Analysis of the effect of CPAP on hemodynamics using clinical data and a theoretical model: CPAP therapy decreases cardiac output mechanically but increases it via afterload reduction

BACKGROUND

Noninvasive positive pressure ventilation (NIPPV) has been established as an effective treatment for heart failure. Positive airway pressure such as continuous positive airway pressure (CPAP) increases cardiac output (CO) in some patients but decreases it in others. However, the mechanism behind such unpredictable responses remains undetermined.

METHODS AND RESULTS

We measured hemodynamic parameters of 38 cases using Swan-Ganz catheter before and after CPAP in chronic heart failure status. In those whose CO increased by CPAP, pulmonary vascular resistance (PVR) was significantly decreased and SpO2 significantly increased, but the other parameters were not changed. On the other hand, PVR was not changed, but systemic vascular resistance (SVR) was increased in those whose CO decreased by CPAP. To explain this phenomenon, we simulated the cardiovascular system using a cardiac model of time-varying elastance. In this model, it was indicated that CPAP decreases CO irrespective of cardiac function or filling status under constant PVR condition. However, when reduction of PVR by CPAP was taken into account, an increase in CO was expected especially in the hypervolemic and low right ventricle (RV) systolic function cases.

CONCLUSIONS

CPAP would increase CO only where PVR can be reduced by CPAP therapy, especially in the case with hypervolemia and/or low RV systolic function. Understanding the underlying mechanism should help identify the patients for whom NIPPV would be effective.

Rodent models of pulmonary embolism and chronic thromboembolic pulmonary hypertension

Pulmonary embolism (PE) is the third most prevalent cardiovascular disease. It is associated with high in-hospital mortality and the development of acute and chronic complications. New approaches aimed at improving the prognosis of patients with PE are largely dependent on reliable animal models. Mice, rats, hamsters, and rabbits, are currently most commonly used for PE modeling because of their ethical acceptability and economic feasibility. This article provides an overview of the main approaches to PE modeling, and the advantages and disadvantages of each method. Special attention is paid to experimental endpoints, including morphological, functional, and molecular endpoints. All approaches to PE modeling can be broadly divided into three main groups: 1) induction of thromboembolism, either by thrombus formation in vivo or by injection of in vitro prepared blood clots; 2) introduction of particles of non-thrombotic origin; and 3) surgical procedures. The choice of a specific model and animal species is determined based on the objectives of the study. Rodent models of chronic thromboembolic pulmonary hypertension (CTEPH), which is the most devastating complication of PE, are also described. CTEPH models are especially challenging because of insufficient knowledge about the pathogenesis and high fibrinolytic activity of rodent plasma. The CTEPH model should demonstrate a persistent increase in pulmonary artery pressure and stable reduction of the vascular bed due to recurrent embolism. Based on the analysis of available evidence, one might conclude that currently, there is no single optimal method for modeling PE and CTEPH.

Mechanical loading of isolated cardiac muscle with a real-time computed Windkessel model of the vasculature impedance

To date, the mechanical loads imposed on isolated cardiac muscle tissue in vitro have been oversimplified. Researchers typically applied loads that are time-invariant, resulting in either isometric and auxotonic contractions, or flat-topped (isotonic shortening) work-loops. These contraction types do not fully capture the dynamic response of contracting tissues adapting to a variable load, such as is experienced by ventricular tissue in vivo. In this study, we have successfully developed a loading system that presents a model-based, time-varying, continuously updated, load to cardiac tissue preparations. We combined a Windkessel model of vascular fluid impedance together with Laplace's Law and encoded it in a real-time hardware-based force-length control system. Experiments were carried out on isolated rat left ventricular trabeculae; we directly compare the work-loops arising from this protocol with those of a typical simplified isotonic shortening work-loop system. We found that, under body conditions, cardiac trabeculae achieved greater mechanical work output against our new loading system, than with the simplified isotonic work-loop protocol. We further tested whether loading the tissue with a mechanical impedance defined by "diseased" Windkessel model parameters had an effect on the performance of healthy trabeculae. We found that trabecula shortening decreased when applying the set of Windkessel parameters describing the hypertensive condition, and increased in the hypotensive state. Our implementation of a real-time model of arterial characteristics provides an improved, physiologically derived, instantly calculated load for use in studying isolated cardiac muscle, and is readily applicable to study various disease conditions.

The pulsatile component of left atrial pressure has little effect on pulmonary artery impedance estimation in normal rats

Pulmonary artery impedance (PAZ) that measures the pulsatile properties of the vasculature provides diagnostic and prognostic information in patients with pulmonary vascular diseases. While downstream pressure [i.e., left atrial (LA) pressure] should be considered when calculating static properties of pulmonary vasculature, PAZ is commonly estimated without taking into account the pulsatile component of LA pressure. We examined whether PAZ can be estimated with reasonable accuracy without using LA pressure. Pulmonary artery (PA) flow, PA pressure, and LA pressure were measured under irregular pacing in eight normal Sprague-Dawley rats. PAZ was estimated by analyzing a one-input, one-output system (I1O1 analysis) that does not include LA pressure, and a two-input, one-output system (I2O1 analysis) that includes LA pressure. Using a tube and 3-element Windkessel model, PAZ was parameterized as peripheral resistance (RP ), arterial compliance (CP ), characteristic impedance (ZC ), and transmission time to the reflection site (TD ). These parameters were not significantly different between the I1O1 and I2O1 analyses (RP : 0.286 ± 0.040 vs. 0.274 ± 0.038 mmHg·min/mL, CP : 0.352 ± 0.049 vs. 0.343 ± 0.041 mL/mmHg, ZC : 0.115 ± 0.005 vs. 0.117 ± 0.005 mmHg·min/mL, TD : 13.2 ± 1.8 vs. 12.9 ± 1.7 msec). In conclusion, the I1O1 analysis that does not use LA pressure estimates PAZ with reasonable accuracy compared with the I2O1 analysis that uses LA pressure in normal rats. Our finding that the pulsatile component of LA pressure contributes little to PAZ estimation may justify the clinical use of the I1O1 analysis.