Overall cardiac function (cardiac output) is the result of a combination of four things:
- Heart rate
Preload is the amount that the heart muscle is stretched when filled with blood just before a contraction. You can think of preload as the load or stretch in the ventricle pre contraction. It is related to the volume of blood in the ventricle at the end of diastole, just before the ventricle contracts.
Afterload is the resistance that the heart must overcome to eject blood from the left ventricle, through the aortic valve and into the aorta. You can think of afterload as being the load or resistance after the aortic valve – how much resistance there is to pushing blood through the aortic valve. Common causes of raised afterload are hypertension and aortic stenosis.
Contractility refers to the strength of the heart muscle contraction.
Heart rate is the number of heartbeats per minute.
Systemic vascular resistance is the resistance in the systemic circulation that the heart must overcome to pump blood around the body.
Stroke volume is the volume of blood ejected during each beat.
Cardiac output is the volume of blood ejected by the heart per minute. The formula is:
Cardiac output = stroke volume x heart rate
Mean arterial pressure (MAP) is the average blood pressure throughout the entire cardiac cycle, including both systole and diastole. Mean arterial pressure is a product of cardiac output and systemic vascular resistance. Low arterial pressure may be the result of low cardiac output or low systemic vascular resistance.
An adequate mean arterial pressure is essential for tissue perfusion throughout the body. Low mean arterial pressure results in tissue hypoperfusion, leading to hypoxia, anaerobic respiration, lactate production and damage to the tissue.
Cardiac function can be monitored closely in the intensive care unit. The extent of monitoring depends on the individual patient and clinical problems.
Basic non-invasive monitoring involves:
- Heart rate
- Peripheral blood pressure
- Pulse oximetry (oxygen saturations)
- Continuous ECG monitoring
More intense monitoring can involve:
- Invasive blood pressure monitoring via an arterial line (a special cannula inserted into an artery)
- Arterial blood gas analysis taken from an arterial line
- Central venous pressure via a central venous catheter in the vena cava/right atrium
- Central venous oxygen saturation measured using blood samples from a central venous catheter
- Pulmonary wedge pressure via a pulmonary artery catheter (indicates the left atrial pressure) (rarely done)
- Pulmonary artery oxygen saturation via a pulmonary artery catheter (rarely done)
- Echocardiogram (transoesophageal or transthoracic)
Cardiac output monitoring can involve:
- Pulse contour cardiac output (PiCCO) monitors cardiac output via a central venous catheter and thermodilution arterial line
- Oesophageal Doppler monitor assesses the blood flow through the thoracic aorta to estimate stroke volume and cardiac output
An important part of maintaining cardiac function is optimising the fluid status. This is the first step before considering inotrope and vasopressor medications.
The central venous pressure is often used as an estimate of preload. In simple terms, it shows how much blood is available to fill the heart before a ventricular contraction. If the central venous pressure is low, the heart has less blood filling the ventricles for each contraction.
The central venous pressure helps guide fluid resuscitation. Giving additional IV fluids helps increase the central venous pressure, helping the heart fill with blood during diastole. Preload and stroke volume are improved, ultimately improving the cardiac output.
Too much fluid can lead to fluid overload, creating congestion in the circulation. This results in congestive heart failure, pulmonary oedema and increased mortality. Therefore, fluid balance needs to be carefully monitored and optimised.
Inotropes are medications that alter the contractility of the heart.
Positive inotropes act to increase the contractility of the heart. This increases cardiac output (CO) and mean arterial pressure (MAP). They are used in patients with a low cardiac output, for example, due to heart failure, recent myocardial infarction or following heart surgery.
Most positive inotropes are catecholamines. Catecholamines stimulate the sympathetic nervous system via alpha and beta-adrenergic receptors.
Examples of positive inotropes that are catecholamines are:
- Noradrenaline (weak inotrope and mostly a vasopressor)
- Dopamine (not an inotrope at lower infusion rates)
Milrinone is a positive inotrope that works as a phosphodiesterase-3 inhibitor.
Levosimendan is another positive inotrope that works by increasing the heart muscle’s sensitivity to calcium.
Positive inotropes are given through a central venous catheter. They are only used where patients can be closely monitored (e.g., ICU). Doses are titrated in response to changes in the patient’s clinical condition, mean arterial pressure, central venous pressure and cardiac output.
Negative inotropes act to reduce the contractility of the heart. Examples are:
- Calcium channel blockers
Vasopressors are medications that cause vasoconstriction (narrowing of blood vessels). This increases the systemic vascular resistance and consequently mean arterial pressure (MAP).
Vasopressors are commonly used by anaesthetists as a bolus dose or in ICU as an infusion to improve patient’s blood pressure and, therefore, tissue perfusion. Severe sepsis is a common example of a condition where they may be used.
Common vasopressors are:
- Noradrenaline (given as an infusion via a central line)
- Vasopressin (given as an infusion via a central line)
- Adrenaline (given as an infusion via a central line or as a bolus in an emergency)
- Metaraminol (given as a bolus or an infusion)
- Ephedrine (given as a bolus)
- Phenylephrine (given as a bolus or an infusion)
Vasopressin is antidiuretic hormone (ADH). It acts as a vasopressor by causing contraction of smooth muscle in blood vessels. It also stimulates water reabsorption from the collecting ducts in the kidneys.
Glycopyronium is an antimuscarinic medication used to treat bradycardia, often during operations. Antimuscarinic medication work by blocking acetylcholine receptors.
Atropine is another antimuscarinic medication used to treat bradycardia.
Intra-Aortic Balloon Pump
An intra-aortic balloon pump is a device used in a number of cardiac conditions, such as:
- Cardiogenic shock
- Acute coronary syndrome (unstable angina and myocardial infarction)
- Immediately following heart surgery
An intra-aortic balloon pump is a temporary measure used while the underlying condition is managed.
A catheter is inserted into the arterial system, usually via the femoral artery, up to the descending thoracic aorta. At the tip of the catheter is an inflatable balloon. The balloon is intermittently inflated and deflated by a special machine, synchronised to the heart contractions. Helium is used to inflate the balloon.
During diastole, when the heart is relaxing, the balloon is inflated. This pushes blood backwards into the coronary arteries, improving coronary perfusion.
During systole, when the heart is contracting, the balloon is deflated. Deflating the balloon creates a vacuum effect, as empty space is created that the balloon previously filled. This reduces the afterload and increases cardiac output.
In summary, an intra-aortic balloon pump:
- Increases coronary blood flow
- Reduces afterload
- Increases cardiac output
Last updated August 2021