The Dept of Anaesthesia & Intensive Care, CUHK thanks

for an unrestricted education grant
BASIC instructor/provider course, Hong Kong, July 2nd-4th
Other upcoming courses
Home Feedback Contents

CVS monitoring

Up CVS monitoring Mechanical ventilation e-Lectures Respiratory failure Sedation Shock


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

Detecting a problem

  • blood pressure
  • compensatory sympathetic stimulation
    • tachycardia
    • sweating
    • vasoconstriction
  • organ dysfunction
    • mental state
    • urine output
  • tissue hypoxia
    • pH
    • lactate

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
 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
Copyright policy    Contributors