Blood Pressure

What pressure values are normal for each chamber of the heart?

  • A systolic blood pressure of 120 mmHg and diastolic of 80 mmHg gives a mean arterial pressure of 93 mmHg via the following equation
  • On the graph the MAP line makes the areas A and B equal
  • The bump in the descending limb is the dicrotic notch, caused by closure of the aortic valve

Tell me about coronary perfusion pressure and flow

  • Coronary perfusion pressure is the pressure driving the blood flow to the coronary arteries, which predominantly occurs during diastole for the left ventricle, and during both systole and diastole for the right ventricle
    • CPP = Aortic pressure – Ventricular wall pressure (compressing the arteries)
      • For the left ventricle this is therefore:
        • Aortic diastolic pressure – Left Ventricular End Diastolic Pressure
  • Coronary blood flow depends on pressure and resistance via Ohm’s law
    • Pressure = Flow x Resistance
      • Coronary blood flow = Coronary Perfusion Pressure / Coronary vascular resistance
        • Flow is greatest to the left ventricle during diastole as not only is the CPP at its highest, but the vessels are also at their least compressed, and therefore resistance is at its lowest
          • Left ventricular CBF = 100ml/100g/min
          • Right ventricular CBF = 10ml/100g/min
    • During isovolumetric contraction, CBF is zero as the coronary vessels are maximally compressed

What are Starling forces?

These are the forces that determine the flow of fluid across the capillary wall, and are comprised of hydrostatic and oncotic forces or pressures.

Hydrostatic forces act to push fluid out of a given area while oncotic forces act to draw fluid into a given area, with higher solute concentrations generating more oncotic pressure.

The relationship between the forces is described by the Starling equation.

At the arterial end of the capillary, the hydrostatic pressure is much higher inside the artery than outside, so there is a net outward driving pressure, meaning fluid tends to leak into the interstitium.

At the venous end, the hydrostatic pressure is lower, so the oncotic pressure draws fluid back into the vein.

Net driving pressure = (Difference between hydrostatic pressures in and out) – (difference between oncotic pressures in and out)

To convert this into net fluid flux across the membrane, we have to add in two coefficients

  • K tells us how much fluid crosses the membrane per unit of pressure
  • σ tells us how inherently permeable the membrane is to the solutes and plasma proteins of the fluid

Net fluid flux = K[(Pc – Pi) – σ(πc – πi)]

The Starling Equation

How would Starling forces be affected by a sudden loss of a litre of blood?

  • There would be a greatly reduced arterial capillary hydrostatic pressure
    • This means the net driving pressure would favour recruitment of fluid from the interstitium back into the vasculature
      • This helps to retrieve up to 1000ml per hour into the circulating volume

What is shock and what are the different types?

Shock is a state of inability to adequately perfuse the vital organs to meet the metabolic demands, and it can be categorised into four groups:

Causes of Shock

CardiogenicObstructiveHypovolaemicDistributive
The pump isn’t working properlyThere’s a blockage in the pipesThere’s not enough fluid to pump around the bodyThe fluid is being pumped to the wrong place
Acute Coronary SyndromeCardiac TamponadeBleedingSepsis
ArrhythmiasPulmonary embolismDehydrationAnaphylaxis
BurnsNeurogenic

Management of shock

CardiogenicObstructiveHypovolaemicDistributive
InotropesThrombectomyIV fluidsVasopressors
Left ventricular assist deviceBypass surgeryBlood Products
Intra-aortic balloon pumpThrombolysis

Shock severity scoring

Grade of ShockIIIIIIIV
Blood loss (ml)up to 750750-15001500-2000>2000
% Blood lossup to 15%15 – 30%30 – 40%>40%
Heart Rate (bpm)<100100-120120-140>140
Blood PressureNormalSlightly ReducedReducedSeverely Reduced
Respiratory Rate (bpm)<2020 – 3030 – 40>40
Urine output (ml/h)>3020 – 305 – 150
CognitionNormalAnxiousConfusedDrowsy

What is systemic vascular resistance?

  • The opposition or resistance to blood flow in the systemic circulation against which the ventricle must push blood
  • Units are dynes.s.cm^-5
    • A dyne is the amount of force that will accelerate 1g by 1cm per second squared
      • It is one one-hundred-thousandanth of a Newton

What is the equation for pulmonary vascular resistance?

  • PVR = (Mean pulmonary artery pressure – Left atrial pressure) / cardiac output x 80
    • Usually around 100 – 150 dynes.s.cm^-5

Please describe what happens during a Valsalva maneouvre

  • The Valsalva maneouvre is forced expiration against a closed glottis for ten seconds
    • It has four phases
      • Phase 1 – beginning at the start of compression and lasting for a few seconds
        • The valsalva manoeuvre increases intrathoracic pressure, which initially increases venous return, causing a rise in blood pressure, and a reflex bradycardia
      • Phase 2 – from the end of phase 1 until release of the pressure
        • The sustained increased intrathoracic pressure now reduces venous return, causing a reduction in blood pressure and a reflex tachycardia, which gradually restores blood pressure
      • Phase 3 – from the release of pressure for a few seconds
        • The sudden release of pressure reveals a large empty pool of venous capacitance vessels, causing a drop in blood pressure and a sustained tachycardia
      • Phase 4 – from the end of phase 3 until normal parameters return
        • Once venous return is restored, blood pressure returs to normal, causing a reflex bradycardia, which then gradually resolves back to normal

What is central venous pressure and why do we use it?

  • This is the hydrostatic pressure exerted on the walls of the great veins by the venous blood, and can be measured in the inferior or superior vena cava using a pressure transducer on a central venous catheter
  • It can be used as an estimate for right atrial pressure, which in turn gives us an idea of the filling status of the right ventricle and therefore the heart as a whole
  • A wave
    • Atrial contraction
    • Absent in atrial fibrillation
    • Cannon A waves seen in tricuspid stenosis and complete heart block
  • C wave
    • Tricuspid valve bulges back into the atrium during right ventricular systole
  • X descent
    • As right ventricle contracts and right atrium relaxes, the tricuspid valve is dragged downwards, reducing the pressure in the right atrium
  • V wave
    • Atrial filling when tricuspid still closed
    • Giant V waves in tricuspid regurgitation
  • Y descent
    • Tricuspid valve opens and blood drains passively into the ventricle

What factors affect pulmonary vascular resistance?

Increase PVRDecrease PVR
Reduced cardiac outputIncreased cardiac output
Increased viscosityReduced viscosity
Lung volume much higher or lower than functional residual capacityLungs at functional residual capacity have lowest PVR, as at this point there are minimal compression or distensive forces acting on the pulmonary blood vessels
VasoconstrictorsVasodilators
Histamine and serotoninAcetylcholine
HypoxiaHyperoxia
HypercapnoeaHypocapnoea
AcidosisNormal pH
Sympathetic stimulationParasympathetic stimulation

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