Preload
Preload
-end diastolic volume
-dependant on ventricular filling
Starlings law of the heart:
-relationship between cardiac output and left ventricular end diastolic volume (LVEDV or preload)
-if heart rate is constant then cardiac output is generally proportional to preload
Setting of excess preload:
-cardiac output may no longer be directly proportional
-instead cardiac output may not change or even decrease
Determinants of Ventricular Filling:
Factors which may affect ventricular filling:
-venous return
-volume of blood
-blood volume distribution
-rhythm of heart
-rate of heart
Venous return:
-most important factor to influence ventricular filling
-increases with increases in metabolic activity
-increases with decreased venous capacitance
Common causes of perioperative changes in ventricular filling and cardiac output
-changes in venous tone
-changes in blood volume
Venous pressure gradients affecting venous return may involve changes in:
-intrathoracic pressure
-pericardial pressure
-posture
Heart rate may affect ventricular filling by:
increased heart rate: proportional decrease in ventricular filling time
decreased heart rate: proportional increase in ventricular filling time
heart rate greater than 120 beats per minute impair left ventricular filling
Abnormal atrial rhythm which may impair ventricular filling by 20 – 30 % includes:
-atrial fibrillation
-atrial flutter
-low atrial rhythm
-junctional rhythm
Diastolic Function and Ventricular Compliance
Imaging techniques to help assess LVEDV:
-transesophageal echocardiography
-radionucleotide imaging
-contrast ventriculography
Approximate correlation:
LVEDP may be used a measure of LVEDV if:
the relationship between the ventricular pressure and volume is constant
LVEDV ≅ LVEDP ≅ PAOP ≅ CVP
Factors affecting ventricular compliance may be related to:
-rate of relaxation: early diastolic compliance ex. hypertrophy, ischemia, asynchrony
-passive stiffness of the ventricle: late diastolic compliance ex. hypertrophy, fibrosis
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