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

oSensitivity is the minimum input parameter that creates a detectable output change

oRange is the difference between maximum and minimum values of the applied parameter that can be measured.

oPrecisionis the degree of reproducibility of the measurements

oResolutionis 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

•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

•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

•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
3rd sound / Rapid ventricular filling in early diastole
4th sound / Ventricular filling due to atrial contraction

The heart sounds. The 1st and 2nd heart sounds are most prominent.

QUANTIFICATION OF FLOW

Electromagnetic flow probes

Ultrasound flowmetry

Laser-Doppler flowmetry

Echocardiography

Flow measurements

Principle of an electromagnetic flowmeter

Ultrasonic flow measurements

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

•1. Blood loss due to disconnection

•2. Arterial thrombosis

•3. Infection

•4. Haematoma formation

•5. True and false aneurysm formation

•6. Distal and central embolisation

Laser-Doppler Flowmetry

Doppler shift on thered blood cells.

Bernoulliequation: 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 causesa fall in pressure

Poiseuille equation: flow-radius relationship

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.Arterial line

b.Central venous pressure

c.Pulmonary artery catheter

Indications

What is being measured

Technique- positioning, sites

Complications

Arterial line

•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

•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

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

Measurement of cardiac output

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.Ischaemic heart disease with recent myocardial infarction

2.Symptomatic valvular heart disease

3.Cardiomyopathy

4.Congestive heart failure and low ejection faction

5.Shock- septic or hypovolaemic

6.Pulmonary hypertension

7.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 thecentral venous injection of a cold (< 8°C) or room-tempered (< 24°C) saline bolus

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**

Parameters derived from cardiac output

•Cardiac index: CO / body surface area

•Stroke volume: CO / frequency

•Total vascular resistance:MAP-CVP

CO

•Pulmonary vascular resistance:PAP-LAP

CO

•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

•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

•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

Transgastric View

Assessment of Wall Motion

•Normal

–All wall segments contract equally

•Hypo kinesis

–Global

–Segmental

•A kinesis

–Segmental

Intra-Cardiac Air

•Possible with open heart surgery

•Most serious on the left side of the heart

–Lungs usually clear right sided air

Assessment of the Aorta

•Rotated from the Basal View

•Appears Round

•Look for

–Plaque

–Dissections

Complications / TEE

Less severe

•Esophagus perforation

•GI bleeding

•Esophageal burn injury

•Temporary vocal cord edema

Application of Color Doppler Flow

•Mitral Valve Regurgitation (MR)

•Tricuspid Valve Regurgitation (TR)

•Aortic Insufficiency (AI)

•Pulmonary Valve Insufficiency (PI)