Hospitals & Services

Haemodynamic Priciples

Introduction

 

When faced with a patient who may have some haemodynamic impairment you should have a systemic approach to assessment and management.

Ask four questions:

  • Is the blood pressure actually low?
  • Is there any evidence of poor tissue perfusion?
  • Does this patient require more fluid?
  • Do I need to introduce an inotrope, a chronotrope or a vasopressor substance?

 

Diagnosing Hypotension

 
Low blood pressure is variously defined as:
Systolic BP < 90 mmHg or Mean arterial pressure < 60 mmHg

These pressures are implied limits at which vital organs can continue to auto regulate blood flow.

Is there any evidence of hypoperfusion?

 
Bedside Indicators

  • Cerebral perfusion:  restlessness or confusion
  • Renal blood flow: oliguria (<0.5 ml/kg/hr)
  • Cool peripheries (unreliable)

 
Simple Investigation

  • ECG:  evidence of regional ischaemic changes
  • ↓pH on arterial blood gas and base excess

Does this patient require more fluid resuscitation?

 
This is the hardest question to answer.  You will be asked to adequately volume resuscitate the patient, while simple to say, it is difficult to fulfil.

Should I give more fluid?

Patients on the volume responsive part of the Starling curve should increase their cardiac output in response to further intravenous fluid.

How Do I know where the Patient is on the curve?
 
In an number of patients this is not problematic as they have haemorrhagic shock, vomiting, diarrhoea or some other reasons for absolute or relative (eg. Epidural, anaphylaxis) intra-vascular volume contraction.

Clinical estimates of hydration (moist mucous membranes, skin turgor….) are almost useless in the intensive care unit.  Estimation of JVP, or indeed invasive measures of right heart (CVP) or left heart (pulmonary capillary wedge pressure) filling pressures have some relevance in patients with hearts that have normal structure and function.  Often this is not the case in the intensive care.

So what should I do?

Often clinical practice relies on your impression and an assessment of the risk of giving more fluid than not doing so, ie. It would be "easier" to give fluid to a hypotensive person with a clear chest who is ventilated, than one with chest crackles who is developing respiratory failure, even if this is due to some other pathology.

If in doubt give fluid.  The decision to implement a fluid challenge is inextricably linked with a duty to closely observe the results and act accordingly.

 

Do I need to Introduce an Inotrope, a Chronotrope or a Vasopressor?

 
Once the first three questions above have been addressed adequately, it may become necessary to use an agent to bring about an increase in blood pressure and therefore organ perfusion, ie:
Organ perfusion is dependant on:

  • Cardiac Output
  • Arterial Oxygen Content HbSaO2
  • Blood therapy
  • BP = CO x SVR

SV x HR x SVR

Manipulation of blood pressure and therefore organ perfusion relies on changing one of the three parameters given above.

 

Page last reviewed: 13 May 2014
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