The utility of novel non-invasive technologies for remote hemodynamic monitoring in chronic heart failure

Abstract

Monitoring a patient’s hemodynamic status may be a revolutionary way to aid a ‘health maintenance’ strategy in which the physician strives to therapeutically keep the patient in an ideal hemodynamic range. Currently, home telemonitoring employs a ‘crisis-prevention’ approach. This strategy is still based on easily acquired measures such as heart rate, weight and blood pressure – measurements that are useful to help implement guideline-directed therapy but provide little information about impending decompensation or the risk of hospitalisation. Current systems provide limited information to personalize and adapt medication therapy for heart failure. Several innovative technologies that can remotely monitor estimates of cardiovascular hemodynamics, such as cardiac index, systemic vascular resistance, augmentation index and added heart sounds may enable earlier detection of heart failure decompensation. This editorial presents an overview of the innovative technologies that are available for non-invasive hemodynamic monitoring and maybe adapted for home telemonitoring for chronic heart failure.

Keywords::

• bio-impedance
• cardiac output
• heart failure
• telemonitoring

The increasing burden of managing chronic diseases such as heart failure (HF) has important ramifications on the delivery of care and clinical outcomes as well as health care spending. Pioneering efforts have led to telemonitoring (TM) being proposed as a potential solution to deal with the increasing prevalence of chronic disease among an ageing population [1,2]. Home TM of patients with HF may reduce mortality [3], improve symptoms and reduce the need for hospitalization. [4]. Furthermore, the TM approach recruits the patient to become a central ‘partner’ in their own management through frequent self-monitoring and improved interaction with health professionals [5].

An approach to TM, which employs a ‘health maintenance’ strategy which is being investigated in the heartcycle program [6,7], has developed in the last decade [8,9]. Enabling patients to measure their hemodynamic status on a regular basis in their own home with the transmission of information to an expert system and clinician overview could transform the way care for HF is given. In this concept, non-invasive surrogate measures of cardiovascular function and lung congestion such as bioelectrical impedance, systolic time intervals, pulse-wave characteristics, cardiac index and systemic vascular resistance (SVR), which are well-established clinical measures, are combined with medication adaptation algorithms to tailor treatment to optimize the individual patient's clinical status (Figure 1) [10]. This strategy aims to maintain cardiovascular function in an ideal steady-state range. Rather than trying to detect something going seriously wrong and fixing it, a health maintenance strategy declares an ideal range of values and adjusts treatment to try to maintain the patient in a safe envelope [6].

Figure 1. Noninvasive hemodynamic monitoring concept.

The strategy is advantageous for several reasons that include motivating patients to be more centrally involved in their care and thereby improving their engagement as well as improving daily adjustments of some medications, such as diuretics. Furthermore, maintaining cardiovascular function in an ideal range may have favorable effects on the natural history of disease compared with crisis management and may positively influence adverse outcomes, such as pulmonary hypertension, atrial fibrillation and sudden death [9].

Cardiac hemodynamics: the basis for hemodynamic monitoring in chronic pump failure

Fundamentally, the hallmark of symptomatic HF is decreased cardiac output. The determinants of cardiac output are predominantly the heart rate and stroke volume. The stroke volume is in turn determined by the interplay between preload, afterload and cardiac contractility. Hemodynamic changes in chronic heart failure (CHF) often comprise a sympathetically mediated increased heart rate and decreased stroke volume. Additionally, the preload is often increased, while the afterload, which is the SVR to the flow of blood, will often be increased. Myocardial contractility will often be decreased (Figure 2).

Figure 2. Hemodynamics of heart failure. This figure illustrates cardiac hemodynamic changes in symptomatic heart failure which are decreased cardiac output, decreased stroke volume, increased or decreased heart rate, increased preload, increased afterload as well as decreased myocardial contractility.

Decompensating HF hemodynamics

Abnormal left ventricular (LV) loading conditions leading to pulmonary congestion usually precede HF hospitalization [11,12]. Weight, a traditionally monitored variable in HF, poorly correlates with increase in pulmonary pressure and worsening symptoms of acute HF [13,14]. Interplay between reduced capacitance in the venous system leading to increased preload, increased arterial stiffness and resistance and subsequent increased afterload is thought to drive intrathoracic fluid accumulation [15,16]. Conceptually, continuous monitoring of fluid status in HF patients would therefore aid in identification of volume overload, thus providing an opportunity to intervene at an early stage and possibly avert hospital admission for acute decompensated HF.

Health maintenance hemodynamics

This concept leans on continually restoring hemodynamic homeostasis in a patient with congestive HF. The maintenance of cardiovascular function in an ideal range steady state would aim to balance the preload and afterload and improve cardiac contractility as illustrated in Figure 3.

Figure 3. Illustration of ideal range hemodynamics in heart failure – parameters to measure and potential therapeutic strategy. This figure illustrates the underlying reason for the health maintenance strategy and the possible actionable measurements which can be taken from the decompensating heart failure patient. The possible interventions which a clinician could undertake are shown. Within the action range phase, telemonitoring of TBW, BIM, S3, S4, SVR, CPI, LVST, EMAT, dt/ptcould be undertaken. Then treatments with diuretics, ACEI, ARB, nitrates etc. could be instituted to prevent deterioration to the danger zone.

Novel sensors for remote monitoring in HF

The ideal novel sensor for remote hemodynamic monitoring should be simple to use, especially given that the majority of HF patients are elderly. Furthermore, it should have been tested under robust clinical conditions and shown not only to accurately detect hemodynamic perturbations consummate with decompensating pump failure but also to detect and track subtle changes, even with changes in patients’ activities of daily living. Such modalities include non-invasive devices for measuring cardiac hemodynamics using impedance cardiography, finger plethysmography, phonocardiography as well as thoracic impedance vests. Interestingly, in a pilot study using impedance cardiography and finger plethysmography to measure noninvasive estimates of hemodynamics, under different environmental conditions in patients with CHF, subtle differences in noninvasive blood pressure and SVR could be reliably detected and tracked during the study day [17]. We will now review how these modalities may be useful for remote hemodynamic monitoring of patients with HF.

Impedance cardiography

Thoracic electrical bio-impedance BIM is inversely proportional to lung congestion. It is a validated non-invasive method that measures cardiac hemodynamic parameters [18,19], including in the ambulatory setting [20]. Briefly, impedance cardiography uses low-amplitude, high-frequency alternating signal to determine changes in electrical impedance, cardiac ejection and the velocity of blood flow within the aorta [21] from which stroke volume, cardiac output, SVR and total body water can be derived. Whole-body BIM monitoring using a Non-Invasive Cardiac System has been used to estimate cardiovascular function in an outpatient CHF population, providing good predictive information of readmissions with CHF [22]. The PREDICT study [23], which was also an outpatient clinic based study, further consolidated the findings that when performed regularly in stable patients with HF with a recent clinical decompensation, impedance cardiography can identify patients at increased risk of recurrent decompensation.

Spectroscopic bioimpedance

Bioimpedance spectroscopy has emerged as a potential method to detect subtle changes in thoracic fluid content HF patients [24,25]. A feasibility study using the BIM sensor for home TM has been conducted, where it was shown to predict episodes of hospitalization for worsening HF [Schauerte P, Unpublished Data]. Multimodal assessments of CHF patients with natriuretic peptides, BIM and clinical evaluation have been shown to identify those with thoracic congestion and more adverse prognosis [26]. This modality of lung congestion monitoring holds great promise in providing ‘snap-shot’ measurements, which may be easily performed at home on a daily basis, to provide crucial information about patients at risk of ‘danger zone congestion status.’ Such information could then be remotely transferred to the health care provider who would in turn swiftly optimize therapy and thereby prevent a ‘crisis’ hospitalization.

Finger plethysmography

Non-invasive hemodynamic monitoring using pressure pulse contour analysis has been validated in patients with advanced HF [27]. In this method, continuous beat-to-beat cardiac output, blood pressure and SVR are determined from the pulsatile systolic area of the blood pressure curve [28,29]. This method is promising but remains to be tested in the ambulatory setting.

Pulse-wave characteristics

Peripheral arterial tonometry using ankle-brachial sphygmomanometers combined with finger-toe plethysmography (Enverdis) enable easy, noninvasive determination of vascular dysfunction. Disordered pulse-wave amplitude is linked to endothelial dysfunction which is present in patients with early asymptomatic [30] as well as symptomatic [31] HF. Endothelial dysfunction, increased oxidative stress and baroreceptor dysfunction contribute to the pathophysiology of CHF [32,33]. Moreover, patients with impaired endothelial function are at increased risk for cardiovascular events and death [33]. Pulse-wave characteristics such as aortic pulse-wave velocity, augmentation index and mean arterial pressure can therefore be determined noninvasively to aid identification of high-risk groups which can be aggressively treated both non-pharmacologically with exercise and advice to stop smoking and pharmacologically with blood pressure reduction using angiotensin-converting enzyme inhibitors and statin therapy – measures which are known to positively influence endothelial function. This method is likely more suitable for snap-shot assessments in outpatient clinics and general practice to provide simple noninvasive markers of vascular dysfunction.

Phonocardiography

Acoustic cardiography enables detection of normal and abnormal heart sounds and simultaneously correlates the timing of those heart sounds in every cardiac cycle to the onset of the P wave and QRS complex on the ECG [34]. Parameters determined using this modality include the strength of the third heart sound (S3) graded from 0 to 10, the strength of the fourth heart sound (S4), electromechanical activation time (EMAT) (interval from Q wave to the first heart sound) and systolic dysfunction index (a combination of EMAT/RR, S3 score, QRS duration and QR interval). S3 strength >5 has been shown to correlate with LV end-diastolic pressure >15 mm Hg [35,36]. Diastolic heart sounds provide diagnostic and prognostic information in HF. The third heart sound has been shown to be independently associated with adverse outcomes, including progression of HF [37]. In addition, an S4 is associated with increased LV stiffness and also elevated LV end-diastolic pressure [38]. Systolic time intervals are validated indicators of HF [39,40]. In patients suffering from HF with reduced ejection fraction, EMAT is prolonged whilst LV contractility (dP/dt) is low. Overall, these highly specific measures have been found to improve the diagnostic accuracy for decompensated HF and LV dysfunction [41,42]. Ambulatory acoustic cardiography holds promise as it provides an assessment of the electromechanical performance of the heart. There could be a role for use in HF follow-up in the home setting to aid optimization of therapy.

Summary

Intrathoracic fluid congestion, changes in LV-filling pressures and reduced cardiac function are common pathophysiologic events that occur prior to HF hospitalizations. Interestingly, these events may occur with little changes in traditionally monitored variables such as weight, heart rate and blood pressure. Conceptually, enabling patients to measure estimates of their hemodynamic status on a regular basis in their own home with transmission of information to an expert system and clinician overview could transform the way that care for HF is given. An added advantage of the strategy is that patients are likely to become more empowered and thus become more proactive in managing their disease.

An ideal innovative TM sensor should be simple and easy to use in the home scenario, inexpensive and easily operated by patients who can then perform daily checks themselves. Furthermore, it should acquire vital cardiac signals and extract prognostic cardiac information that can then be transferred to an expert system with clinician overview. With such technology in mind, TM could be transformed from a concept of ‘crisis detection and management’ strategy to a ‘health maintenance’ strategy.

Monitoring intrathoracic impedance, abnormal diastolic heart sounds and systolic time intervals have clearly been shown to correlate with early signals of HF decompensation. Conceptually therefore, novel innovative sensors employing these technologies could be useful in maintaining individual patients in an ideal hemodynamic range. Further studies to assess the impact of a health maintenance strategy in HF on morbidity and mortality as well as any economic benefits are required.

Financial & competing interests disclosure

J Cleland acted as Chief Medical Officer for the Heartcycle study. This work has received funding from the European Community’s Seventh Framework Programme under grant agreement n° FP7–216695. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

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