Esophagus
The transition between these two type of epithelium is visible as a
zig-zag line. Most of the muscle is smooth muscle although striated
muscle predominates in its upper third. It has two muscular rings or
sphincters in its wall, one at the top and one at the bottom. The
lower sphincter helps to prevent reflux of acidic stomach content. The
esophagus has a rich blood supply and vascular drainage. Its smooth
muscle is innervated by involuntary nerves (sympathetic nerves via the
sympathetic trunk and parasympathetic nerves via the vagus nerve) and
in addition voluntary nerves (lower motor neurons) are carried in the
vagus nerve to innervate its striated muscle.
Sympathetic nervous system
Abstract
To characterize the efferent pathway from the hypothalamic
paraventricular nucleus (PVN) to peripheral autonomic neurons and
finally to selected effector organs, we stimulated the PVN in 10
conscious rats at frequencies ranging from 0.05 to 2.0 Hz.
Simultaneously, blood pressure, heart rate, splanchnic sympathetic
nerve activity, and mesenteric artery blood flow were measured. The
sinus node of the heart responded to PVN stimulation via the
parasympathetic pathway (during beta 1-adrenergic blockade) up to a
stimulation frequency of 2.0 Hz, whereas the sympathetically mediated
response (during muscarinic blockade) was limited to stimulation
frequencies < 0.5 Hz. The splanchnic nerve responded to PVN
stimulation with synchronous discharges up to stimulation frequencies
of 2.0 Hz, whereas the oscillatory component of the vasoconstrictor
response of the mesenteric artery was negligible beyond stimulation
frequencies of 1.0 Hz. We conclude that sympathetic transmission to
the heart is at least four times slower than parasympathetic
transmission. In addition, the time-limiting step in sympathetic
transmission from the hypothalamus to vascular smooth muscle
contraction and pacemaker activity of the sinus node may be located at
the site of synaptic transmission to the adrenergic receptors.
This monitors the electrical activity of the heart with electrical
potentials of 0.5-2 mV at the skin surface. It is useful in
determining the heart rate, ischaemia, the presence of arrhythmias and
conduction defects. It should be emphasized that it gives no
assessment of cardiac output.
The bipolar leads (I, II, III, AVR, AVL and AVF) measure voltage
difference between two electrodes. The unipolar leads (V1-6) measure
voltage at different electrodes relative to a zero point.
Components
1.
Skin electrodes detect the electrical activity of the heart (Fig.
10.5). Silver and silver chloride form a stable electrode combination.
Both are held in a cup and separated from the skin by a foam pad
soaked in conducting gel.
Colour-coded cables to transmit the signal from electrodes to the
monitor. Cables are available in 3- and 5-lead versions as snap or
grabber design and with a variety of lengths. All the cables of a
particular set should have the same length to minimize the effect of
electromagnetic interference.
3.
The ECG signal is then boosted using an amplifier. The amplifier
covers a frequency range of 0.05-150 Hz. It also filters out some of
the frequencies considered to be noise. The amplifier has ECG filters
that are used to remove the noise/artifacts from ECG and produce a
'clean' signal.
4.
An oscilloscope that displays the amplified ECG signal. A
high-resolution monochrome or colour monitor is used.
Mechanism of action
1.
Proper attachment of ECG electrodes involves cleaning the skin, gently
abrading the stratum corneum and ensuring adequate contact using
conductive gel. Skin impedance varies at different sites and it is
thought to be higher in females. The electrodes are best positioned on
bony prominences to reduce artifacts from respiration.
2.
Modern ECG monitors use multiple filters for signal processing. The
filters used should be capable of removing the unwanted frequencies,
leaving the signal intact (Fig. 10.6). Two types of filters are used
for this purpose:
a)
high-pass filters attenuate the frequency components of a signal below
a certain frequency. They help to remove lower frequency noise from
the signal. For example, the respiratory component from ECG can be
removed by turning on a 1-Hz high pass filter on the amplifier. The
filter will centre the signal around the zero isoline
b)
low-pass filters attenuate the frequency components of a signal above
a certain frequency. They are useful for removing noise from lower
frequency signals. So an amplifier with a 35-Hz low-pass filter will
remove/attenuate signals above 35 Hz and help to 'clean' the ECG
signal.
The ECG monitor can have two modes:
a)
the monitoring mode has a limited frequency response of 0.5-50 Hz.
Filters are used to narrow the bandwidth to reduce environmental
artifacts. The high-frequency filters reduce distortions from muscle
movement, mains current and electromagnetic interference from other
equipment. The low-frequency filters help provide a stable baseline by
reducing respiratory and body movement artifacts
b)
the diagnostic mode has a wider frequency response of 0.05-150 Hz. The
high-frequency limit allows the assessment of the ST segment, QRS
morphology and tachyarrhythmias. The low-frequency limit allows
representation of P- and T-wave morphology and ST-segment analysis.
4.
There are many ECG electrode configurations. Usually during
anaesthesia, three skin electrodes are used (right arm, left arm and
indifferent leads). The three limb leads used include two that are
'active' and one that is 'inactive' (earth). Sometimes five electrodes
are used. Lead II is ideal for detecting arryhthmias. CM5
configuration is able to detect 89% of ST-segment changes due to left
ventricular ischaemia. In CM5, the right arm electrode is positioned
on the manubrium (chest lead from manubrium), the left arm electrode
is on V5 position (fifth interspace in the left anterior axillary
line) and the indifferent lead is on the left shoulder or any
convenient position the area in zig-zag is at times counter with other
THC affects to relax to accept the ECG that record is on file of the
Frequency used and can be used to cause a Heart Attack is a lead into
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