Cardiology in a Heartbeat, second edition

Chapter 1

Anatomy and Physiology of the Cardiovascular System

by A. Vaswani, L. Chan and V. Zamvar

1.1 Introduction

The heart is a muscular organ that pumps blood throughout the circulatory system.

At a heart rate of 70 beats per minute, a human heart will contract approximately 100 800 times a day, more than 36 million times a year and nearly 3 billion times during an 80-year lifespan.

Figure 1.1 – The anatomy of the heart.

1.2 Chambers of the heart

The heart has four chambers: two atria and two ventricles that function to return deoxygenated blood to the lungs and oxygenated blood to the rest of the circulation.

Figure 1.2 – The internal anatomy of the heart.

1.2.1 Right atrium

Forms the entire right border

Receives venous blood from the:

○ superior vena cava superiorly, draining the azygos, subclavian and jugular veins

○ inferior vena cava inferiorly, draining the lower body

○ coronary sinus inferiorly, draining the heart

○ anterior cardiac veins anteriorly, draining the anterior heart

Posterior part of wall is smooth

Anterior part of wall is rough with trabeculations known as musculi pectinati (pectinate muscles) derived from the true fetal atrium

Crista terminalis is a muscular ridge that runs vertically downwards, separating the smooth and rough parts

Right auricle/atrial appendage is a cone-shaped muscular pouch-like extension of the right atrium

Fossa ovalis is a shallow oval depression in the interatrial septum

○ an embryonic remnant of the fetal foramen ovale.

// PRO-TIP //

Unlike the conduction system in the ventricles, the conduction across the atria is less well defined. Bachmann’s bundle refers to an interatrial bundle comprising parallel arranged myocardial fibres connecting the right and left atria with regard to electrical conduction. Three other tracts known as the anterior, middle and posterior tracts run from the SA node to the AV node.

1.2.2 Right ventricle

Forms most of the inferior border and anterior surface of the heart

Connected to the right atrium by the tricuspid valve which has three cusps: anterior, posterior and septal

Connected to the pulmonary trunk by the pulmonary valve comprising three semilunar cusps

Each cusp is connected to its corresponding papillary muscle by chordae tendineae (heart strings)

Trabeculae carneae are irregular ridges lining the wall

Moderator band is a muscular bundle connecting the interventricular septum to the anterior wall

○ conveys the right bundle branch to the ventricular muscle

Infundibulum is the smooth-walled outflow tract directed upwards and right towards the pulmonary trunk.

// PRO-TIP //

The moderator band carries part of the right bundle branch from the interventricular septum from the apex of the RV to the anterior papillary muscle, and was so named because it was thought to have prevented overdistension of the ventricle based on anatomical attachments on its own (hence the name moderator). Interestingly, this structure was first described by Leonardo da Vinci.

Figure 1.3 – Superior view of heart valves.

1.2.3 Left atrium

Forms most of the base of the heart

Smaller, but thicker walled than right atrium

Receives oxygenated blood from the four pulmonary veins which open into the cavity on its posterior wall (two from each lung: superior and inferior)

Left auricle/atrial appendage is an ear-shaped muscular pouch extending forwards and to the right

○ this is a common site for thrombus formation

// WHY? //

The left atrial appendage is a unique component of the left atrium, distinct in both anatomy and physiology.

From a physiological standpoint, this is where flow appears to be at a minimum (recall that Virchow’s triad consists of hypercoagulability, endothelial dysfunction, and in the case of the left atrial appendage (LAA), stasis).

Anatomically, the LAA is not simply an embryological remnant, but is thought to play a role in fluid regulation (via natriuretic peptide mediation) as well. It is, however, also the site of up 90% of thrombus formation in non-valvular AF.

Apart from blood flow stasis, the pathogenesis of why this area appears to be especially thrombogenic is unclear, but the shape and character of its trabeculations are also thought to play a role.

Mainly smooth-walled except for ridges in the auricle due to underlying pectinate muscles.

// WHAT’S THE EVIDENCE? //

In patients who are unsuitable for or unable to tolerate anticoagulation, LAA closure with a percutaneous device is also an alternative option. Surgical closure has been attempted as early as the 1950s or 60s. The LAAOS study was the first of its kind (a single-centre trial) that randomised patients with CABG to suture/staple vs. a control group, but only 14% had AF, and only two-thirds of patients actually had a successful closure. LAAOS II was better designed, but showed no difference in the primary endpoint, with variable success in surgical closure. Newer surgical techniques include epicardial closure during cardiac surgery, which has markedly better closure rates (98% and above), and LAA closure has thereafter been associated with a lower risk of stroke.

The ESC and ACC/AHA guidelines recommend LAA closure to be considered in patients who are undergoing cardiothoracic surgery. Percutaneous closure (such as with devices like the WATCHMAN device) has been evaluated in the PROTECT-AF trial, which showed it was non-inferior to warfarin that was dose adjusted, with fewer bleeding events, but more serious safety issues such as the development of a pericardial effusion in 5%.

Because of this, the PREVAIL trial was conducted, for which the operators for the device were newer, and patients needed more than one risk factor for stroke to be enrolled. In this trial only 2.2% had safety events, and the data suggested that it was non-inferior to warfarin, with lower risk of haemorrhagic events, but also an increased risk of ischaemic stroke compared to warfarin, and thus can be considered as an alternative to anticoagulation in patients who are unable to tolerate it.

1.2.4 Left ventricle

Forms most of the left border and the apex of the heart

Longer, more conical and thicker walled than the right atrium (three times thicker, between 6 and 10 mm in a normal heart)

Wall is lined by thick trabeculae carneae

Joined to left atrium by mitral valve

Mitral valve has two cusps: anterior and posterior

Chordae tendineae connect each cusp to its corresponding papillary muscle

Communicates with aorta via the aortic valve:

○ 3 semilunar cusps: anterior, right and left posterior

○ anterior and left posterior aortic sinuses above the valve give rise to the right and left coronary arteries respectively.

// EXAM ESSENTIALS //

There are five papillary muscles in the heart that arise from the ventricular wall. Three attach to the tricuspid valve, and two of these attach to the mitral valve. These muscles hold on to chordae tendineae and are responsible for preventing valvular regurgitation in the first instance.

Attached to the tricuspid valve, the three complexes are referred to as anterior, posterior and septal; and the two complexes attached to the mitral valve are known as the anterolateral and posteromedial complexes. Rupture of these papillary muscle complexes can occur partially or completely, leading to valvular regurgitation and in the case of complete ruptures, more likely haemodynamic instability.

Of note, it is important to remember that the posteromedial papillary muscle complex is the most likely to be affected, given its single blood supply from the posterior descending artery, unlike the anteromedial papillary muscle, which has a dual blood supply, and is less likely to rupture.

// PRO-TIP //

‘Pectinate’ means ‘like a comb’ in Latin

The mitral valve gains its name from its resemblance to a bishop’s mitre

A patent foramen ovale is found in 20% of adults and is usually asymptomatic

Rupture of any papillary muscle, such as following a myocardial infarction, will cause the valve cusp to prolapse, resulting in severe regurgitation.

1.3 Coronary circulation

1.3.1 Coronary arteries

The heart has a high oxygen demand from continuous pumping of blood. This demand is met by the left and right coronary arteries.

Right coronary artery

Originates from the anterior aortic sinus

Passes forwards between pulmonary trunk and right auricle

Runs along the atrioventricular (AV) groove

Continues to the inferior border of heart to anastamose with the circumflex branch of the left coronary artery

Major branches:

○ sinoatrial (SA) nodal artery runs posteriorly between the right auricle and aorta, supplying the SA node

○ marginal artery along the inferior border of the heart

○ posterior descending/posterior interventricular artery descends in the posterior interventricular groove to anastamose with the left anterior descending artery at the apex

○ AV nodal artery arises from the characteristic loop where the posterior descending artery originates to supply the AV node.

Left coronary artery

Originates from the left posterior aortic sinus

Larger than the right coronary artery

Left main stem varies in length (4–20 mm)

The left anterior descending artery is known as the ‘widowmaker’ because occlusion leads to rapid death

○ the term ‘widowmaker’ usually refers to LAD; some variations include the left main stem as the artery in question

Initially passes behind then to the left of the pulmonary trunk

Reaches the left part of the AV groove

Runs laterally around the left border as the left circumflex artery to reach the posterior interventricular groove

Major branches:

○ left anterior descending (LAD)/anterior interventricular:

– descends in the anterior interventricular groove to anastamose with the posterior descending artery at the apex

○ left circumflex (LCX):

– continues round the left side of the heart in the atrioventricular groove, giving off various ventricular and atrial branches

– anastamoses with the terminal branches of the right coronary artery.

Figure 1.4 – The coronary vessels.

// PRO-TIP //

Knowledge of the major and minor branches of the coronary arteries is essential for coronary angiogram interpretation. Figure 1.5 is a schematic of the arteries as visualised in a typical coronary angiogram.

RCA = right coronary

RDP = posterior descending

RPL = right posterolateral branch

LCX = left circumflex

LAD = left anterior descending

D1 = first diagonal branch of LAD

M1 = first marginal branch of LCX

MO1 = first marginal obtuse branch of LCX

Figure 1.5 – Schematic of coronary arteries as visualised in a typical angiogram.

Anatomical variations

90% of the population are ‘right dominant’ – where the posterior descending/posterior interventricular artery branches from the right coronary artery. In the ‘left dominant’ 10%, the left coronary and circumflex arteries may be larger and branch off the posterior descending artery before anastomosing with an unusually smaller right coronary artery.

The SA node is supplied by the right coronary in 60% and the left circumflex artery in nearly 40% of the population. Dual supply is present in 3%.

The AV node is supplied by the right coronary artery in 90% of the population, with the remaining 10% by the left circumflex.

// PRO-TIP //

The coronary arteries originate from the aortic sinuses – small openings superior to the cusps of the aortic valve. This makes coronary arteries the first branches of the aorta

Coronary blood flow is about 250 ml/min at rest (5% of cardiac output) and rises to 1 L/min during exercise

During systole (contraction), the aortic valves open and blood is ejected into the aorta. When the aortic valves close during diastole (when the heart is at rest), blood in the aorta flows into the aortic sinuses then into the coronary arteries to supply the heart

Coronary flow takes place mainly during diastole. It is reduced during systole when the intramyocardial arteries are compressed by the contracting muscle

Therefore, diastolic perfusion time is important for the coronary circulation. This is shortened by a rapid heart rate which may result in inadequate perfusion.

1.3.2 Coronary veins

Most of the heart’s venous drainage is fulfilled by the tributaries of the coronary sinus.

The coronary sinus runs in the posterior atrioventricular groove. It opens into the right atrium just to the left of the opening of the inferior vena cava. Its orifice is guarded by the Thebesian valve. Its tributaries are:

the great cardiac vein in the anterior interventricular groove

the middle cardiac vein in the inferior interventricular groove

the small cardiac vein that accompanies the marginal artery along the inferior border of the heart

the oblique vein which descends obliquely on the posterior side of the left atrium.

The remaining venous drainage is fulfilled by:

the anterior cardiac veins (3 or 4 of them) draining a large proportion of the anterior surface of the heart directly into the right atrium

small veins (venae cordis minimae) within each chamber wall draining directly into their respective chambers.

1.4 Conducting system of the heart

Conducting system In A Heartbeat

SA node depolarisation → atrial contraction → AV node activation (0.1 s delay) → bundle of His → left and right bundle branches → Purkinje fibres → ventricular contraction

Figure 1.6 – The conducting system of the heart.

The sinoatrial (SA) node (the ‘pacemaker of the heart’) is situated at the junction of the superior vena cava and the right atrium. This is where the electrical cycle begins.

1. The SA node initiates contraction by depolarising both atria, causing them to contract and pump blood into the ventricles.

2. The atrial action potential activates the AV node which lies in the interatrial septum immediately above the opening of the coronary sinus.

3. The AV node introduces a delay of 0.1 seconds before relaying the impulse to the bundle of His. This delay allows the ventricles to fill.

4. Depolarisation then spreads through the bundle of His (which then divides into the left and right bundle branches) and Purkinje fibres to reach the ventricular muscle.

5. This activates the ventricles and causes them to contract.

In a normal heart, the total time taken for the action potential to travel from the SA node to the end of the Purkinje fibres is about 0.22 seconds. A normal sinus rhythm is approximately 72 beats per min, which equates to 0.83 seconds for each cardiac cycle.

1.5 Nerve supply of the heart

While the conductive system of the heart has its proprietary intrinsic pacemaker, the autonomic nervous system is important in the rate of impulse formation, conduction

and contraction strength

The heart’s nerve supply is derived from the vagus nerve (parasympathethic cardio-inhibitor) and the C1-T5 sympathetic ganglia (cardio-accelerator) via the superficial and deep cardiac plexuses

Many drugs used in cardiology target the receptors shown in Table 1.1 (e.g. beta-blockers).

Table 1.1 – Actions of the autonomic nervous system on the heart

// WHY? //

Cardiac pain is not found exclusively in the chest but often radiates down the medial side of the left arm and up to the neck and jaw. This is because radiation occurs to areas that send sensory impulses to the same level of the spinal cord that receives cardiac sensation.

The sensory fibres from the heart travel up to T1-4, hence radiating to the medial left arm via dermatomes T1-4. Cardiac vagal afferent fibres synapse in the medulla and descend to the upper cervical spinothalamic tract cell, contributing to the pain felt in the neck and jaw.

1.6 The cardiac cycle

Isovolumetric means that the ventricles contract or relax with no corresponding change in ventricular volume. There are periods during these phases when all valves are closed. Conversely, there is never a point in the cardiac cycle when all valves are open.

Figure 1.7 – The Wiggers diagram, named after Dr Carl Wiggers who in 1915 described the events occurring during each cardiac cycle.

Table 1.2 – The cardiac cycle

1.7 Heart sounds

1.7.1 S1 lub

Reflects closure of the mitral and tricuspid valves

Loud in: thin patients, hyperdynamic circulation (e.g. pregnancy), mitral stenosis

Soft in: obesity, emphysema, pericardial effusion, mitral regurgitation or severe mitral stenosis

Best heard at the apex.

1.7.2 S2 dub

Reflects closure of the aortic and pulmonary valves

Physiological splitting of the sound may occur during inspiration

Split into: Aortic before Pulmonary (A2–P2 intervals)

Because inspiration increases venous return → increases stroke volume → prolongs the right ventricular ejection period

Loud in: systemic hypertension, hyperdynamic circulation

Soft in aortic stenosis.

1.7.3 S3 KENTUCKY: KEN = S1, TUCK = S2, Y = S3

Reflects rapid ventricular filling, when blood strikes a compliant left ventricle

Occurs in early diastole, just after S2 when the mitral valve opens

May be physiological in young, fit patients or children

In older patients, may be due to heart failure or volume overload

Best heard at the apex.

1.7.4 S4 TENNESSEE: TEN = S4, NES = S1, SEE = S2

Reflects surge of ventricular filling with atrial systole

Occurs in late diastole, immediately before S1, when the atria contract to force blood into a non-compliant left ventricle

Always pathological

Never present in atrial fibrillation (there is no atrial contraction)

Indicates increased ventricular stiffness or hypertrophy

E.g. hypertension, aortic stenosis, acute myocardial infarction (MI)

Like S3, best heard at the apex.

1.8 Cardiac muscle contraction

Cardiac muscle contraction In A Heartbeat

Contraction: Calcium-induced calcium release

Depolarisation → Little Ca²+ enters cytosol → Ca²+ enters sarcoplasmic reticulum → SR releases more Ca²+ into cytosol → Ca²+ binds to troponin → cross-bridge formation between actin-myosin → contraction

Relaxation:

Ca²+ is pumped from cytosol back into SR → Ca²+ levels drop → Ca²+ dissociate from troponin → relaxation

Cardiac muscle cells are known as cardiac myocytes. Following depolarisation, calcium (Ca²+) enters the cell through L-type voltage-gated Ca²+ channels in the sarcolemma. This supplies only 20% of the Ca²+ required, insufficient for muscle contraction. The rest is released from the sarcoplasmic reticulum (SR) where Ca²+ is stored in high concentration in a calcium-induced calcium release process (see Figure 1.8) unique to cardiac muscle. The sequence of events is as follows:

Initiation of contraction

Depolarisation causes Ca²+ to enter the myocyte via L-type voltage-gated Ca²+ channels

This causes a rise in Ca²+ concentration in the gap between sarcolemma and SR

This activates Ca²+-sensitive Ca²+ release channels (ryanodine receptors) in the SR

Ca²+ floods into the cytoplasm down its concentration gradient. This releases a large amount of Ca²+, sufficient to initiate contraction.

Contraction

Calcium ions in the sarcoplasm then interact with troponin C to initiate cross-bridge formation

When Ca²+ binds to troponin C, the actin binding site is exposed, allowing the myosin head to bind to the actin filament

Just as in skeletal muscle contraction, using adenosine triphosphate (ATP) hydrolysis, the myosin head pulls the actin filament towards the centre of the sarcomere.

Relaxation

When the concentration of Ca²+ exceeds resting levels, ATP-dependent Ca²+ pumps (Ca²+-ATPase) in the tubular part of the SR are activated and pump Ca²+ from the cytosol back into the SR

As the action potential (AP) repolarises and Ca²+ channels inactivate, this mechanism reduces Ca²+ towards resting levels, Ca²+ disassociates from troponin C and the muscle relaxes.

Figure 1.8 – Calcium-induced calcium release process in a cardiac myocyte leading to contraction of cardiac muscle.

1.9 Cardiac output

1.9.1 Definitions

The cardiac output is the volume of blood ejected by each ventricle per minute

Cardiac output (CO) = Stroke volume (SV) × heart rate (HR)

= Mean arterial pressure ÷ systemic vascular resistance

At rest, the average cardiac output is about 5 L/min

During exercise, this can exceed 25 L/min as the heart rate increases two to three times and stroke volume doubles.

The stroke volume (SV) is the volume of blood pumped by each ventricle per beat

It is about 75 ml in a normal heart and may double during exercise

Stroke volume (SV) = End diastolic volume (EDV) – end systolic volume (ESV)

End diastolic volume (EDV) = maximum ventricular volume immediately before systole (~120 ml)

End systolic volume (ESV) = minimum ventricular volume (~40 ml) immediately before diastole.

The ejection fraction (EF) is the percentage (%) of blood ejected by the ventricle relative to its filled volume

1.9.2 Regulation of cardiac output

Adequate cardiac output is important for ensuring that each organ receives its minimum required blood flow. The body regulates cardiac output by regulating heart rate and stroke volume.

Heart rate

A normal heart rate is between 60 and 100 bpm

○ <60 bpm is bradycardia

○ >100 bpm is tachycardia

This is affected by the heart’s intrinsic rhythmicity and extrinsic stimulation by the autonomic nervous system