Lemson_subbed.qxp 16/2/09 14:22 Page 94
Paediatric Haematology
Figure 1: Basic Haemodynamics Figure 3: Example of a Model of the Circulation
AB
Controlled variable (baroreflex)
Aortic resistance
Aorta
e volume
Blood pressure = SVR x CO
Strok
Compliance Peripheral resistance
Fluid-responsive
HR x SV
Preload, afterload, contractility
Right atrium
Pre-load
CO = cardiac output (l/minute); HR = heart rate; SV = stroke volume.
Figure 4: Arterial-pressure-based Continuous
Cardiac Output Methods
Figure 2: Flow and Pressure Recording of a 5.7kg Lamb at
Hypovolemic Shock (A), After Fluid Resuscitation (B) and
During Dobutamine Therapy (C)
A. Pulse contour B. PulseCO
Pulse contour
ABC
P(t) dP
∆V/∆bp = cal*250*e
-kbp
Cardiac output = K*HR*
+C(p)* dt
8
SVR dT The algorithm corrects any arterial pressure
7
HR = heart rate
signal to a standardised volume waveform
6 K = factor reflecting specific patient characteristics K = factor reflecting specific patient
Opa
5 P = pressure characteristics (determined with use of TPTD)
(1/min)
4
t = time Cal = a calibration factor derived from compliance
3
SVR = systemic vascular resistance bp = blood pressure
2
1
C = compliance of aorta k = constant
0
-1
120
C. PRAM D. FlowTrac
100
A
80
Cardiac output = HR* Cardiac output= PR*sd(AP)*X
Opa
P/
t
*F
(1/min)
60 HR = heart rate
PR = pulse rate
40
A = area under the systolic pressure curve
Sd(AP) = pulsatility using the standard deviation
P = pressure
of the arterial pressure wave
20
t = time
X = constant (arterial compliance, vascular
0 F = dimensional factor inversely related to the resistance)
0 0.2 0.4 0 0.2 0.4 0 0.2 0.4 instantaneous acceleration of the vessel cross-
section area
Sec Sec Sec
Qpa = flow pulmonary artery; Pao = aortic pressure.
area under the systolic portion of the arterial pressure waveform and
opportunity to decrease the afterload of the left ventricle during applies an equation as shown in Figure 4A. It requires calibration using
the ejection phase, while at the same time helping to increase diastolic transpulmonary thermodilution in order to establish the individual
pressure, enabling diastolic flow through, for instance, the left characteristics of the aortic compliance. The PulseCO method is
coronary artery. However, the windkessel function is strongly incorporated in the LidCO device (LidCO, Cambridge, UK) and uses a
influenced by the (individually different) compliance of the aorta. pulse power analysis to derive the SV. This method uses measurements
of the beat and ejection duration and a series of approximations
Figure 2 shows an example of an animal experiment. A 5kg lamb is regarding the relationship between radial artery pressure, aortic
subjected to hypovolaemia, subsequent volume resuscitation and pressure, aortic flow and CO. By using the entire pressure waveform
dobutamine therapy. The flow measured with an ultrasound flow probe rather than just the systolic portion of the curve, the PulseCO system
around the pulmonary artery reflects actual CO. It can clearly be seen incorporates the influence of peripheral resistance and the reflected
that the arterial curve shows an increase in absolute pressure and pulse wave from the periphery (see Figure 4B). The derived SV is corrected
pressure concomitant with the increase in CO. However, the curve itself using a calibration factor based on the transpulmonary lithium
also changes in shape and magnitude. This change is caused by dilution technique.
alterations in peripheral vascular resistance, resonance, pulse pressure
and CO. These effects can be incorporated into an algorithm to Currently, there are two systems that do not require calibration. The
calculate absolute cardiac stroke volume. A simple model is shown in pressure recording analytical method (PRAM, Mostcare, BioSi, Florence,
Figure 3. A method of determining CO using the arterial pressure curve Italy) uses a routine that identifies the characteristic points of the pressure
was described long ago.
20
A simple research device was built by wave during each beat (diastolic, systolic, dicrotic notch and resonant
Wesseling et al.
21
pressure points during the systolic and end-diastolic phases). The
morphological analysis of the beat allows determination of the stroke
As a result of the unknown individual characteristics of the aorta, a volume (see Figure 4C).
22
In the FloTrac system (Vigileo, Edwards, US) SV
second CO technique is often used to calibrate an APCCO method. is derived using the patient’s vascular resistance and arterial compliance
There are currently two methods that require calibration with an based on sex, height, weight and age and the pulse pressure (PP)
established CO method. The pulse contour method (PCCO) that is waveform characteristics, while pulsatility is derived from continuous
incorporated into the PiCCO device (Munich, Germany) calculates the analysis of the shape of the arterial pressure waveform (see Figure 4D).
94 EUROPEAN HAEMATOLOGY
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