9.8 Cardiac Output Monitoring

Question 1

How does SV and Pos Pressure Ventilation affect BP

Answer 1

In a spontaneously ventilating patient,

systolic blood pressure fluctuates with ventilation by 5–10 mmHg.

This is known as the respiratory swing.

Pulsus paradoxus is when the difference in systolic blood pressure between
inspiration and expiration is greater than 10 mmHg.

In patients undergoing positive pressure ventilation,

reverse pulsus paradoxus occurs.

During inspiration, stroke volume (SV) from the
right ventricle decreases and
that from the left ventricle increases
due to the increased intrathoracic pressure.

This causes an increase in blood pressure
during inspiration and decrease during expiration.

Question 2

What do you mean by dynamic fluid responsiveness?

Answer 2

Stroke volume variation (SVV) is one method of measuring patient’s dynamic
fluid responsiveness.

It is defined by:
SVmax – SVmin
_____________
SV mean

It is represented as a percentage and reflects the change in SV during the
respiratory cycle and can be assessed continuously by any beat-to-beat
cardiac output monitor.

SVV of greater than 10% suggests that patient is fluid-responsive
as it indicates that SV is sensitive to fluctuations in preload
caused by the respiratory cycle.

Question 3

What are the specific causes of central venous pressure (CVP) inaccuracies?

Answer 3

Patient factors
* Altered in ventilated patients due to increase in intrathoracic pressure

• Tricuspid valve disease
  (e.g. tricuspid regurgitation can cause discrepancy between
  digital display and end diastolic, end expiratory pressure)

Equipment related
* Transducer height

• Damping/resonance
• Pressured bag—sufficient pressure?

Question 4

Draw an arterial line trace and explain what information can be obtained from it. see Figure 9.7

Answer 4

Upstroke

Dicrotic notch

AUC

Pulse Pressure

Question 5

Upstroke

Dicrotic Notch

Answer 5

Upstroke
* dP/dT
* Marks the stage of ventricular ejection
* Represents contractility
* Slope can be slurred in aortic stenosis

Dicrotic notch
* Closing of aortic valve
* Represents systemic vascular resistance (SVR)
* If very low, suspect low SVR (e.g. septic shock)

Question 6

Area under the curve

Pulse Pressure

Answer 6

Area under the curve
* Up to dicrotic notch represents SV

Pulse pressure
* Widened pulse pressure suggests aortic regurgitation
(in diastole, the arterial pressure drops to
fill the left ventricle through the regurgitating aortic valve)

• Narrow pulse pressure suggests cardiac tamponade or low-output state

(e.g. aortic stenosis, severe cardiogenic shock, massive pulmonary
embolism, or tension pneumothorax)

Question 7

How can cardiac output be monitored?

Answer 7

• Pulmonary artery catheter—still the gold standard
• Pulse pressure analysis (e.g. PiCCOTM, LiDCoTM, FloTracTM/VigileoTM)
• Oesophageal Doppler
• Applied Fick’s principle (e.g. NICOTM system)
• Bioimpedance

Question 8

What are the properties of an ideal cardiac output monitor?

Answer 8

• Accurate
• Easily reproducible results
• Quick and easy to use—minimal setup and interpretation of information
• Operator-independent—skill of operator should not affect information
• Continuous measurement
• Minimal drift
• Safe to staff and patients
• Noninvasive
• Cost-effective
• Fast response time

Question 9

Discuss LiDCO TM and how it works.

Answer 9

LiDCo TM uses pulse pressure analysis to
track continuous changes in stroke volume.

It follows an algorithm that is based on the assumption that the net
power change in the system in a heartbeat is the difference between the
amount of blood entering the system (SV) and the amount of blood flowing
out peripherally.

It uses the principle of conservation of mass (power) and assumes that,
following correction for compliance, there is a linear relationship between
net power and net flow.

Some studies describe LiDCoTM as a pulse power analysis.

LiDCoTM plus requires calibration using lithium indicator dilution technique
(performed via a peripheral cannula).

LiDCoTM rapid uses nomograms for cardiac output monitoring and does not
require lithium calibration.

Question 10

Explain the working of oesophageal Doppler.

Principal

Answer 10

When sound waves are reflected from a moving object,
their frequency is altered.

This is the Doppler effect.

By using an ultrasound probe to visualise directional blood flow,
the change in frequency before and after reflection
of moving red blood cells can be determined.

This, together with the cross-sectional area of the
blood vessel being observed,
can be used to determine flow using the formula:

Flow = Area × Velocity

Aortic area is not measured using this method; it is estimated using an
algorithm based on body surface area.

Question 11

Explain the working of oesophageal Doppler.

Practical

Answer 11

The Doppler probe is passed into the oesophagus

and rotated until the transducer faces the descending aorta (

the oesophagus and descending aorta run close and parallel to each other)

+

The resultant characteristic waveform is studied.

Question 12

Whats the formula?

Answer 12

The Doppler shift (Fd) produced by moving blood flow
is calculated by the ultrasound system using the following equation:

Fd = 2FtVCos θ / C

Ft is the transmitted Doppler frequency,

V is the speed of blood flow,

Cos θ is the Cosine of the blood flow to beam angle,

and C is the speed of sound in tissue