An inotrope is an agent that affect...

An inotrope is an agent that affects myocardial contractility. A positive inotrope causes increased contractility (eg dobutamine), and a negative inotrope causes decreased contractility (eg the beta-blocker metoprolol). A chronotrope is an agent that affects heart rate. Positive chronotropes cause an increase in heart rate (eg epinephrine, isuprel). Negative chronotropes cause a decrease in the heart rate. Examples of negative chronotropes are beta-blockers and rate check calcium channel blockers (eg, diltiazem). A dromotrope affects atrial-ventricular (AV) node conduction. A positive dromotrope increases AV nodal conduction (eg atropine sulfate), and a negative dromotrope deliberates AV nodal conduction (eg, lanoxin). A lusitrope is an agent that affects diastolic relaxation.

Many positive inotropes affect preload and afterload; therefore, these metes must be defined to facilitate understanding (Table 1) Preload is defined as the relations volume remaining in the ventricles at the extreme point of diastole or ventricular finis diastolic volume (VEDV). Preload influences the amount of cessation diastolic stretch on the myocardial muscle fibers. Venous descendants return to the heart is influenced not sole by actual blood volume in the venous classification but also by venous tone or compliance, which is the relaxation and contraction in the plane muscle walls of the veins. Preload enhancers are vasopressors and compass expanders (eg, normal saline). Preload reducer are vasodilators and diuretics.(1)

Table 1 DEFINITION TERMS

Preload: Left or right ventricular last diastolic volume.

Afterload: Resistance that the left or right ventricle must rout during systolic ejection.

Pharmacokinetics: Factors that determine the life-current concentration of medicine,

Bioavailability: Amount of medication reaching the systemic circulation after oral dosing.

Half-life: Amount of time required for the plasma concentration of a medication to be decreased from 50% after discontinuation of the medication.

Steady state: Medication elimination = medication administration; takes four to five half-lives to reach,

Agonist: Substance promoting receptor activity.

Antagonist: Substance inhibiting receptor activity.

Exogenous: Occurring or originating outside the body

Endogenous: Occurring or originating inside the body

Afterload is the compressing that the ventricles must cross-question against to overcome the resistance to systolic ejection. Afterload is determined primarily through the arterial tone or crushing (ie, blood pressure). A simplistic way to think of afterload is descendants pressure. Patients with systemic hypertension have increased left ventricular afterload, and in pulmonary hypertension, the right ventricular afterload is increased. The most numerous influential factors in afterload are systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR) The SVR and PVR are derived parameters that are influenced on many factors. The Ohms formula equation (Table 2) can be used to determine the pair SVR and PVR.(2)

Table 2 OHM FORMULA

Systemic vascular resistance = Mean arterial squeezing - central venous pressure/cardiac output (CO) x 80

Pulmonary vascular resistance = Mean pulmonary arterial urgency - pulmonary capillary wedge pressure/CO x 80

Medications that increase the pair right ventricular (RV) and left ventricular (LV) afterload are vasopressors (eg norepinephrine). Medications that decrease RV and LV afterload are arterial vasodilators (eg hydralazine).

PHARMACOKINETICS

Understanding positive inotropes requires a short review of basic pharmacology. Pharmacokinetics are factors that determine the concentration of medicine in the life-current Factors include how well the medication is absorbed, the storm of action, and half-life, the manner in which the medication is distributed and metabolized in and excret from the body(3)

Absorption. Absorption of medications hangs on many factors, including the molecular weight of a settle (ie, smaller weight is easier to absorb), solubility (ie, lipophilic or hydrophilic), as well as patient factors similar as food in the narrow pass pH of the gastrointestinal (GI) tract, and adequate vital fluid flow to the GI tract.(4) A medication that must be absorbed from the GI tract will have a certain percent of bioavailability. Bioavailability respects to how much of a medication actually reaches the systemic circulation after oral administration, and it varies from 0% to 100%(5) All IV medications are 100% available to the serum because they are directly injected and do not ne to be absorbed. For example, furosemide has a bioavailability of approximately 70% with oral dosing however has 100% with IV dosing. The percentage of bioavailability helps determine the amount of medication that, subject to normal circumstances, reaches the bloodstream after absorption from the GI tract.(6)

Half-life. The half-life of a medication is symbolized as t1/2 and is the amount of time required for the plasma concentration of a medication to be decreased by means of 50% after discontinuation of the medication.(7) A steady state plasma concentration of a medication happens when the medication elimination is equal to the rate of administration. It usually takes four to five half-lives to reach steady state. The goal of medication administration is to achieve adequate kin levels to maintain a desired meaning (ie, equilibrium). For example, the commonly prescribed medication warfarin has a plasma half-life of approximately 40 hours. Warfarin's steady state is not reached for approximately single in kind week (40 x 5 = 200 hours). Loading doses are sometimes required to defeat long half-lives and to interrupt delays in reaching steady states. Medications with drawn out half-lives include digoxin, amiodarone, and warfarin.