultrasonic signals correlate with Pearson coefficients ranging from -0.83 to -0.98, dependent on the pulsatile frequency. Average flow determined by ultrasound and the optical fiber sensor showed a parabolic relationship with R² = 0.99. An abrupt step change in flow induced by occlusion and release of the circuit tubing demonstrated that the optical fiber and ultrasound sensor had similar response. The method described is capable of intrapulse blood flow measurement under pulsatile flow conditions, with potential applications in medicine where continuous blood flow sensing is desired.

Surviving sepsis in the critical care environment

The management of sepsis and septic shock in the intensive care environment is a complex task requiring the cooperation of a multidisciplinary team. The Surviving Sepsis Campaign provides systematic guidelines for the recognition, early intervention, and supportive management of sepsis. Critical care nurses are instrumental in ensuring that these guidelines and other sources of evidence-based practice are used for patients with severe sepsis or septic shock. This article discusses the pathophysiologic processes in severe sepsis and septic shock and discusses the appropriate interventions as recommended by the Surviving Sepsis Campaign. Recommended early treatments are reviewed along with interventions related to hemodynamics, perfusion, and supportive care in the critical care environment.

Changes in volumetric hemodynamic parameters induced by fluid removal on hemodialysis in critically ill patients

Management of volume status is difficult in critically ill patients with renal failure. Volumetric hemodynamic indices are increasingly being used to guide fluid therapy in the intensive care unit (ICU), but are not established to monitor hemodialysis-induced fluid removal in critically ill patients. Using volumetric hemodynamic monitoring, changes in extravascular lung water index (EVLWI) and intrathoracic blood volume index (ITBVI) were measured immediately before and after hemodialysis sessions in 35 ICU patients. Additional hemodynamic and oxygenation related parameters were recorded at the same time, and online relative blood volume (RBV) monitoring was performed during hemodialysis. EVLWI decreased significantly with fluid removal (median 10.0 vs. 9.6 mL/kg, P = 0.001), whereas ITBVI remained stable (median 1012 vs. 1029 mL/m(2) , P = 0.402). Significant changes were also observed in stroke volume variation (median 12.0 vs. 13.0 %, P = 0.012), cardiac index (median 4.2 vs. 3.5 mL/min/m(2) , P = 0.003), mean arterial pressure (median 77 vs. 85.5 mmHg, P = 0.006), norepinephrine dose (median 0.092 vs. 0.114 μg/kg per min, P = 0.043), and hemoglobin values (median 9.5 vs. 10.4 gm/dL, P = 0.036). RBV decreased by 7.8% (median); there was no correlation with either the volumetric measurements or the other hemodynamic parameters recorded. EVLWI reduction with dialysis reflects the removal of excess body fluid, whereas preservation of cardiac preload is indicated by ITBVI stability. Volumetric hemodynamic measurements provide additional information concerning fluid status and are thus potentially useful to guide fluid removal on hemodialysis in critically ill patients.

The role of ultrasound as an adjunct to arterial catheterization in critically ill surgical and intensive care unit patients

OBJECTIVE

To review the evidence behind Ultrasound (US) guided placement of arterial cannulae and its use in the critically ill population.

DATA SOURCES

We performed a computer-aided literature search using set search terms and electronic data bases of PubMed and EMBASE from their commencement date through the end of July 2013.

SUMMARY OF REVIEW

Insertion of intra-arterial catheters is a commonly performed invasive procedure in the peri-operative and intensive care setting that facilitates invasive blood pressure and cardiac output monitoring as well as frequent blood sampling. Arterial catheterization can be particularly challenging in critically ill and high-risk surgical patients with circulatory collapse, low cardiac output state and peripheral edema, all of which can limit the ability to successfully palpate and cannulate the artery. There is a convincing body of evidence suggesting a decrease in complication rate and first-pass success rate in US guided central venous catheter (CVC) insertion compared with the landmark technique. While most intensivists and peri-operative physicians are familiar with US guided CVC placement, fewer use US to guide arterial access.

CONCLUSIONS

Most studies have demonstrated a higher success rate when using US guidance for arterial cannulation. Moreover, the technique permits more rapid access and establishment compared with the conventional palpation technique. However, there is evidence opposing the routine use of US to guide arterial cannula insertion. Further studies are required to ascertain the benefits and cost effectiveness of US guided arterial catheterization in peri-operative and critical care.