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| | Introduction to haemodynamic monitoring for junior ICU trainees and nurses
and medical students
Functions of monitoring
- to detect that there is a problem
- to give information to allow diagnosis of the problem
- blood pressure
- compensatory sympathetic stimulation
- tachycardia
- sweating
- vasoconstriction
- organ dysfunction
- mental state
- urine output
- tissue hypoxia
The importance of clinical assessment cannot be overemphasized. Clinical
signs of organ dysfunction and sympathetic stimulation tend to precede
hypotension.
Blood pressure monitoring
Non-invasive monitoring
- tends to under-read at high pressures and over-read at low pressures
- less accurate during arrhythmias
- non-continuous
- less suitable for haemodynamically unstable patients
- cuff width most important determinant of the accuracy of the pressure
reading. Should be 40% of mid-circumference of limb (the length should be
twice the width). Cuffs which are too narrow tend to overestimate BP while
those which are too wide tend to underestimate
- complications include: ulnar nerve injury (usually associated with cuff
being placed too low on upper arm), oedema of the limb, petechiae and
bruising, friction blisters, failure to cycle and drip failure.
Invasive pressure monitoring
This system consists of
- arterial line connected by
- saline filled non-compressible tubing to a
- pressure transducer. This converts the pressure waveform into an
electrical signal which is displayed on the
- bedside monitor
- pressurized saline for flushing
Sources of error
- failure of any one of the components in system
- transducer position
- pressure displayed is pressure relative to position of transducer
- in order to reflect blood pressure accurately transducer should be at
level of heart. Over-reading will occur if transducer too low and
under-reading if transducer too high
- transducer must be zeroed to atmospheric pressure
- damping. Important to have appropriate amount of damping in the system.
Inadequate damping will result in excessive resonance in the system and an
overestimate of systolic pressure and an underestimate of diastolic
pressure. The opposite occurs with overdamping. In both cases the mean
arterial pressure is the most accurate. An underdamped trace is often
characterized by a high initial spike in the waveform.
The damping in the system can be formally
tested but it is often simpler to first check for underdamping by
determining the true systolic pressure with a simple test:
- place a manual (not automated) blood pressure cuff on the limb in
which the arterial line has been inserted
- inflate the cuff until the arterial trace is flat
- slowly deflate the cuff until pulsation can just be detected on the
arterial trace. The pressure displayed on the mercury manometer is the
true systolic pressure
Complications
- distal ischaemia
- arterial thrombosis
- embolism. May manifest as splinter haemorrhages
- infection
- haemorrhage
- disconnection
- around line
- accidental drug injection
- damage to artery eg aneurysm
Which pressure?
Which pressure should be monitored depends on the clinical situation.
- systolic pressure is important when there is a high risk of bleeding
- diastolic pressure is important in determining coronary perfusion pressure
and therefore is particularly important in patients with coronary disease
- mean arterial pressure is a major determinant of perfusion pressure of
other organs
Why is the patient shocked?
- The simplest way to approach this problem is from a physiological
standpoint
- Mean arterial pressure=Cardiac output x Total peripheral resistance
- Cardiac output=Heart rate x Stroke volume
- Stroke volume is dependent on preload, afterload and contractility.
Preload can be assessed from JVP or CVP but it is not possible to assess
afterload and contractility easily.
- Therefore the important parameters to assess are:
- total peripheral perfusion. Are the peripheries warm or cold
- heart rate
- JVP or CVP
Causes of shock
| |
Heart rate |
JVP or CVP |
Peripheries |
| Cardiac |
Moderate tachycardia usual.
Severe bradycardia or tachycardia in arrhythmia-induced shock |
Raised or normal |
Cold |
| Hypovolaemic |
Moderate tachycardia |
Decreased |
Cold |
| Distributive |
Moderate tachycardia |
Decreased |
Warm |
| Obstructive |
Moderate tachycardia |
Markedly raised |
Cold |
Interpretation of CVP
- CVP gives a crude estimate of left atrial pressure (LAP)
- LAP approximates to left ventricular end-diastolic pressure (LVEDP)
- preload is more closely related to left ventricular end-diastolic volume
(LVEDV) rather than LVEDP. It follows that it is necessary to know the
relationship between LVEDP and LVEDV
- the orange line below describes the relationship in a patient with normal
ventricular compliance. A fall in compliance (eg due to ischaemia) results
in a shift to the left and up
- as it is difficult to determine compliance clinically it is difficult to
distinguish between a patient with a low preload and low compliance and a
patient with a high preload and normal compliance. By giving fluid one can
make this distinction. In a patient with a low preload giving fluid does not
alter the CVP, while giving fluid to a patient with a high preload results
in a dramatic increase in CVP.
- the following can be used as a rule of thumb: change in CVP measured
before and 5 min after bolus of fluid:
- 0-3 mmHg - underfilled
- 3-5 mmHg - adequately filled
- 5-7 mmHg - overfilled
Complications of CVP lines
- pneumothorax
- haemothorax
- chylothorax
- damage to vein or adjacent structures
- vein thrombosis or thrombophlebitis
- infection
- catheter or guidewire embolization
- cardiac arrhythmias
- haemorrhage
- air embolism
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