-
Blood Pressure is
Cardiac Output multiplied by Peripheral Resistance.
-
Cardiac Output is Heart Rate
times Stroke Volume.
-
Hypotension is caused by either
inadequate Cardiac Output or inadequate Peripheral Resistance
-
Heart Rate, Stroke Volume and
Total Peripheral Resistance exist in dynamic equilibrium: these
interactions maintain blood pressure. If one of the three becomes
abnormal, the other two compensate. This represents the cardiovascular
physiologic reserve.
-
Hypotension is an indication of
1) an abnormality of Heart Rate, Stroke Volume or Peripheral Resistance, &
2) failure of the others to compensate.
-
Shock is acute circulatory
failure leading to inadequate tissue perfusion and end organ injury: it
classified as being due to malfunction of 1) the Pump (cardiogenic), 2 )
the Tubing (distributive), or 3) the Fluid (hypovolemic).
-
The heart rate is a fundamental
element of hypotension both in terms of cause (tachyarrhythmias /
bradyarrhythmias) and compensation – hypotension should be accompanied by
a tachycardia.
-
Low Stroke volume is
caused by a problem with reception or a problem with ejection.
-
Problems with reception are:
inadequate venous return or cardiac inflow obstruction.
-
Fluid loss is caused by either
absolute hypovolemia (e.g. blood loss) or relative hypovolemia (“third
spacing”).
-
Cardiac inflow obstruction is
caused by a pericardial (tamponade) or intrathoracic process (PEEP), or a
lesion within the heart itself (mitral stenosis).
-
Problems with ejection include
pump failure (ischemia, overload, contusion, inflammation) and outflow
obstruction (embolism, aortic stenosis, aortic crossclamps).
-
Shock caused by low peripheral
vascular resistance is caused by loss of tonic vasoconstriction
(vasoplegia), due to sympathectomy, anaphylaxis or sepsis, leading to
relative hypovolemia.
-
Vasodilation associated with
septic shock occurs due to increased synthesis of nitric oxide, activation
of ATP-sensitive potassium channels in vascular smooth muscle, and
deficiency of vasopressin.
