IEEE EMBC 2006, 30 August – 3 September, 2006, New York City - We hypothesize that intracardiac...

IEEE EMBC 2006, 30 August – 3 September, 2006, New York City

- We hypothesize that intracardiac impedance can provide real-time hemodynamic information to automatically control the upper and lower pacing rate limits of a rate adaptive pacemaker.

- A number of technical challenges must be overcome to incorporate bioimpedance based automatic control of rate within the constraints of an implantable device.

- We will introduce some basic results of our collaborative study in this subject.

Intracardiac Electrical BioimpedanceIntracardiac Electrical Bioimpedanceas a Basis for Controlling of Pacing Rate Limitsas a Basis for Controlling of Pacing Rate Limits

Andres KinkAndres Kink, Rodney W. SaloRodney W. Salo, Mart Min Mart Min , Toomas ParveToomas Parve, and Indrek RätsepIndrek Rätsep

Guidant Corporation (USA), Smartimplant Ltd, and Tallinn University of Technology (ESTONIA)

IEEE EMBC 2006 30 Aug – 3 Sept 2006, New York City 2 A.Kink, R.W.Salo, M.Min, T.Parve, I.Rätsep. Intracardiac Electrical Bioimpedance as a Basis for Controlling of Pacing Rate Limits

1. Over pacing and Under pacing

Fig. 1. A rate-adaptive pacing system

The pacing rate PR should follow the body workload Wbody, which correlates with minute ventilation MV = RR x TV estimated, e.g., through a transthoracic bioimpedance (Fig. 1).

Problem: Inadequate estimation can lead to dangerous, too high or too low PR

Automatic control of upper and lower PR limits prevents both, over and under pacing.

Cardiac output CO = HR SV

Veins

P

LLuunnggss

O2

Arteries

Pacing Rate PR

Skeletal Muscles and other tissues

Activity

Acceleration Minute Volume MV

Myocardium

P R

Heart Rate

Stroke Volume

Respiration Rate Tidal Volume

Minute Volume MV = RR TV

Over pacing takes place when- the cardiac cycle is too short to permit adequate filling, thus limiting cardiac output, or - cardiac output is maintained, but at the cost of excessive oxygen consumption.

Under pacing occurs when the sensed demand is low and the paced rate is either - insufficient to meet first cardiac or overall metabolic demand, or - the cardiac output is maintained only by an excessive increases in preload and high filling pressures.


2. Why the paced heart rate is critical?

Fig. 2. Ventricular pressure-volume loop (a),

and variation of arterial pressure (b):

1. HR = Hrest 2. HR = 2·Hrest

Metabolic demand can exceed the capabilities of a damaged heart (W > E).

Problem: artificial pacing may drive the heart into failure.

A “positive” balance E ≥ W must be maintained !

Pacing rate limits are needed, but incorrect, fixed limits can exacerbate problems such as: - Postural hypotension, caused by shifts in blood volume to lower extremities, which can lead to syncope, and - Neurogenic syncope, which is even more insidious problem because a sudden drop in blood pressure may occur minutes after the precipitating event.Dynamically adaptive limits are needed!

Sdem < Ssup

Wext ≈ ΔP · SV E ≈ ΔP · tdiast

external work energy supply


3. Myocardial Energy ?

Fig. 3. P-V diagram with energy consumption areas Sdem and Spot , where ESPVR is the end-systolic P-V relationship line.

The area

Sdem ≈ ΔP · SVin Fig. 3 represents the myocardium’s

external work Wext.

The area

Spot ≈ ΔP·(Ves ‑ V0 ) ∕ 2 is proportional to the myocardium’s

internal work Wint ,

Total energy consumption W of myocardium can be expressed as

Scons = Sdem + Spot = = ΔP·(2·SV + Ves ‑ V0) ∕ 2


4. Myocardial Energy and Impedance

The myocardial energy balance can be expressed as

Ssup = Sdem ∙ kpot , where kpot expresses the role of internal work and

depends almost linearly on relative stroke volume SV/Ves ,

but only very slightly on relative pressures Pas/Pad (Fig.4).

It can be approximated as

kpot = (1.7-1.8) SV/Ves

Rough estimation of the overall energy balance

E = Wext + Wint does not require pressure measurements.

We can extract the needed information (time intervals and relative ventricular volumes) solely from the measured intracardiac impedance waveform !

Fig. 4 Relationship of the relative energy demand Sdem/Spot

versus relative stroke volume SV/Ves

for some values of Pas /Pad .


5. Experiments and Modeling

Fig. 5. A comparison between right ventricular stroke volume measured by aortic flow, and right ventricular peak-to-peak resistance, ΔR, in a dog during an infusion of dobutamine.

1) the actual stroke volume in ml measured by aortic flowmeter in an intact canine model,

and

2) the peak-to-peak resistance change, ΔR (in ohms), during a cardiac cycle measured with a four electrode right ventricular catheter.

In Fig. 5. is shown an experimental comparison between


Fig. 6. A comparison between the stroke volumes computed from resistances (vertical axis) measured across the left ventricle (from right ventricular apex to left ventricular free wall)

and the actual left ventricular volumes (horizontal axis).

Fig. 6 shows the relationship between actual volumes, and the volumes that were calculated from impedances (vertical axis).

A finite-difference model of heart conductance containing a standard implantable electrode system was used to generate these data.

The near-to-linear relationship indicates the feasibility of following changes in relative stroke volume using a standard electrode system.


Fig. 7. Dependence of the stroke volume SV and diastolic time tdiast on pacing rate PR.

Stabilizing the Stroke Volume

The energy supply diminishes with the higher rate because the dia-stolic time tdiast

shortens (Fig.7).

When stabilizing the stroke volume at the predetermined reference value

SV = SVref , (see Fig.8) the myocardium’s

energy consumption will remain constant.

The energy balance can be expressed directly through the pacing rate.

Substituting tdiast = (60/PR) – tsyst in kpot·(SVref) · ΔP ≤ (tdiast) · ΔP

we obtain that PR ≤ 60 SVref ⁄ (1 + kpot · SVref · tsyst).

t cycle , s

0 60 120 180 PR, bpm

1 s

Rest 100W 400 Body work, W

0.5

tsyst

SV, ml

1.5

50 ml SVrest

SV – stroke volume

tdiast

60 1 tcycle = ------- = ------- = tsyst + tdiast PR fbeat


Fig. 8. A closed-loop system to control pacing rate

by maintaining a fixed stroke volume.

A closed loop stabilizer of Stroke Volume

Pacing rate is controlled automatically as a tool to stabilize the stroke volume

SVactual = SVref

using a closed-loop or feedback system.


Modeling of the Heart and Intracardiac Impedance

Fig. 9. 3D digital model of the heart with 16 measurement electrodes.

Fig. 10. Experimentation on a virtual heart reflecting impedance variations during a cardiac cycle

measured between different pairs of electrodes:

a) 1 and 4, b) 2 and 4, c) 3 and 4,

applying excitation current to electrodes 1 and 4.


Experimentation with an isolated pig’s heart

A picture of an experimental setup with an isolated pig heart is shown in Fig. 11.

The resuscitated heart was continuously perfused by an artificial blood circulation system permitting active experimentation for several hours.

An original impedance measurement and data acquisition system was developed for performing in vivo experiments.

PCT Patent Application WO2005/122889 A1 (29.12.2005).

Fig. 11. Experimental setup with an isolated pig’s heart.

http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=WO2005122889&F=0&QPN=WO2005122889


6. Discussion

As a result of our study, we assume that:

A. it becomes possible to use 1) hemodynamic information derived from bioimpedance measurements alone for feed-forward control of pacing rate, while2) simultaneously monitoring myocardial energy balance to preclude potentially damaging rates using feedback corrections.

B. the pacing rate is only an intermediate parameter, available as a control tool to 1) satisfy the patient’s metabolic demands, and2) fulfilling the myocardium’s energy supply needs.

C. a potential benefit of stabilizing the stroke volume we would maintain a relatively constant preload and myocyte “stretch”, minimizing hypertrophic signaling and subsequent cardiac remodeling.


Conclusions

Our experimental studies and theoretical speculations confirm that:

- Increased concern over cardiac efficiency and maintenance of energy balance within the heart may be addressed by novel pacing control algorithms that require only relative stroke volume information, derivable from bioimpedance measurements applied to a feedback control system.

- New impedance measurement methods can permit more reliable results to make such feedback systems feasible for rate control.

- Model based design appears to be a fruitful tool for the synthesis of complicated and nonlinear closed loop systems for pacing rate control.

Thank you for your attention !

IEEE EMBC 2006, 30 August – 3 September, 2006, New York City - We hypothesize that intracardiac...

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