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Arterial Pressure: A Personal View
D. Bennett
Three thousand years ago, the Chinese, during the reign of The Yellow Emperor,
realized that there was an association between a pulse that was difficult to com-
press and the subsequent development of stroke. However, it was not until more
than 2500 years later that blood pressure was first quantified. In 1731, the Rever-
end Stephen Hales measured the blood pressure of a horse by inserting a brass
tube one sixth of an inch in diameter connected to a glass tube which was nine feet
in length into the crural artery. After releasing the ligature, which had previously
been tied around the artery, he found that the blood rose in the tube to a height of
eight feet above the level of the left ventricle.
Hales made a further series of measurements in animals and calculated that the
blood pressure in humans would be about seven feet. Some progress occurred over
the next 100 years in developing techniques for measuring blood pressure in
patients but it was not until 1876 that Von Basch made a simple sphygmomanome-
ter which allowed him to assess systolic pressure with a fair degree of accuracy and
for the first time made it possible to collect data on blood pressure from a large
number of patients. Twenty years later, Rocci developed the mercury sphygmoma-
nometer, which has changed little in the last 100 years. Probably the most important
development in the measurement of blood pressure was the recognition by
Korotkoff that it was possible to define accurately both systolic and diastolic
pressure by listening with a stethoscope over the brachial artery below the inflated
cuff as the pressure was slowly lowered. It is worth noting that Korotkoff\u2019s descrip-
tion was in 1904 so that this year is the 100th anniversary of that event. The principle
involved in non-invasive blood pressure has changed little in the last 100 years
although several technological developments have occurred during this period.
These developments include the automated auscultatory method that uses a
microphone to detect the Korotkoff sounds but this method was sensitive to noise
artifact and was found to be inaccurate when measuring in patients with low blood
pressure. This technique measures the systolic and diastolic pressures from which
the mean pressure is calculated.
In contrast, the oscillatory method, which was devised to overcome the inaccu-
racies of the auscultatory method, measures the mean pressure from which the
systolic and diastolic pressures are calculated, calculations which are prone to
error. Other technologies introduced to overcome these problems include infra
sound to detect the very low frequency components of the Krotokoff sounds below
50 Hz, which are inaudible. Ultrasonic technology has been used combined with
other technologies to measure blood pressure but these techniques tend to be very
operator dependent.
More recently two other techniques have been developed in the hope of over-
coming some of the problems that clearly exist with all the existing methods.
Impedance plethysmography as its name implies monitors the change in electrical
impedance at the measurement site. This changes with local volumetric changes
associated with local pulsatile arterial distension occurring with each cardiac cycle.
Arterial tonometry applies a carefully measured compressing pressure to the
arterial site The applied pressure is measured by sensors and this allows an arterial
waveform to be constructed using an algorithm which is claimed to be very similar
to that directly recorded intra-arterially. A recent report using this technique,
however, has not shown a good correlation with directly measured pressure [1].
Non-invasive measurement of blood pressure is one of the most widely under-
taken procedures in clinical medicine and the data it provides are crucial in
monitoring patients with hypertension. However non-invasive techniques are only
used in a minority of intensive care unit (ICU) patients and this is for several
reasons [2\u20134].
Accuracy of measurement is of utmost importance in managing critically ill
patients particularly when they are cardiovascularly unstable when blood pressure
is low. It is vital to know that the mean arterial pressure is 65 mmHg and not 75
mmHg as this is likely to make a fundamental difference to the treatment given [5].
Clinical experience has demonstrated that in these circumstances, particularly if
the peripheral circulation is shut down, intra-arterial pressure measurement is
much more precise. In addition, it allows continuous monitoring of pressure which
none of the invasive techniques can offer. Even in less sick patients with stable
circulations, intra-arterial monitoring has the advantage of comfort. Frequently
repeated cuff inflations cause significant discomfort and adds to the level of anxiety
in an already anxious patient. Finally the presence of an intra-arterial line allows
almost unlimited blood sampling mainly for blood gas analysis but also for routine
blood tests. Intra-arterial cannulation has therefore become a routine procedure
in the vast majority of ICU patients. It is not within the scope of this chapter to
discuss the various techniques of arterial cannulation or indeed of the technology
behind the measurement of intra- arterial pressure.
Blood pressure is of course determined by the relationship between flow and
peripheral resistance and therefore plays a fundamental role in determining per-
fusion to various organs, particularly the kidneys, heart, and brain. Thus, in
situations where global flow may be normal or high, for example in septic shock, a
low peripheral resistance must be associated with low mean pressure which inevi-
tably leads to reduced flow to the kidneys once pressure falls below the lower limit
of the auto-regulatory mechanism. This lower limit in the typical older patient
commonly seen in the ICU, may be relatively high and may be one of the reasons
that acute renal failure is a common finding in such patients. The adjustment of
the mean pressure to some `optimal\u2019 level is vital in trying to minimize the risk of
developing acute renal failure although pressure alone is not the only factor leading
to the development of renal dysfunction [6].
90 D. Bennett
What level of mean pressure should be targeted is a controversial question about
which there is not a clear consensus. Most clinicians aim for a pressure that results
in urine production and is associated with the reduction in the metabolic acidosis
commonly seen in these circumstances. The majority of clinicians now feel that
manipulation of pressure in such patients should only be undertaken with knowl-
edge of cardiac output. This is to prevent vasoconstrictors being administered
where pressure is low due to hypovolemia and a low cardiac output. Similarly,
careful manipulation of blood pressure plays an essential role in the management
of patients with significantly impaired left ventricular function, for example, post
infarction or in patients with severe left ventricular failure.
The normal left ventricle is able to maintain a constant cardiac output over a
wide range of mean blood pressure. This is not true when left ventricular function
is significantly impaired so that as blood pressure rises cardiac output rapidly falls.
This is the reason that afterload reduction can be so effective in treating patients
with left ventricular failure. Furthermore, as peripheral resistance is a prime
determinant of myocardial oxygen consumption its reduction can play an impor-
tant role in the management of myocardial ischemia. The question then arises again
as to what should the target blood pressure be in such patients with significantly
impaired left ventricular function with or without evidence of continuing myocar-
dial ischemia This emphasizes why it is so important that when manipulation of
blood pressure and cardiac output are to be undertaken, they should be performed
with continuous and accurate measurements of both