3. Physiology
        3.2. Cardiovascular
            3.2.2. Heart
                3.2.2.3. Heart pump
3.2.2.3.2. Stroke volume

Stroke volume

 

Preload

... the load on the myocardial muscles just prior to the onset of contraction

Preload influence contractility due to:

 

Best index of preload

Other indices of preload

Pulmonary capillary wedge pressure

Pulmonary capillary wedge pressure (PCWP) (or pulmonary artery occlusion pressure (PAOP)) correlates well with LAP most of the time

 

Afterload

... the impedance to the ejection of blood from the heart into the arterial circulation

Best index of afterload

Other index of afterload

Contractility

[KB2:p47] ... the factor that is responsible for changes in myocardial performance which are not due to changes in heart rate, preload, or afterload.

[BL8] Performance of the heart at a given preload and afterload.

Best index of contractility

Other index of contractility

 

Factors influencing contractility

All mechanisms that increase contractility do so by increasing intracellular [Ca2+]

e.g. decrease in sodium gradient, increase in extracellular [Ca2+]

NB:

Neural influences

Sympathetic influence

e.g.

Sympathetic stimulation

--> beta1 receptor stimulation

--> Increased cAMP

--> Increased contractility and faster relaxation

Also, catecholamines also increase sensitivity of myofilaments to Ca2+

(See ANS innervation of the heart)

Parasympathetic influence

e.g.

Parasympathetic stimulation

--> Muscarinic acetylcholine

--> Decreased cAMP

--> Decreased Ca2+ influx

--> Decreased contractility

(See ANS innervation of the heart)

Hormonal influences

Adrenomedullary hormones

Mainly epinephrine, and some norepinephrine

Adrenocrotical hormones

i.e. corticosteroids

Possibly potentiating in human [BL8:p109]

But is permissive for the permissive effect of catecholamine [WG21: p372]

Thyroid hormones

Enhance contractility via

  1. Increased Ca2+ uptake by SR
  2. Increased protein synthesis in heart
    --> Hypertrophy
  3. Affect composition of myosin isoenzymes in cardiac muscle (esp the ones with greatest ATPase activity)
Insulin

Prominent, direct, positive inotropic effect [BL8:p111]

Glucagon

Potent positive inotropic and chronotropic effect on the heart

Mechanism very similar to catecholamines.
* i.e. also increases cAMP via adenylyl cyclase

NB:

Other factors

Digitalis glycoside

Inhibits Na-K ATPase pump

--> Decrease Na+ chemical gradient

--> Decreased Na+ influx

--> Decrease Ca2+ efflux via Na/Ca exchanger

--> Increase in intracellular Ca2+ level

--> Increase contractility

Also increases inward Ca2+ current

--> Also increase intracellular Ca2+ level

--> Increase contractility

Xanthines (e.g. caffeine, theophylline)

(e.g. caffeine, theophylline)

Inhibit breakdown of cAMP

--> Inotropic effect

About
Created20050607
Reviewed20050815


Table of contents  | Bibliography  | Index