Heart Mechanics: Systole, Diastole & Blood Flow

• The heart is a system of two pressure pumps (chambers) with two atria just before them.

Mechanics of Heart Work

• Heart valves (mitral, tricuspid, pulmonary, and aortic) ensure unidirectional blood flow
• The heart works as a pressure pump; heart muscle contraction increases pressure
• Systole is the contraction, while diastole is the release (relaxation) of the heart muscle
• Atrial pressure is higher during diastole, creating a pressure gradient for blood flow from atria to ventricles.
• Systole and diastole alternate regularly in a heart cycle with blood volume (ΔV) pushed out by pressure (p) from the heart performing mechanic work (W = p ΔV)
• Kinetic energy (Ek) of systolic cardiac output: Ek = 1/2 * ρ * v^2 * ΔV, where ρ is blood density and v is blood speed
• Total mechanic work of the heart is the sum of mechanic work and kinetic energy: W heart = W + Ek
• Example: Left ventricle displaces 70 ml at 120 mmHg, work is 1.117 J; right ventricle at 15 mmHg, work is 0.140 J
• With a blood density of 1048 kg/m^3 and flow rate of 0.5 m/s, the kinetic energy of both chambers is 0.018 J
• The total work of the heart at rest is 1.275 J per cycle (1.117 J + 0.140 J + 0.018 J)

Cardiac Cycle

• Represents the heart's performance between two successive beats
• During atrial diastole:
  • Right atrium (atrium dextrum) receives deoxygenated blood from vena cava superior and inferior
  • Left atrium (atrium sinistrum) receives oxygenated blood from pulmonary veins (venae pulmonales)
• Ventricular diastole and atrial systole are associated with blood flow from right atrium to right ventricle (through tricuspid valve) and from left atrium to left ventricle (through mitral valve, or AV valves)
• Ventricular systole pushes blood from right ventricle (ventriculum dextrum) to pulmonary arteries (arteriae pulmonales) and from left ventricle (ventriculum sinistrum) to the aorta

Cardiac Cycle Phases

• Atrial contraction: Increased atrial pressure pushes blood "actively" to ventricles
• 75% of blood flows "passively" to ventricles (during diastole), atrial contraction forces 25% of remaining blood
• In resting conditions, the heart is filled passively up to 90%, with only 10% from atrial contraction
• Blood cannot flow back to the venous system due to venous return inertia and chamber contraction towards the AV valve
• End-diastolic volume refers to the maximized volumes of both ventricles, typically 110-120 ml each
• Isovolumic contraction: The pressure increases while the volume remains the same
• AV valves close to prevent backward flow, causing the first heart sound
• Intraventricular pressure increases without volume change, the aortic and pulmonary valves open
• Rapid ejection: Accumulated energy forces blood to the aorta and pulmonary arteries to reach maximal systolic pressures
• Initially, atrial pressure decreases, blood flows to atria from venous tracts, increasing pressure to phase 5
• Slow ejection: Ventricular pressure and volume decrease, atria are filled with blood

Heart Sounds

• Isovolumic relaxation: Chamber pressure decreases, reversed pressure gradient closes valves, second heart sound
• A small amount of blood escapes back to the ventricles, slightly decreasing the blood pressure (dicrotic notch)
• End-systolic volume: Volume of blood remaining in ventricle after systole (40-50 ml typically)
• Stroke volume is the difference between end-systolic and end-diastolic volumes (70-80 ml)
• Rapid filling: Ventricular muscle fibers relax causing inner pressure decrease
• AV valves open when ventricular pressure falls below atrial pressure, blood flows "passively" from atria to ventricles, third heart sound
• Reduced filling: Ventricles become less compliant, increasing pressure
• Decreased pressure gradient across AV valves slows blood flow, ventricles fill to 70-90% before a new cycle begins.

Mechanics of Blood Flow

• Blood circulation is a closed system: heart, vessels (elastic arteries, veins), and blood
• The heart acts as an engine pushing blood into the distributional (vascular) system
• Unidirectional blood flow and pressure gradients between heart/vessels and arterial/venous systems are necessary for continuous flow
• The vascular system is flexible and capable of active contraction (especially arterioles) changing lumen and affecting blood flow
• Blood flow (Q) depends directly on pressure gradient and inversely on vascular resistance and blood viscosity
• Poiseuille - Hagen's law expresses the relationship between blood flow, pressure, viscosity, and vascular resistance

Poiseuille - Hagen's Law

• Q = (π * r^4 * (P1 - P2)) / (8 * η * l)
  • Q is the amount of blood (volume)
  • r is the vessel radius
  • P1-P2 is the pressure difference
  • η is blood viscosity
  • l is vessel length
• Used clinically to calculate total resistance of vascular bed based on cardiac output (Q) and pressure gradient (P1-P2)
• Increased vascular resistance increases systemic blood pressure and heart workload
• Nature of blood flow and velocity affect the use of Poiseuille's law; blood flow can be laminar or turbulent depending on velocity, blood viscosity, and vessel anatomy
• Biochemical energy converts to hydromechanic energy to maintain blood pressure
• Heart performs work by pushing blood against the aorta's pressure, changing static to kinetic work

Bernoulli's Equation

• Bernoulli's equation applies to unit volume of fluid (blood) flow: p + (1/2) * ρ * v^2 = constant, where p is fluid pressure, ρ is fluid density, and v is flow rate
• Energy conservation applies since increased kinetic energy (flow in a smaller tube diameter S2) causes a drop in fluid pressure

Mechanic Properties of Vessels

• Systolic blood pressure pushes blood from left ventricle to aorta and large arteries
• Arteries are elastic due to elastic fibers which need to be stretched
• Artery walls store kinetic energy as potential energy, converting it back during diastole due to elastic tension to maintain diastolic blood pressure
• Elastic vessels maintain blood flow during diastole
• Kinetic to potential energy transformations occur in the bloodstream, known as the elastic effect of blood vessels
• Stiffer (less elastic) arteries result in higher systolic pressure and lower diastolic flow
• Atherosclerosis (decreased vessel elasticity) increases both systolic and diastolic arterial pressures, overloading the heart

Blood Pressure

• Zero pressure exists in ventricles during diastole as they prepare to fill
• Positive pressure is in the pulmonary artery and aorta during diastole due to arterial elasticity
• Arterial elasticity maintains blood pressure during diastole, allows continuous flow (systole and diastole), and increases efficiency
• A vascular elasticity model includes a jar with fluid representing the left ventricle, pressurized with a balloon
• Task to demonstrate vessel elasticity using the model
• Pressure in arteries varies due to heart activity
• Cardiac systole energy speeds blood in arteries (longitudinal pressure) applying to vessel walls (lateral pressure)
• Systemic blood pressure rises during systole, blood is pushed to aorta (systolic pressure) and as kinetic energy is stored in aorta/arteries as potential energy; arteries regain shape and convert potential energy back to kinetic energy (diastolic pressure)
• Systolic pressure is the maximum pressure during left ventricular systole and diastolic pressure is the lowest during cardiac diastole
• Mean blood pressure is diastolic pressure plus one-third of the pressure amplitude (systolic - diastolic)
• Blood pressure is expressed as a fraction of systolic/diastolic pressure

Blood Pressure Measurement

• WHO recommends mmHg or kPa (1 kPa = 7.5 mmHg = 7.5 Torr = ~10 cm H2O)
• Direct (invasive) method measures pressure changes directly in the vessel lumen, disrupting skin/vessel wall
  • A catheter with heparinized solution (to prevent coagulation) is inserted (often arteria subclavia or arteria femoralis)
  • Pressure transmits to a sensor (electromanometer) transducing pressure wave to electrical signal recognizable by computers
  • Accurate, used for research, experimental procedures, or intensive care but not normal clinical practice
• Indirect (non-invasive) method measures pressure within a cuff, less accurate but is more widespread due to its non-invasive nature
• Two indirect methods: palpation and auscultation, or their combination
• Palpation measures systolic pressure without a stethoscope
  • Examiner palpates pulse on a. radialis, inflates cuff until pulse disappears, cuff fully compresses a. brachialis which prevents blood flow
• Examiner increases pressure by 3 kPa and gradually reduces pressure, and systolic pressure is determined when pulse reappears on a. radialis
• Auscultation method:
  • Wrap cuff around the arm, palpate pulse on radialis to increase pressure 20mmHg above the pressure when pulse disappeared
  • Korotkov's sound phenomena (turbulent blood flow) occur under the cuff
  • Place Recklinghausen's cuff in the middle third of arm (on a. brachialis) and stethoscope below cuff
  • Increase cuff pressure until blood circulation stops, slowly open valve and listen to artery. No sound at first because there is no flow since artery is completely collapsed
• As pressure drops the first short tapping sounds (Korotkoff sound I) indicate partial blood flow and correspond to systolic pressure
  • Further decrease leads to louder sounds turning to murmurs (Korotkoff sound II) changing to clear sounds (Korotkoff sound III) until sounds disappear (Korotkoff sound IV) indicating free flow and diastolic pressure
• Blood pressure is expressed as systolic/diastolic with units kPa, mmHg, or Torr (e.g., 120/80 [mmHg]); accuracy is about 5 mmHg
• Systolic pressure typically ranges from 12 to 20 kPa (90–140 mmHg) and diastolic from 8 to 12 kPa (60–90 mmHg)
• Systolic pressure above 140 mmHg and diastolic above 90 mmHg indicate hypertension

Blood Pressure Measurement Principles

• Avoid smoking/caffeine/alcohol and exercise before
• Measure in sitting/recumbent position
• Person should be at rest for 10-15 min
• Sleeves should be loose to not restrict blood flow
• Room should be quiet, with reasonable temperature
• The sphygmomanometer should face the examiner
• The patient must sit with limb and sphygmomanometer at heart level
• Must respect correct cuff and sphygmomanometer position at heart level
• Cuff width affects values, standard adult cuff is ~12 cm wide
• Cuff width should be 40% of limb circumference, and length should encircle arm, covering at least 2/3 of circumference
• Recommended NIBP has a range (cm), width, length and max circumferences for Newborn, Infant, Child, Small adult, Adult and Large adult
• Must consider correct cuff size for bodybuilders, newborns, and children
• Mercury (alcohol) tonometer includes the Riva-Rocci sphygmomanometer with Recklinghausen's cuff, balloon, and mercury/alcohol scale
• Aneroid (spring) tonometer replaces the mercury column with a spring; resistance spring is susceptible to damage, requiring frequent checks
• Digital tonometer is automatic/semi-automatic and very popular
• WHO prefers mercury/alcohol tonometer as the accurate method, because digital relies on oscillometric methods, thus measuring change through vessel circumference
• Oscillometric tonometers work on oscillometric principle, measuring pressure-volume changes translated through the artery’s (blood pressure) relationship to the artery surface (cuff pressure)
• The pulse is maximal in the artery, meaning the cuff pressure equals mean arterial pressure; systolic/diastolic pressures calculated by software

Blood Pressure Measured Techniques

• Auscultation
  • Seat patient at table and prepare sphygmomanometer and stethoscope
  • Measure pulse on radial artery and record values and compare
• Digital Tonometer
  • Seat person at the table and prepare a digital sphygmomanometer
  • Measure records values and compares