Stroke volume and the three phase cardiac output rate relationship with ventricular pacing

Applying systems thinking to unravel the mechanisms underlying orthostatic hypotension related fall risk

Orthostatic hypotension (OH) is an established and common cardiovascular risk factor for falls. An in-depth understanding of the various interacting pathophysiological pathways contributing to OH-related falls is essential to guide improvements in diagnostic and treatment opportunities. We applied systems thinking to multidisciplinary map out causal mechanisms and risk factors. For this, we used group model building (GMB) to develop a causal loop diagram (CLD). The GMB was based on the input of experts from multiple domains related to OH and falls and all proposed mechanisms were supported by scientific literature. Our CLD is a conceptual representation of factors involved in OH-related falls, and their interrelatedness. Network analysis and feedback loops were applied to analyze and interpret the CLD, and quantitatively summarize the function and relative importance of the variables. Our CLD contains 50 variables distributed over three intrinsic domains (cerebral, cardiovascular, and musculoskeletal), and an extrinsic domain (e.g., medications). Between the variables, 181 connections and 65 feedback loops were identified. Decreased cerebral blood flow, low blood pressure, impaired baroreflex activity, and physical inactivity were identified as key factors involved in OH-related falls, based on their high centralities. Our CLD reflects the multifactorial pathophysiology of OH-related falls. It enables us to identify key elements, suggesting their potential for new diagnostic and treatment approaches in fall prevention. The interactive online CLD renders it suitable for both research and educational purposes and this CLD is the first step in the development of a computational model for simulating the effects of risk factors on falls.

A Computationally Efficient Approach to Simulate Heart Rate Effects Using a Whole Human Heart Model

Computational modeling of the whole human heart has become a valuable tool to evaluate medical devices such as leadless pacemakers, annuloplasty rings and left ventricular assist devices, since it is often difficult to replicate the complex dynamic interactions between the device and human heart in bench-top and animal tests. The Dassault Systèmes Living Heart Human Model (LHHM) is a finite-element model of whole-human-heart electromechanics that has input parameters that were previously calibrated to generate physiological responses in a healthy heart beating at 60 beat/min (resting state). This study demonstrates that, by adjusting only six physiologically meaningful parameters, the LHHM can be recalibrated to generate physiological responses in a healthy heart beating at heart rates ranging from 90−160 beat/min. These parameters are as follows: the sinoatrial node firing period decreases from 0.67 s at 90 bpm to 0.38 s at 160 bpm, atrioventricular delay decreases from 0.122 s at 90 bpm to 0.057 s at 160 bpm, preload increases 3-fold from 90 bpm to 160 bpm, body resistance at 160 bpm is 80% of that at 90 bpm, arterial stiffness at 160 bpm is 3.9 times that at 90 bpm, and a parameter relating myofiber twitch force duration and sarcomere length decreases from 238 ms/mm at 90 bpm to 175 ms/mm at 160 bpm. In addition, this study demonstrates the feasibility of using the LHHM to conduct clinical investigations in AV delay optimization and hemodynamic differences between pacing and exercise. AV delays in the ranges of 40 ms to 250 ms were simulated and stroke volume and systolic blood pressure showed clear peaks at 120 ms for 90 bpm. For a heart during exercise, the increase in cardiac output continues to 160 bpm. However, for a heart during pacing, those physiological parameter adjustments are removed that are related to changes in body oxygen requirements (preload, arterial stiffness and body resistance). Consequently, cardiac output increases initially with heart rate; as the heart rate goes up (>100 bpm), the increasing rate of cardiac output slows down and approaches a plateau.

Effects of pacing modality on noninvasive assessment of heart rate dependency of indices of large artery function

Studies investigating the relationship between heart rate (HR) and arterial stiffness or wave reflections have commonly induced HR changes through in situ cardiac pacing. Although pacing produces consistent HR changes, hemodynamics can be different with different pacing modalities. Whether the differences affect the HR relationship with arterial stiffness or wave reflections is unknown. In the present study, 48 subjects [mean age, 78 ± 10 (SD), 9 women] with in situ cardiac pacemakers were paced at 60, 70, 80, 90, and 100 beats per min under atrial, atrioventricular, or ventricular pacing. At each paced HR, brachial cuff-based pulse wave analysis was used to determine central hemodynamic parameters, including ejection duration (ED) and augmentation index (AIx). Wave separation analysis was used to determine wave reflection magnitude (RM) and reflection index (RI). Arterial stiffness was assessed by carotid-femoral pulse wave velocity (cfPWV). Pacing modality was found to have significant effects on the HR relationship with ED (P = 0.01), central aortic pulse pressure (P = 0.01), augmentation pressure (P < 0.0001), and magnitudes of both forward and reflected waves (P = 0.05 and P = 0.003, respectively), but not cfPWV (P = 0.57) or AIx (P = 0.38). However, at a fixed HR, significant differences in pulse pressure amplification (P < 0.001), AIx (P < 0.0001), RM (P = 0.03), and RI (P = 0.03) were observed with different pacing modalities. These results demonstrate that although the HR relationships with arterial stiffness and systolic loading as measured by cfPWV and AIx were unaffected by pacing modality, it should still be taken into account for studies in which mixed pacing modalities are present, in particular, for wave reflection studies.

Pharmacokinetics and pharmacodynamics of glycopyrrolate following a continuous-rate infusion in the horse

Glycopyrrolate (GLY) is an antimuscarinic agent that is used in humans and domestic animals primarily to reduce respiratory tract secretions during anesthesia and to reverse intra-operative bradycardia. Although GLY is used routinely in veterinary patients, there is limited information regarding its pharmacokinetic (PK) and pharmacodynamic (PD) properties in domestic animals, and an improved understanding of the plasma concentration-effect relationship in racehorses is warranted. To accomplish this, we characterize the pharmacokinetic-pharmacodynamic (PK-PD) actions of GLY during and after a 2-h constant-rate intravenous infusion (4 μg/kg/h) and evaluate potential PK-PD models for cardiac stimulation in adult horses. Measurements of plasma GLY concentrations, heart and respiration rates, and frequency of bowel movements were performed in six Thoroughbred horses. The time course for GLY disposition in plasma followed a tri-exponential equation characterized by rapid disappearance of GLY from blood followed by a prolonged terminal phase. Physiological monitoring revealed significant (P < 0.01) increases in heart (>70 bpm) and respiratory rates accompanied by a marked and sustained delay in the frequency of bowel movements (1.1 ± 0.2 h [saline group] vs. 6.0 ± 2.0 h [GLY group]). Two of six horses showed signs of colic during the 8-h observation period after the end of the GLY infusion, but were treated and recovered without further complications. The relationship between plasma GLY concentration and heart rate exhibited counterclockwise hysteresis that was adequately described using an effect compartment.

Effects of a muscarinic type-2 antagonist on cardiorespiratory function and intestinal transit in horses anesthetized with halothane and xylazine

OBJECTIVE

To evaluate the cardiorespiratory and intestinal effects of the muscarinic type-2 (M2) antagonist, methoctramine, in anesthetized horses.

ANIMALS

6 horses.

PROCEDURE

Horses were allocated to 2 treatments in a randomized complete block design. Anesthesia was maintained with halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, i.v.) and mechanical ventilation. Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of methoctramine or saline (0.9% NaCl) solution (control treatment). Methoctramine was given at 10-minute intervals (10 microg/kg, i.v.) until heart rate (HR) increased at least 30% above baseline values or until a maximum cumulative dose of 30 microg/kg had been administered. Recovery characteristics, intestinal auscultation scores, and intestinal transit determined by use of chromium oxide were assessed during the postanesthetic period.

RESULTS

Methoctramine was given at a total cumulative dose of 30 microg/kg to 4 horses, whereas 2 horses received 10 microg/kg. Administration of methoctramine resulted in increases in HR, cardiac output, arterial blood pressure, and tissue oxygen delivery. Intestinal auscultation scores and intestinal transit time (interval to first and last detection of chromium oxide in the feces) did not differ between treatment groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Methoctramine improved hemodynamic function in horses anesthetized by use of halothane and xylazine without causing a clinically detectable delay in the return to normal intestinal motility during the postanesthetic period. Because of their selective positive chronotropic effects, M2 antagonists may represent a safe alternative for treatment of horses with intraoperative bradycardia.

Effects of glycopyrrolate on cardiorespiratory function in horses anesthetized with halothane and xylazine

OBJECTIVE

To evaluate cardiopulmonary effects of glycopyrrolate in horses anesthetized with halothane and xylazine.

ANIMALS

6 horses.

PROCEDURE

Horses were allocated to 2 treatment groups in a randomized complete block design. Anesthesia was maintained in mechanically ventilated horses by administration of halothane (1% end-tidal concentration) combined with a constant-rate infusion of xylazine hydrochloride (1 mg/kg/h, i.v.). Hemodynamic variables were monitored after induction of anesthesia and for 120 minutes after administration of glycopyrrolate or saline (0.9% NaCl) solution. Glycopyrrolate (2.5 microg/kg, i.v.) was administered at 10-minute intervals until heart rate (HR) increased at least 30% above baseline or a maximum cumulative dose of 7.5 microg/kg had been injected. Recovery characteristics and intestinal auscultation scores were evaluated for 24 hours after the end of anesthesia.

RESULTS

Cumulative dose of glycopyrrolate administered to 5 horses was 5 microg/kg, whereas 1 horse received 7.5 microg/kg. The positive chronotropic effects of glycopyrrolate were accompanied by an increase in cardiac output, arterial blood pressure, and tissue oxygen delivery. Whereas HR increased by 53% above baseline values at 20 minutes after the last glycopyrrolate injection, cardiac output and mean arterial pressure increased by 38% and 31%, respectively. Glycopyrrolate administration was associated with impaction of the large colon in 1 horse and low intestinal auscultation scores lasting 24 hours in 3 horses.

CONCLUSIONS AND CLINICAL RELEVANCE

The positive chronotropic effects of glycopyrrolate resulted in improvement of hemodynamic function in horses anesthetized with halothane and xylazine. However, prolonged intestinal stasis and colic may limit its use during anesthesia.

How smart should pacemakers Be?

The concept of the "smart" pacemaker has been continuously changing during 40 years of progress in technology. When we talk today about smart pacemakers, it means optimal treatment, diagnosis, and follow-up for patients fitting the current indications for pacemakers. So what is smart today becomes accepted as "state of the art" tomorrow. Originally, implantable pacemakers were developed to save lives from prolonged episodes of bradycardia and/or complete heart block. Now, in addition, they improve quality of life via numerous different functions acting under specific conditions, thanks to the introduction of microprocessors. The devices have become smaller, with the miniaturization of the electrical components, without compromising longevity. Nevertheless, there are still some unmatched objectives for these devices, for example, the optimization of cardiac output and the management of atrial arrhythmias in dual-chamber devices. Furthermore, indications continue to evolve, which in turn require new, additional functions. These functions are often very complex, necessitating computerized programming to simplify application. In addition, the follow-up of these devices is time-consuming, as appropriate system performance has to be regularly monitored. A great many of these functions could be automatically performed and documented, thus enabling physicians and paramedical staff to avoid losing time with routine control procedures. In addition, modern pacemakers offer extensive diagnostic functions to help diagnose patient symptoms and pacemaker system problems. Different types of data are available, and their presentation differs from one company to the other. This huge amount of data can only be managed with automatic diagnostic functions. Thus, the smart pacemaker of the near future should offer high flexibility to permit easy programming of available therapies and follow-up, and extensive, easily comprehensible diagnostic functions.

Adequacy of pacing rate during exercise in rate responsive ventricular pacing

Our objective was to determine the adequate pacing rate during exercise in ventricular pacing by measuring exercise capacity, cardiac output, and sinus node activity. Eighteen patients with complete AV block and an implanted pacemaker underwent cardiopulmonary exercise tests under three randomized pacing rates: fixed rate pacing (VVI) at 60 beats/min and ventricular rate-responsive pacing (VVIR) programmed to attain a heart rate of about 110 beats/min or 130 beats/min (VVIR 110 and VVIR 130, respectively) at the end of exercise. Compared with VVI and VVIR 130, VVIR 110 was associated with an increased peak oxygen uptake (VVIR 110: 20.3 +/- 4.5 VVI: 16.9 +/- 3.1; P < 0.01; and VVIR 130: 19.0 +/- 4.1 mL/min per kg, respectively; P < 0.05 and a higher oxygen uptake at anaerobic threshold (15.3 +/- 2.7, 12.7 +/- 1.9; P < 0.01, and 14.6 +/- 2.6 mL/min per kg; P < 0.05). The atrial rate during exercise expressed as a percentage of the expected maximal heart rate was lower in VVIR 110 than in VVI or VVIR 130 (VVIR 110: 75.9% +/- 14.6% vs VVI: 90.6% +/- 12.8%; P < 0.01; VVIR 110 vs 130: 89.1% +/- 23.1% P < 0.05). There was no significant in cardiac output at peak exercise between VVIR 110 and VVIR 130. We conclude that a pacing rate for submaximal exercise of 110 beats/min may be preferable to that of 130 beats/min in respect to exercise capacity and sympathetic nerve activity.