26 Phase Coherence Between Cardiovascular Oscillations in Malaria …
405
Microvascular blood flow was recorded by laser-Doppler flowmetry (LDF), which
provided a continuous measurement of the microcirculation in the skin, thus reflecting the perfusion in the capillaries, arterioles, venules and dermal vascular plexus.
The instrument (moorLAB, Moor Instruments Ltd, UK) used in the present study
transmits a near-IR laser light from a temperature-stabilized laser diode operating at
a wavelength of 780 nm and with a maximum power of 2.5 mW into the skin through
an MP1-V2 probe (Moor Instruments Ltd, UK), which has two optical fibres. A time
constant of 0.1 s was selected and the LDF processor bandwidth was between 18 Hz
and 22.5 KHz. A flexible probe holder (PH1-V2, Moor Instruments Ltd., UK) was
attached to the skin surface on the outer side of each ankle (lateral malleolus) using
double-sided adhesive discs. One fibre delivers light to the site under observation,
while the backscattered (reflected) light is collected by the other fibre. The light
reflected back from moving RBCs is Doppler-shifted in frequency by an amount
related to the blood flow in the illuminated volume of tissue, the frequency shift
being proportional to red cell speed, while the frequency of the light reflected from
stationary cells and tissue remains unchanged [29]. The difference between incident
light and the Doppler-shifted back-scattered light yields the LDF signal, known as
the blood perfusion signal. The LDF output is semi-quantitative and is expressed in
perfusion units (PU) of output voltage (typically 1 PU = 10 mV) [29]. The sampling
frequency was 40 Hz.
An electrocardiogram (ECG) was used to record the electrical activity of the
heart with a sampling frequency of 1 kHz. The ECG was measured using a bipolar
precordial lead. The electrodes were attached on both shoulders and the lowest left
rib, as this maximizes the sharpness of the R-peak.
The respiration was measured using an elasticated belt fastened across the chest
and fitted with a Biopac TSD201 Respiratory Effort Transducer (Biopac Systems
Inc., CA, USA).
Skin temperature was monitored using two high-sensitivity, low-heat-capacity
thermistors—YSI 709B Thermilinear sensors (YSI Inc, Yellow Springs, OH, USA)
of 8.5 mm diameter, which were taped onto the skin. The thermistors were positioned
outside the left ankles, over the lateral malleolus, close to the LDF probes.
The individual time series were recorded simultaneously using a signal conditioning system (Cardiosignals, Institute Jožef Stefan, Slovenia) over an interval of
30 min.
Blood pressure was measured prior to the initiation of signal acquisition. A Digital
Automatic Blood Pressure Monitor (Omron, M10-IT) was used, with a cuff wrapped
on the subjects’ upper right arm while they were seated. The subject then moved to
a supine position on a comfortable bed, where the necessary sensors were installed.
In this way, subjects relaxed in a supine position for 15–20 min of acclimatisation,
prior to the recordings. The equipment was either battery supplied or connected to
the electrical supply via a mains filter.
405
Microvascular blood flow was recorded by laser-Doppler flowmetry (LDF), which
provided a continuous measurement of the microcirculation in the skin, thus reflecting the perfusion in the capillaries, arterioles, venules and dermal vascular plexus.
The instrument (moorLAB, Moor Instruments Ltd, UK) used in the present study
transmits a near-IR laser light from a temperature-stabilized laser diode operating at
a wavelength of 780 nm and with a maximum power of 2.5 mW into the skin through
an MP1-V2 probe (Moor Instruments Ltd, UK), which has two optical fibres. A time
constant of 0.1 s was selected and the LDF processor bandwidth was between 18 Hz
and 22.5 KHz. A flexible probe holder (PH1-V2, Moor Instruments Ltd., UK) was
attached to the skin surface on the outer side of each ankle (lateral malleolus) using
double-sided adhesive discs. One fibre delivers light to the site under observation,
while the backscattered (reflected) light is collected by the other fibre. The light
reflected back from moving RBCs is Doppler-shifted in frequency by an amount
related to the blood flow in the illuminated volume of tissue, the frequency shift
being proportional to red cell speed, while the frequency of the light reflected from
stationary cells and tissue remains unchanged [29]. The difference between incident
light and the Doppler-shifted back-scattered light yields the LDF signal, known as
the blood perfusion signal. The LDF output is semi-quantitative and is expressed in
perfusion units (PU) of output voltage (typically 1 PU = 10 mV) [29]. The sampling
frequency was 40 Hz.
An electrocardiogram (ECG) was used to record the electrical activity of the
heart with a sampling frequency of 1 kHz. The ECG was measured using a bipolar
precordial lead. The electrodes were attached on both shoulders and the lowest left
rib, as this maximizes the sharpness of the R-peak.
The respiration was measured using an elasticated belt fastened across the chest
and fitted with a Biopac TSD201 Respiratory Effort Transducer (Biopac Systems
Inc., CA, USA).
Skin temperature was monitored using two high-sensitivity, low-heat-capacity
thermistors—YSI 709B Thermilinear sensors (YSI Inc, Yellow Springs, OH, USA)
of 8.5 mm diameter, which were taped onto the skin. The thermistors were positioned
outside the left ankles, over the lateral malleolus, close to the LDF probes.
The individual time series were recorded simultaneously using a signal conditioning system (Cardiosignals, Institute Jožef Stefan, Slovenia) over an interval of
30 min.
Blood pressure was measured prior to the initiation of signal acquisition. A Digital
Automatic Blood Pressure Monitor (Omron, M10-IT) was used, with a cuff wrapped
on the subjects’ upper right arm while they were seated. The subject then moved to
a supine position on a comfortable bed, where the necessary sensors were installed.
In this way, subjects relaxed in a supine position for 15–20 min of acclimatisation,
prior to the recordings. The equipment was either battery supplied or connected to
the electrical supply via a mains filter.