Download Direct (invasive) measurement of pressure and flow

INVASIVE MONITORING
Direct (invasive) measurement of pressure and flow
1. Pressure
2. Flow
- Electromagnetic flow meters
- Ultrasonic flow meters
- Fick method
- Indicator-dilution method
3. Calculated parameters
Sensor / transducer
A sensor / transducer is a device that converts energy from some other form (e.g. heat,
light, sound, pressure, motion, flow), into electrical energy for the purposes of measurement
or control
o Sensitivity is the minimum input parameter that creates a detectable output change
o Range is the difference between maximum and minimum values of the applied
parameter that can be measured.
o Precision is the degree of reproducibility of the measurements
o Resolution is the smallest detectable incremental input parameter that can be detected
in the output signal
Direct (invasive) measurement of pressure
Technique
• Insert catheter/transducer in:
– Antecubital vein  vena cava, right atrium, right ventricle, pulmonary
artery
– Brachial/femoral artery  aorta, left, ventricle, left atrium
• Directly through pressure transducer
Indications of direct (invasive) pressure measurements
•
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direct measurement of blood pressure
accurate technique
continuous haemodynamic information
blood gas measurement
Invasive Pressures
Values are nearly the same
Invasive Pressures - limitations
Damping = removal of energy from a resonant system
Invasive measurement: Flow
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Laminar: the flow is orderly and streamlined
Turbulent: disorderly flow with vortices
Turbulent Flow
• Noisy
• Produces sound
• The murmur heard (narrowed heart valve)
• Can be calculated
whether flow is laminar or turbulent:
Reynolds Number (R)
R = Velocity * Diameter * Density / Viscosity
VD
R

• Values below 2000 define laminar flow
Sounds of the circulation
Sound
Origin
1st sound
Closure of mitral and tricuspid valves
2nd sound
Closure of aortic and pulmonary valves
Rapid ventricular filling in early
diastole
Ventricular filling due to atrial
4th sound
contraction
The heart sounds. The 1st and 2nd heart sounds are most prominent.
3rd sound
QUANTIFICATION OF FLOW
Electromagnetic flow probes
Ultrasound flowmetry
Laser-Doppler flowmetry
Echocardiography
Flow measurements
+
B
B
l
S
N
e = Blu
e

u
Principle of an electromagnetic flowmeter
Ultrasonic flow measurements
Oscillator
Scan head
Image plane
Skin surface
Blood vessel
Doppler
angle
Computer
RF
amplifier
Detector
AF
amplifier
F/V
converter
Ultrasonic flowmeter. The sensor at the scan head transmits the signal from the oscillator and
receives the reflected wave from the blood cells.
Complications / Swan Ganz catheter
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1.
2.
3.
4.
5.
6.
Blood loss due to disconnection
Arterial thrombosis
Infection
Haematoma formation
True and false aneurysm formation
Distal and central embolisation
Laser-Doppler Flowmetry
Doppler shift on the red blood cells.
Bernoulli equation: velocity-pressure relationship
•
the total energy of the fluid at any point is constant
Energy = [V2/2g] + [P/r] + H = constant
•
In an arterial stenosis, the increase in velocity causes a fall in pressure
Poiseuille equation: flow-radius relationship
pR
Q
8L
4
Q = Flow
∆p = Pressure drop across a length of vessel
R = Radius
L = Length of Vessel
Since BP is constant most of the time, flow is controlled by small changes in ‘R’ -the
vessel radius. This is most effective in the arterioles
CLINICAL APPLICATIONS
Invasive cardiovascular monitoring
a.
b.
c.
Arterial line
Central venous pressure
Pulmonary artery catheter
Indications
What is being measured
Technique- positioning, sites
Complications
Arterial line
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•
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direct measurement of blood pressure
accurate technique
continuous haemodynamic information
blood gas measurement
How accurate?
Use correct tubing
Bubbles free (tips)
Tight connections
Level of transducer
Invasive pressure measurements: Indications
Patient factors
• Patient with severe sepsis or shock
• Cardiac diseases: unstable angina, recent AMI, current congestive heart failure or
cardiac arrhythmias, pacemaker
Anaesthetic considerations
– Controlled hypotensive techniques
– Inability to measure blood pressure non-invasively
– Frequent blood sampling required during and after operation
Surgical considerations
• Cardiac surgery
• Major surgery on aorta or carotid artery
• Neurosurgery: craniotomy or aneurysm clipping
• Major surgery with expected high blood loss.
Sites
Setting up an arterial line
Equipment
• Pressure bag
• Collapsible 0.9% 500cc normal saline bag with air expelled
• Pressure transducer and infusion set
• Cannula
• +- heparin (1-2 units /ml)
Measurement of central venous BP
Int. jugular vein
V. subclavia
Value: 2-6 mmHg
- quick, large volume load,
- hypertonic solutions,
- serial blood samples.
Saldinger Technique
1. Introduce a Braunüle into a periferal vein
2. Remove the needle
3. Insert a flexible guide-wire to the central vein
4. Remove the Braunüle cannula
5. Insert – then remove a dilator cannula
6. Insert the central venous cannula
7. Remove of guide-wire
Monitoring of central venous pressure
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One lumen
– long angiocath (16G,14G),
– catafix (375mm, 475mm),
– percutan (7F, 8.5F)
– Swan (8.5F)
Multiple lumen
– 2- 3 lumen
Measurement of cardiac output
•
Principle of the Fick Method
adding 10 beads per minute
concentration is 2 beads per litre
Flow =
Rate added
Concentration
=
10 beads/min
2 beads/litre
= 5 litres/min
Measurement of cardiac output with oxygen content
Adolf Fick (1829-1901)
Blood samples from a peripheral artery and the PA as room air is inspired. The
difference between the oxygen content of inspired air and that measured in the expired
air can then be used to calculate oxygen uptake at the lungs per minute.
West JB: Best and Taylor's Physiological Basis of Medical Practice, 12th edition.
Baltimore, Williams and Wilkins, 1990, , 140 Section 2, Chapter 13, p 245.)
Measurement of cardiac output
Indicator dilution method
Sample dye
concentration
inject bolus
of dye
C
o
nc
en
tr
at
io
n
(g
/L
)
0
Time (min)
0.5
Measurement of cardiac output
Sample dye
concentration
inject bolus
of dye
C
o
nc
en
tr
at
io
n
(g
/L
)
0
Time (min)
0.5
High and Low Cardiac Output
Measurement of cardiac output with dyes
• Practical considerations
– Dye recirculates in the CVS
– Dye must be non-toxic and not immediately absorbed eg indocyanine green
– Injected into pulmonary artery (difficult)
– Measured in brachial artery
– Like the Fick method, is invasive, & discontinuous
• Same principle with heat
– Measure thermodilution of cold saline
The relationship of the temperature gradient and time.
Adapted from Baker, P. D., Orr, J., Westenskow, D. R. and Johnson, R. W. Method and
apparatus for producing thermodilution cardiac output measurements utilizing a neural
network, US Patent, 5,579,778.
When pulmonary artery pressure and cardiac output are measured
simultenaously
Positioning of Swan Ganz catheter
Pulmonary artery and wedge pressures
Indications - Swan Ganz catheter
1.
2.
3.
4.
5.
6.
7.
Ischaemic heart disease with recent myocardial infarction
Symptomatic valvular heart disease
Cardiomyopathy
Congestive heart failure and low ejection faction
Shock- septic or hypovolaemic
Pulmonary hypertension
Cardiac surgery with poor ventricular function
What can be measured? (Swan Ganz catheter)
1.
Central venous pressure
2.
Pulmonary artery systolic and diastolic pressure
3.
Pulmonary capillary wedge pressure
4.
Cardiac output
5.
Mixed venous oxygen saturation
6.
Derived values such as stroke volume, cardiac index, ventricular stroke work,
systemic and pulmonary vascular resistance
Both thermodilution methods
Simply requires the central venous injection of a cold (< 8°C) or room-tempered (<
24°C) saline bolus
Lungs
CVP catheter
Femoral aretery
catheter catheter
Right Heart
RA
RV
EVLW
PBV
EVLW
Advantages of the transpulmonary thermodilution method
Less risk factors and complications
Provides:
• Hemodynamic
• Volumetric: preload parameters (lung water)
• Estimates cardiac contractility*
• On line venous blood gases**
dP/dtmax of arterial pressure curve
P [mm Hg]
t [s]
Left Heart
LA
LV
Parameters derived from cardiac output
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Cardiac index:
CO / body surface area
Stroke volume:
CO / frequency
Total vascular resistance: MAP-CVP
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Pulmonary vascular resistance:
CO
PAP-LAP
CO
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Oxygen delivery
Oxygen consumption
Calculated parameters: Oxygen delivery (DO2) and consumption (VO2 )
DO2 [ml/min] = C.O. x [(1,38 x Hb x SaO2) + (0,003 x paO2)]
Oxygen content = [(1,38 x Hb x SaO2) + (0,003 x paO2)]
VO2 [ml/min] = C.O. x [artrial oxygen content] – [venous oxygen content]
Doppler echocardiography
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Pulsed ultrasound waves emitted
Wavelength of sound is altered as it is reflects off moving red blood cells
Change in pitch indicates velocity of red blood cells
Estimate of aortic cross-section gives blood flow ie. cardiac output
Transesophageal echocardiography (TEE)
Analysis of vetricular wall movements and volume changes
Indications:
• Ischemic states, changes in regional ventricular wall
•
movements and thickness
• Measuerents of ejection fraction,
• Valve function
• Intracardiac air, thrombi, tumor
• Assessing efficacy of surgical repairs
• Can confirm effectiveness of medications
Patient Insertion of TEE probe
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Probe lubricated with water soluble Jell
Insertion in Patients esophagus
Axis of Interrogation
Three Main “Views”
1. Short axis basal view
2. Long axis, (4 chamber) view
3. Transgastric View
Short axis basal View
Long Axis (4-Chamber) View
2-D echocardiography of the long axis view of the right ventrical (RV): (a) the ultrasonic
beam angle through the heart, (b) the cross-sectional diagram of the image and (c) the actual
2-D image. TV = tricuspid valve, AML = anterior mitral leaflet.
Adapted from Rafferty, T. 1992. Transesophageal Two -Dimensional Echocardiography
http://www.gasnet.org/reference/echomanual/html/2-d_echo..html
Transgastric View
Assessment of Wall Motion
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Normal
– All wall segments contract equally
Hypo kinesis
– Global
– Segmental
A kinesis
– Segmental
Intra-Cardiac Air
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Possible with open heart surgery
Most serious on the left side of the heart
– Lungs usually clear right sided air
Assessment of the Aorta
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Rotated from the Basal View
Appears Round
Look for
– Plaque
– Dissections
Complications / TEE
Less severe
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Esophagus perforation
GI bleeding
Esophageal burn injury
Temporary vocal cord edema
Application of Color Doppler Flow
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Mitral Valve Regurgitation (MR)
Tricuspid Valve Regurgitation (TR)
Aortic Insufficiency (AI)
Pulmonary Valve Insufficiency (PI)