Cardiac Output Clinical Trial
Official title:
The Effect of Prone Positioning During Anaesthesia on Liver Blood Flow and Function
Recent occurrence of significant post-operative liver impairment at Nottingham University
Hospitals NHS Trust has prompted investigation into whether interaction between general
anaesthesia and prone positioning (lying face down) for surgery can influence liver
function.
Historical research has shown that the heart does not function as efficiently when an
anaesthetised patient is placed in the prone position for surgery. The techniques used for
anaesthetising and monitoring these patients have changed remarkably, since these studies
were first performed.
It is important to know accurately how the heart functions to enable better understanding of
the changes in blood flow to the liver when in the prone position.
Studies looking at blood flow to the liver when lying face down have been done before but
are mainly reporting patients on the intensive care unit. These intensive care patients are
different to those in theatre undergoing routine surgery. The intensive care patients are
usually received in different types of drugs and monitored with several different types of
monitor at the time. Perhaps most importantly is that they are placed face down on a soft
air cushioned mattress and pillows rather than the rigid support used for surgical patients.
This study will look at whether the function of the liver changes when a patient is
anaesthetised and is then rolled onto their front. The function of the liver will be
measured by looking how it clears a specific drug from the blood. Also this study will look
at how accurate a particular type of heart monitor is when an anaesthetised patient is
placed onto their front.
Recent occurrence of significant post-operative hepatic dysfunction including two fatalities
at Nottingham University Hospitals NHS Trust has prompted investigation into whether
interaction between general anaesthesia, prone positioning for surgery, cardiac output and
the use of vasoactive drugs can influence hepatic function.
Research has shown that cardiac output is decreased when an anaesthetised patient is placed
from the supine to the prone position, and that the magnitude of these changes vary with the
exact method of physical patient support in the prone position.1-3 Anaesthetic technique and
methods of cardiac output monitoring used in these studies are no longer in routine use and
therefore may not accurately reflect current practice of balanced anaesthesia or current
monitoring techniques.
The LiDCOplus minimally invasive cardiac output monitor can be used to accurately calculate
cardiac ouput and its associated derived variables (stroke volume and systemic vascular
resistance). The LiDCOplus monitor uses data already available from an arterial cannula
which is routinely used for monitoring patients in these circumstances. It is calibrated by
using a small (0.3mmol) bolus dose of lithium chloride. This technique has been shown to be
accurate and safe even after repeated calibration procedures over a short time period.4-6
Much of the evidence with regards to liver blood flow in the anaesthetised patient in prone
position is from the critical care literature. There are several differences between this
group of critically ill patients undergoing prone positioning for rescue therapy from
refractory hypoxia due to acute lung injury and healthy patients undergoing elective surgery
in the prone position. Critically ill patients have multiple cardiovascular and respiratory
co-morbidities, are usually receiving infusions of vasoactive drugs and are generally
positioned on soft air cushioned mattresses and pillows. Prone positioning in these patients
does not seem to significantly alter hepatic function as measured by hepatic Indocyanine
Green (ICG) clearance.7 8
ICG is a fluorescent dye, which can absorb infra-red light with a very rare (1:40 000) side
effect profile. After intravenous injection it is almost exclusively eliminated by hepatic
excretion into bile. The rate of hepatic elimination has been shown to correlate accurately
with plasma disappearance rate as measured by transcutaneous infra-red absorbtion.9 It is a
dynamic marker of hepatic blood flow, hepatocellular function and biliary excretion, with
short term variation thought to be attributable to changes in hepatic blood flow rather than
cellular dysfunction.10
The LiMON monitor uses transcutaneous infra-red light absorption (like a pulse oximeter) to
measure and calculate values for ICG plasma disappearance and its associated values such as
clearance rate and plasma retention rate. The technique involves intravenous injection of a
small (0.25mg/kg) bolus dose of ICG. This can be repeated up to 20 times per 24hr period.
It is postulated that simultaneously measuring changes in hepatic blood flow and cardiac
output will allow us to gather greater understanding of the haemodynamic changes associated
with the prone position in the anaesthetised patient using current routine monitoring
modalities.
;
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