40

Prévia do material em texto

Totally Non-invasive Continuous Cardiac Output
Measurement with the Nexfin CO-Trek
A. Perel and J.J. Settels
J.-L. Vincent (ed.), Annual Update in Intensive Care and Emergency Medicine 2011
DOI 10.1007/978-3-642-18081-1, ˇ Springer Science+Business Media LLC 2011
Introduction
Cardiac output is one of the most important physiological parameters as it is the
major determinant of oxygen delivery (DO2). Its measurement is of special impor-
tance since so many of our efforts in the care of critically ill and high-risk surgi-
cal patients are aimed at optimizing its value by various therapeutic means. The
importance of measuring cardiac output is further highlighted by the many stud-
ies that have repeatedly shown that clinical evaluation and conventional monitor-
ing alone are inaccurate and unreliable for the assessment of cardiac output, and
that adequate resuscitation cannot be based on normalization of vital signs alone
[1–4]. Although there is little evidence that the measurement of cardiac output
per se improves outcome, this is also true for all other hemodynamic parameters
that are in common daily use.
The main technology for the measurement of cardiac output has been the ther-
modilution technique used with the pulmonary artery catheter (PAC). However,
many newer technologies offer a less invasive approach than the PAC while measur-
ing cardiac output with similar accuracy. The most recent significant development
in this field is the introduction of uncalibrated continuous cardiac output technolo-
gies. Uncalibrated continuous cardiac output is usually based on pulse contour
technology, by which various formulas are used to compute cardiac output values
from the blood pressure waveform, without using intermittent thermodilution for
calibration. The absolute accuracy of some of these technologies has not been
shown to equal that of intermittent thermodilution [5]. However, most of these
devices have good tracking accuracy which provides a very useful tool for the
assessment of hemodynamic events with short time constants, e.g., fluid loading,
passive leg raising, start of inotropes, etc. The Nexfin HD, which belongs to this
category of continuous cardiac output monitors, offers in addition a unique feature
– total non-invasiveness.
The Nexfin HD monitor (Fig. 1, BMEYE, Amsterdam, The Netherlands) is a
device that measures continuous cardiac output by an inflatable finger cuff which is
the only interface with the patient. The Nexfin HD measures cardiac output by com-
bining continuous blood pressure monitoring and a novel pulse contour method
(Nexfin CO-Trek) which is based on the systolic pressure area and a physiological
three-element Windkessel model. The cardiac output is calculated without external
calibration although it can be calibrated externally. The parameters that are mea-
sured by the Nexfin HD include continuous blood pressure (systolic, diastolic,
mean), heart rate, continuous cardiac output, stroke volume (SV), systemic vascular
resistance (SVR), and an index of left ventricular (LV) contractility (dP/dt).
434
X
Fig. 1. The Nexfin monitor.
The Principles of Continuous Cardiac Output Measurement
using the Nexfin HD
The Nexfin HD applies three major steps in the measurement of continuous car-
diac output: 1. Measurement of continuous beat-by-beat finger blood pressure; 2.
Transformation of the finger blood pressure curve into a brachial arterial blood
pressure waveform; 3. Calculation of the continuous cardiac output from the bra-
chial pressure pulse contour.
Measurement of Continuous Finger Blood Pressure
For measuring continuous beat-by-beat blood pressure, a cuff is wrapped around
the middle phalanx of the 2nd, 3rd or 4th finger (Fig. 2). The finger cuff includes an
LED emitter-detector that measures the diameter of the finger arteries running
on both sides of the finger’s palmar aspect. The cuff pressure is increased and
decreased to keep the diameter of the finger arteries constant throughout the car-
diac cycle (’volume clamp method’) (Fig. 3). Continuous recording of the cuff
pressure therefore generates a real-time arterial pressure waveform. After initial
calibration which typically takes 1–2 minutes, the Nexfin uses an auto-calibration
algorithm (‘Physiocal’) that periodically recalibrates the system. During these
periodic calibrations, the pressure is maintained at three levels resulting in three
different volume changes, the shape of which determines the calibration. Depend-
ing on the stability of the pressure signal, the length of the calibration interval
Totally Non-invasive Continuous Cardiac Output Measurement with the Nexfin CO-Trek 435
X
Fig. 2. The finger sensor which is the only
interface with the patient.
Fig. 3. The finger sensor includes 2 LEDs and an inflatable cuff which measures beat-to-beat real-time
continuous blood pressure.
varies between 5 beats and 70 beats. Intervals & 30 beats indicate a stable mea-
surement condition. Periodic calibration with the Physiocal improves the accu-
racy of the continuous blood pressure measurement during significant changes in
vascular tone. The volume clamp and Physiocal methods were previously used in
the Ohmeda 2300 Finapres continuous blood pressure monitor. A ‘heart reference
436 A. Perel and J.J. Settels
X
system’ (HRS) measures and automatically corrects for the vertical height differ-
ence between the finger cuff and the heart level, allowing free movement of the
hand while continuous cardiac output is monitored.
Transformation of the Finger Blood Pressure Curve Into a Brachial Artery Waveform
The continuous cardiac output measured by pulse contour algorithms may be sig-
nificantly influenced by the site of the blood pressure measurement [5]. As a rule,
the more peripheral the site where the blood pressure is measured, the greater
the chance that the algorithm may not be able to compensate for changes in
shape and amplitude of the waveform in extreme hemodynamic conditions. The
Nexfin HD transforms finger blood pressure to brachial blood pressure waveform
using a transfer function based on a vast clinical database and correcting for the
brachial-finger pressure gradient. Thus the continuous cardiac output measured
by the Nexfin uses the brachial pressure waveform as a robust substitute for aor-
tic pressure as input for continuous cardiac output measurement. Calibration
with an upper arm cuff is no longer needed.
Calculation of Continuous Cardiac Output from the Brachial Arterial Pressure
Waveform
The original concept of the pulse contour method for estimation of beat-to-beat
SV was first described by Otto Frank in 1899 as the classic two-element Windkes-
sel model (see later). According to this model, the interaction between the cardiac
systole and the arterial input impedance (Zin) determines the SV and the systolic
and diastolic arterial pressures. Pulse contour methods in general use this close
interaction in the hemodynamic version of Ohm’s law (ΔP/Q = Zin). Thus, when
the arterial input impedance (Zin) is known, a given pressure (P) allows for the
calculation of the related flow (Q).
Pulse contour algorithms for non-calibrated continuous cardiac output mea-
surement were initially developed by Wesseling and coworkers, who described the
corrected characteristic impedance or cZ method in 1983 [6], and the Modelflow
method in 1993 [7]. The cZ method computes beat-to-beat stroke volume by inte-
grating the area under the systolic portion of the measured arterial pressure pulse
(Fig. 4 A) and dividing this area by the characteristic input impedance (cZ). A cor-
rection to the characteristic impedance, which is derived by a distributed trans-
mission line model, is then applied in order to account for changes in mean arte-
rial pressure (MAP) and heart rate (HR): Zc = K / (a + b × HR + c × MAP). The
limitation of this pulse contour algorithm is that a calibration factor (K) had to be
determined at least once for each patient, and that the non-linear pressure depen-
dency ofcompliance, as well as the effects of strong vasoconstriction and vasodi-
latation in peripheral arterioles, were not modeled.
To overcome these limitations, Wesseling developed the Modelflow method
(1993) [7] by simulation of a flow waveform in a physiological, three-element,
non-linear, age-dependent Windkessel model of the aortic arterial input imped-
ance as a description of the cardiac afterload of the left ventricle. This model
includes the following elements (Fig. 4 B):
1. Characteristic impedance (Zc). During ejection into the blood-filled proximal
aorta, the left heart encounters the combined effects of the proximal aortic
Totally Non-invasive Continuous Cardiac Output Measurement with the Nexfin CO-Trek 437
X
age
gender
height
weight
Z0
Cw
stroke volume
cardiac output
HR
RpPressure
MAP
Zin
A
B
C
D
E
F
compliance (C) and its blood mass, or inertance (L). This combined effect is
called the characteristic impedance and is calculated as Zc = √L/C, where iner-
tia increases the resistance to ejection and compliance facilitates ejection.
2. The total arterial compliance (Cw) equals the sum of the compliances of all
arteries, is determined mainly by the ascending and descending aorta, and
represents the ability of the aorta and larger vessels to elastically store the
ejected stroke volume.
3. The total peripheral resistance (Rp) equals the sum of the resistances of all
small arteries, arterioles and capillaries. It represents the resistance to outflow
of blood to all vascular beds during diastole when inflow into the aorta is zero.
SV is then calculated by the integration of the systolic portion of the resulting
modeled flow waveform. The method can be calibrated for each individual
patient by adjusting a calibration factor to improve absolute precision [7]. Track-
ing changes in SV with this method has been found to be excellent even in the
presence of large changes in pressure, heart rate and strong vasodilation or vaso-
constriction [8]. However, the Modelflow method is less accurate in estimating
the initial absolute level of cardiac output at the start of monitoring and is less
suited for the use of finger blood pressure, which gives less satisfactory results
than invasive radial arterial pressure.
The Nexfin CO-Trek algorithm, which was introduced in 2007, computes beat-
to-beat stroke volume using an updated pulse contour method that contains ele-
ments from these two previous methods (Fig. 4). The Nexfin CO-Trek method
Fig. 4. The Nexfin CO-Trek algorithm integrates the systolic pressure-time integral (A) of the brachial
arterial pressure waveform (C). The heart’s afterload is calculated from the three-element Windkessel
model (B), with Zo (characteristic impedance) and Cw (arterial compliance) derived from the aortic
pressure-diameter relationship using age, gender, height and weight as input parameters (D). The total
peripheral resistance (Rp) is time-dependent and is an outcome of the model computations (E).
438 A. Perel and J.J. Settels
X
integrates the systolic pressure-time integral similar to the cZ method (Fig. 4A),
but in doing so it uses the reconstructed brachial arterial pressure waveform as a
close substitute for aortic pressure as pressure input (Fig. 4C). The heart’s after-
load is calculated from the three-element Windkessel model (Fig. 4B), with Zo
(characteristic impedance) and Cw (arterial compliance) derived from the aortic
pressure-diameter relationship using age, gender, height and weight as input
parameters (Fig. 4D). With the individual input of these parameters for each
patient, the components of the three-element Windkessel afterload are individual-
ized, can be computed for any arterial pressure level, and can further be used for
SV computation. The total peripheral resistance (Rp) is time-dependent and is an
outcome of the model computations (Fig. 4E). Beat-to-beat SV is calculated by
dividing the pressure-time integral by the resulting Zin (Fig. 4F), so that SV =
1 / Zin * � [P(t) – Pd]dt.
Validation of the Nexfin HD
Since the Nexfin HD is a new device there are only a few published validation
studies of its new CO-Trek algorithm. However, many clinical studies have vali-
dated the previous pulse contour methods, namely the cZ and the Modelflow
methods. These studies, done in high risk surgery (e.g., cardiac, liver transplanta-
tion) and intensive care patients, showed excellent correlation with thermodilu-
tion cardiac output, which was measured by an automated series of 4 thermodilu-
tion injections equally spread over the ventilatory cycle for better accuracy [8]. In
a study from 2007, performed in 24 patients undergoing uncomplicated coronary
artery bypass graft (CABG) surgery, excellent results in absolute values as well as
in tracking changes in cardiac output were obtained using the Modelflow method
[9]. The results were found to be better than those of Wesseling’s cZ method, and
of the LiDCO and PiCCO algorithms. In a more recent study from 2009, the same
group of de Wilde et al. studied another small group of patients within 2 h of
arrival in the intensive care unit (ICU) following cardiac surgery [10]. The values
of cardiac output measured by the FloTrac device, the Modelflow method and the
transesophageal ultrasonic HemoSonic system (Arrow), were compared with
accurately performed thermodilution as the reference. Cardiac output values were
measured during and after four interventions: (i) an increase in tidal volume by
50 %; (ii) a 10 cmH2O increase in positive end-expiratory pressure (PEEP); (iii)
passive leg raising; (iv) head-up position. The cardiac output values measured by
Modelflow were found to have the best precision (0.69 l/min) and smallest limits
of agreement (-1.08, +1.68 l/min, 26 %), compared with the FloTrac (-1.47, +2.13
l/min, 34 %) and the Hemosonic (-2.62, +1.80 l/min, 44 %) systems. The authors
concluded that only the Modelflow yielded limits of agreement (26 %) that were
below the 30 % criteria for a theoretically acceptable alternative to thermodilu-
tion cardiac output. The FloTrac was found to overestimate changes in cardiac
output, although directional changes in thermodilution cardiac output were
detected with a high score by all three methods [10].
In another recent study [11], the authors observed a good correlation between
cardiac output values measured by a PAC and by the Nexfin HD, with an r2 =
0.83, a bias of 0.23 l/min, and two SD of ± 2.1 l/min, and a percentage of error
of 29 %. These findings are even more impressive if one takes into account that
the study was done in severely ill patients (4 patients were post-lung transplant,
Totally Non-invasive Continuous Cardiac Output Measurement with the Nexfin CO-Trek 439
X
4 patients were post-liver transplant, and 2 had severe acute respiratory distress
syndrome [ARDS]), all of whom were receiving norepinephrine at the time of
the study. Moreover, data were analyzed retrospectively using hourly Nexfin HD
cardiac output measurements, rather than simultaneously measured cardiac out-
put values. The authors noted that there were no clinical signs of disturbed
microcirculation of the fingers in these patients during application of the finger
cuff, indicating a safe use of the Nexfin HD system. In addition, the authors
noted that the Nexfin HD, being quick to install and easy to use, could offer a
quick initial hemodynamic overview and allow to bridge the time until a longer-
lasting invasive monitoring could be installed in the case of a deteriorating
patient [11].
Clinical Applications
The totally non-invasive nature of the Nexfin HD allows the measurement of con-
tinuous cardiac output in a much wider variety of patients than was hitherto pos-
sible. Originally, the Nexfin HD was introduced in cardiology clinics for the per-
formance of tilt-test for the detection of orthostatic hypotension. Indeed, a recent
study using finger pressure-derived continuous cardiac output has shown that the
early postoperative postural cardiovascularresponse is impaired after radical
prostatectomy with a risk of orthostatic intolerance, limiting early postoperative
mobilization [12]. Both the tilt test and the sit-stand test take advantage of the
fact that the Nexfin HD provides real-time continuous cardiac output, allowing
immediate detection of the instantaneous response to diagnostic and therapeutic
challenges. These include passive leg raising, fluid challenge, start of inotropes,
exercise, etc. The continuous real-time cardiac output measurement may be more
useful and provide more accurate information about the changes in cardiac out-
put than intermittent cardiac output measurements by thermodilution with their
inherent variance.
One of the most interesting areas where the potential of the Nexfin HD can be
fully expressed is perioperative care. It is well recognized that a small group of
patients account for the majority of perioperative morbidity and mortality.
These ‘high-risk’ patients have a poor outcome because of their inability to meet
the oxygen transport demands imposed on them by the nature of the surgical
response during the perioperative period. It has been shown that by targeting
specific hemodynamic and oxygen transport goals at any point during the peri-
operative period, the outcome of these patients can be improved. Most studies
on perioperative optimization have used repetitive fluid challenges in order to
maximize the cardiac output. Cardiac output was measured in most of these
studies by esophageal Doppler or by FloTrac. However, esophageal Doppler can
be used only after induction of anesthesia and for a limited period, while the
FloTrac necessitates the presence of an arterial line. Indeed, finger pressure
derived continuous cardiac output has already been used for this purpose [13].
The non-invasive nature of the Nexfin HD and its semi-disposable finger sensor
make this monitor fit ideally in this important setting. In fact, the Nexfin is per-
fectly suitable for any patient who is sick or at risk enough to warrant the need
for continuous real-time hemodynamic monitoring, but who is not sick enough
to warrant the use of invasive lines and catheters with their associated complica-
tions.
440 A. Perel and J.J. Settels
X
There are some limitations to the use of the Nexfin HD. Because of the continu-
ous variable inflation of the finger cuff, the approved duration of continuous
measurement is restricted to 8 hours. In addition, the Nexfin HD may not per-
form well in the presence of strong vasoconstriction or very edematous fingers.
Currently the device does not provide measures of pulse pressure (PPV) or stroke
volume (SVV) variations, which should be an integral part of any continuous car-
diac output monitor. The additional benefits of the Nexfin HD include a semi-dis-
posable finger cuff which can be used for many patients, a user-friendly touch
screen, and instant data retrieval with a USB flash drive. The second version of
the monitor, the ccNexfin, includes pulse oximetry and non-invasive hemoglobin
(Masimo Rainbow SET technology) making all elements of DO2 available at the
bedside in a non-invasive manner.
Conclusion
The monitoring of continuous cardiac output offers important information that
cannot be reliably obtained from clinical examination and conventional monitor-
ing alone. The new generation of uncalibrated continuous cardiac output moni-
tors that are based on pulse contour analysis has made this measurement more
prevalent in the care of high-risk and critically ill patients. The Nexfin monitor
provides uncalibrated continuous cardiac output in a totally non-invasive manner
with a finger cuff being the only interface with the patient. The non-invasive
nature of this device allows quick and easy measurement of continuous cardiac
output in patients without the need for sedation or arterial cannulation. The Nex-
fin HD can, therefore, be used as a short-term (up to 8 hours) continuous cardiac
output monitor in patients who are not sick enough to warrant more invasive and
advanced monitoring, or to offer an initial hemodynamic overview until a longer-
lasting invasive monitoring can be installed. The Nexfin HD seems most suitable
for the perioperative period, where the ability to track hemodynamic events with
sort time constants has been shown to improve outcome.
References
1. Wo CCJ, Shoemaker WC, Appel PL, Bishop MH, Kram HB, Hardin E (1993) Unreliability of
blood pressure and heart rate to evaluate cardiac output in emergency resuscitation and
critical illness. Crit Care Med 21: 218–223
2. Meregalli A, Oliveira RP, Friedman G (2004) Occult hypoperfusion is associated with
increased mortality in hemodynamically stable, high-risk, surgical patients. Crit Care 8:
R60-R65
3. Veale WN Jr, Morgan JH, Beatty JS, Sheppard SW, Dalton ML, Van de Water JM (2005)
Hemodynamic and pulmonary fluid status in the trauma patient: are we slipping? Am Surg
71: 621–625
4. Grissom CK, Morris AH, Lanken PN, et al (2009) Association of physical examination with
pulmonary artery catheter parameters in acute lung injury. Crit Care Med 37: 2720–2726
5. Camporota L, Beale R (2010) Pitfalls in hemodynamic monitoring based on the arterial
pressure waveform. Crit Care 14: 124
6. Wesseling KH, de Wit B, Weber JA, Smith NT (1983) A simple device for continuous mea-
surement of cardiac output. Adv Cardiovasc Phys 5: 16–52
7. Wesseling KH, Jansen JRC, Settels JJ, Schreuder JJ (1993) Computation of aortic flow from
pressure in humans using a nonlinear, three-element model. J Appl Physiol 74: 2566–2573
8. Jansen JRC, Schreuder JJ, Mulier JP, Smith NT, Settels JJ, Wesseling KH (2001) A compari-
Totally Non-invasive Continuous Cardiac Output Measurement with the Nexfin CO-Trek 441
X
son of cardiac output derived from the arterial pressure wave against thermodilution in
cardiac surgery patients. Br J Anaesth 87: 212–222
9. de Wilde RBP, Schreuder JJ, van den Berg PCM, Jansen JR (2007) An evaluation of cardiac
output by five pulse contour techniques during cardiac surgery. Anaesthesia 62: 760–768
10. de Wilde RBP, Geerts BF, Cui J, van den Berg PCM, Jansen JRC (2009) Performance of
three minimally invasive cardiac output monitoring systems. Anaesthesia 64: 762–769
11. Stover JF, Stocker R, Lenherr R, Neff TA, Cottini SR, Zoller B, Béchir M (2009) Noninva-
sive cardiac output and blood pressure monitoring cannot replace an invasive monitoring
system in critically ill patients. BMC Anesthesiology 9:6
12. Bundgaard-Nielsen M, Jørgensen CC, Jørgensen TB, Ruhnau B, Secher NH, Kehlet H
(2009) Orthostatic intolerance and the cardiovascular response to early postoperative
mobilization. Br J Anaesth 102: 756–762
13. Bundgaard-Nielsen M, Ruhnau B, Secher NH, Kehlet H (2007) Flow-related techniques for
preoperative goal-directed fluid optimization. Br J Anaesth 98: 38–44
442 A. Perel and J.J. Settels
X
	Totally Non-invasive Continuous Cardiac Output Measurement with the Nexfin CO-Trek
	Introduction
	The Principles of Continuous Cardiac Output Measurement using the Nexfin HD
	Measurement of Continuous Finger Blood Pressure
	Transformation of the Finger Blood Pressure Curve Into a Brachial Artery Waveform
	Calculation of Continuous Cardiac Output from the Brachial Arterial Pressure Waveform
	Validation of the Nexfin HD
	Clinical Applications
	Conclusion
	References