So exactly why are we measuring all this?

To, hopefully, detect and avoid SHOCK

Shock is hypotension and consequent lack of oxygenated blood supply that will lead, if untreated, to an organ injury or death:

Due to malfunction of;

  • the Pump (cardiogenic / obstructive)
  • the Tubing (distributive / septic / obstructive)
  • the Fluid (hypovolemic / obstructive)

There are four types of shock (with some additional subgroups), problems with the heart, sometimes called Cardiogenic shock, problems in the vascular system, known as Distributive shock, loss of circulating blood volume, known as Hypovolaemic shock, Obstructive shock is generally associated with external influences on the cardiac or vascular system.

It is safest to use the physiologic approach to shock:

shock is due to inadequate blood pressure (low MAP)

low blood pressure is due to inadequate cardiac output or low systemic vascular resistance

low cardiac output is caused by a problem with heart rate or stroke volume (SV)

heart rate abnormalities: too fast (tachycardia), too slow (bradycardia)

stroke volume abnormalities: failure to receive, failure to eject, inadequate volume

low systemic / peripheral vascular resistance is due to inappropriate vasodilation

and obstruction, if absolute, means death

Cardiogenic Shock

Blood flow decreased due to an intrinsic defect in cardiac function either the heart muscle or the valves are dysfunctional, an example is acute anterior myocardial infarction (AMI), when the amount of damaged ischaemic muscle may be so great that the heart cannot pump anymore.  The decreased contractility causes a decrease in stroke volume.

Shock can be caused by arrhythmias like Complete Heart Block (CHB), ventricular tachycardia (VT), Supra-Ventricular Tachycardia (SVT) etc and too much cardio toxic medications like Beta Blockers or digoxin etc

Typical haemodynamic picture:

  • decreased cardiac output and therefore blood pressure
  • high left ventricular filling pressures (backward failure)
  • increased systemic vascular resistance (SVR from vasoconstriction, which is a sympathetic compensatory response to the low blood pressure)
  • increased heart rate (sympathetic compensatory response to the low blood pressure)
  • other features of cardiogenic shock such as the cool peripheries, decreased urine output and sweating can also be explained by the sympathetic compensatory response


Distributive Shock

Occurs when the peripheral vascular dilatation causes a fall in Systemic Vascular Resistance (SVR or the tone of blood vessels that decreases resistance and venous return)

Common causes:

  • septic shock (infection)
  • anaphylactic shock (allergies)
  • acute adrenal insufficiency (hormonal)
  • neurogenic shock (broken feedback mechanisms / cord damage)

Cardiac output is often increased but the perfusion of many vital organs is compromised because the blood pressure is too low and the body is unable to distribute blood properly

Criteria for Septic Shock:  Sepsis and (despite adequate volume resuscitation) both of:

  • Persistent hypotension requiring vasopressors to maintain MAP greater than or equal to 65 mm Hg, and
  • Lactate greater than or equal to 2 mmol/L

With these criteria, hospital mortality is in excess of 40%

Haemodynamic issues are usually characterised by:

  • normal or increased cardiac output with a low SVR / capillary refill
  • low to normal left ventricular filling pressures
  • low blood pressure

Clinical features include:

  • warm peripheries
  • bounding pulses
  • tissue dysfunction despite this attempt to compensate leads to
    • mental status changes (anxiety, drowsy, loss of consciousness)
    • oliguria (low or absent urine output)
    • lactic acidosis (lack of oxygen rich blood supplying muscles)


Hypovolemic shock

The result of intravascular blood volume depletion

Common causes:

  • haemorrhage
  • vomiting
  • diarrhoea
  • dehydration
  • third-space (into interstitium generally) losses during major operations, pancreatitis, Diabetic Ketoacidosis (DKA) etc

Primary issue is a decrease in preload. The decreased preload causes a decrease in stroke volume

Typical haemodynamic picture:

  • decreased cardiac output and blood pressure
  • low right and left ventricular filling pressures (because the ventricles  are pumping while relatively empty)
  • increased SVR (from vasoconstriction, which is a sympathetic compensatory response to the low blood pressure)
  • increased heart rate (sympathetic compensatory response to the low blood pressure also)

Other features of hypovolaemic shock are similar to those seen in cardiogenic shock and include cool peripheries, decreased urine output and sweating that can also be explained by the sympathetic compensatory response


Obstructive Shock

Cardiac tamponade (fluid, usually blood, filling the sack around the heart)

  • extra-cardiac obstructive shock
  • mechanical obstruction to cardiac filling
  • pressures of the right cardiac chambers, the pulmonary artery, and the left cardiac chambers equilibrate in diastole

We should always consider cardiac tamponade when the CVP is high and BP low. (1 of the 4 H&Ts of cardiac arrest)

Shown as pulsus paradoxus

  • an exaggeration of normal physiology in which there is a decrease (obvious fluctuation) of >10 mm Hg (about 10mm Hg can be normal) or >15% in systolic blood pressure during inspiration
  • important clinical finding in patients with suspected cardiac tamponade

Other forms

  • tension pneumo/haemothorax (air or blood fills the chest cavity causing twisting of the major vessel eventually stopping blood return to heart)
  • massive pulmonary embolus (large clot obstructing blood flow from right to left heart)
  • aortic stenosis (adds huge resistance to cardiac outflow)
  • constrictive pericarditis (pericardium becomes non compliant, heart cannot expand normally so pressures effected across systolic and diastolic phase)

Image –

Image –

So that is basic ICU Haemodynamic Monitoring with a bit of shock thrown in, very well done!




Oh’s Intensive Care Manual 7th Edition 2014 by Andrew Bersten, Neil Soni

The ICU Book 4th Edition 2015 by Paul L Marino


More reading


Arterial Catheters & MAP

Central Venous Catheters & CVP