Principles of haemodynamic measurements: Pressure and flow measurements calculation of cardiac output Calculation of shunts valve area. DEEPAK NANDAN. Definition of Hemodynamics.
Hemodynamics is concerned with the physical and physiological principles governing the movement of blood through the circulatory system.
1. Kinetic and potential energy provided by the cardiac pump
3. Hydrostatic Pressure
4. Pressure gradients, or differences, between two any points
2. Inertial mass
3. Volume of blood to be moved
1. Size of blood vessel
2. Condition of blood vessel
3. Smoothness of lumen
4. Elasticity of muscular layer (tunica media)
5. Destination of blood (distal vascular bed)
PRESSURE: the ratio of a force acting on a surface to the area of the surface
(force per unit area).
Units : Newtons/m², pascal(Pa), atmospheres(atm), mmHg.
FLOW RATE: Amount of fluid passing a given point over a given period of time
Described as either flow volume or flow velocity.
Flow volume is measured in mI/mm or cm3/sec -defined by Poiseuille’s
Flow velocity is measured in cm/sec or m/sec -described by Bernoulli’s principle.
VISCOSITY: The internal friction between adjacent layers of fluid.
Blood is 1.5 times as viscous as water and its viscosity is directly related to hct level
In hemodynamics-described as the forward movement of blood.
POTENTIAL ENERGY: stored energy.
Kinetic energy is transferred into potential energy when it produces a lateral pressure or stretching of vessel walls during systole.
The potential energy is converted back into kinetic energy when the arterial walls rebound during diastole.
Energy in the blood stream exists in three interchangeable forms: 1)pressure arising from cardiac output and vascular resistance, 2)hydrostatic pressure from gravitational forces,
3)kinetic energy of blood flow
TE = (P per + P grav) + ½ ρ V²
PRESSURE IN STATIC FLUIDS
Rest- the pressure caused by liquid is α to the depth of the liquid & density
pressure is exerted equally in all directions in a static liquid
External pressure exerted on an enclosed liquid changes the overall pressure.
Pascal’s Principle states that any change of pressure in an enclosed fluid is transmitted undiminished to all parts of the fluid.
ΔP = ρg Δh
magnitude of the gravitational effect (ΔP) is the product of the specific gravity or density of the blood (ρ), the acceleration due to gravity (980 cm/s/s) (g), and the vertical distance above or below the heart (Δh) which is 0.77 mm Hg/cm at the density of normal blood.
The pressure at a given depth in a static liquid is a result of the weight of the liquid acting on a unit area at that depth plus any pressure acting on the surface of the liquid.
The pressure in any blood vessel below the level of the heart is increased, and the
pressure in any vessel above heart level is decreased by the effect of gravity.
Eg:-in the upright position, when the MAP 100
mm Hg, the mean pr in a large artery in the head (50 cm above ) is 62 mm Hg (100 - [0.77 x 50]) an
the pressure in a large artery in the foot (105 cm
below ) is 180 mm Hg (100 + [0.77 x 105]).
pressure in a flowing liquid depends on the details of the flow process, in contrast to the case of the static liquid
pressure gradient, which is defined as a pressure drop/ unit length.
For a uniform horizontal tube the pg will be the same at all points in the tube if a perfectly uniform flow pattern is maintained
Pressure gradient = P1 – P2 /L
( P1 = entry pressure,P2 = exit pressure, L = length of tubing)
Drops in pressure during flow represent losses in energy.
These losses are largely attributable to friction effects
Viscosity- force that oppose the flow
The Bernoulli Effect
When fluid flows steadily (without acceleration or deceleration) from one point in a system to another further downstream, its total energy content along any given streamline remains constant, provided there are no frictional losses.
Bernoulli’s equation -rel between kinetic energy, gravitational potential energy, and pressure in a frictionless fluidsystem.
predicts flow velocity and explains the presence of high-velocity jets obtained with Doppler instruments
(Q = volume flow P1 = entry pressure P2 = exit pressure l = length of tubing r = radius of tube ή = viscosity of fluid)
As pressure difference or diameter of vessel ↑, volume flow ↑
As length or viscosity ↑, volume↓
predicts volume flow under laminar flow conditions in straight tubes where wall friction is not a significant factor.
relationship bet bf , resistance, and pressure - modification of Ohm’s law for the flow of electrons in an electrical circuit:
Flow(Q) = pressure gradient (P)/resistance(R)
relationship is known as Poiseuille’s law
Resistance = 8 × viscosity × length/ π× radius4
Blood flow = π × radius4 × difference in pressure/8 × viscosity × length
Viscosity is also important in determining resistance. Exp relative to water.
In the mammalian circulation, resistance is greatest at the level of the arterioles. effective area is much larger (capillaries)
Transition from laminar to turbulent flow can be predicted by calculatingtheReynold’s number, which is the ratio of inertial forces to viscous forces
R = diameter × velocity × density/viscosity
(viscosity (ή) of blood is 0.004 Pa s, density (ρ) of blood is approximately 1050 kg/m3, velocity (V) of blood is in m/s, and the diameter of the tube is in m. Reynold’s number is dimensionless.)
1 .force of the contracting chamber
2.surrounding structures - contiguous chambers of the heart pericardium, lungs, vasculature
3.Physiological variables - heart rate, respiratory cycle
The complex waveform- (Fourier analysis)-
Mathematical summation of a series of simple sine waves of differing amplitude and frequency- harmonics
Fourier found that each pressure wave is a summation of a series of simple sine waves of differing amplitude and frequency
To record pressure accurately, a system must respond with equal amplitude for a given input throughout the range of frequencies contained within the pressure wave
ratio of the amplitude of the recorded signal to the amplitude of the input signal
Damping- dissipation of the energy of oscillation of a pressure measurement system owing to friction
The pressure transducer -calibrated against a known pressure, and the establishment of a zero reference undertaken at the start of the catheterization procedure
Intersection of the 4th ICS and
½ the anterior-posterior diameter of the chest
DETERIORATION OF FREQUENCY RESPONSE
Motion of tip of the catheter within the measured chamber→ Enhance the fluid oscillations of the transducer system
May produce superimposed waves of ±10 mm Hg
Particularly common in PA
An end-hole catheter measures an artificially elevated pressure
because of streaming or high velocity of the pressure wave
Flowing bld- K.E- sudden halt- converted to pressure
This added pressure may range from 2-10 mm Hg
When the catheter is struck by the walls or valves of the cardiac chambers
Common with the pigtail cath in the LV, where the MV hits the cath as they open in early diastole
arterial O2 content - mixed venous (PA) O2 content
Metabolic rate meter by Waters instruments
Parts: oxygen hood /mask
Polarographic oxygen sensor cell
V o2=O2 content in the room air – O2 content in the air flowing past the polarographic cell
Respiratory quotient is assumed
Paramagnetic sensor for measuring O2
Adjusts for temperature and partial pressure of water vapour
Calculates respiratory Q for each patient
Deltatrac II made by sensormedics
Kendrick AH.DirectFick CO: are assummed values of O2 consumption acceptable?Eur Heart J 1988;9:37
“An indicator mixed into a unit volume of constantly flowing blood can be used to identify that volume of blood in time, provided the indicator remains in the system between injection and measurement and mixes completely in the blood”
Average dye concentration = 2 mg/L
Therefore the volume that diluted the dye =
5mg/2mg per L = 2.5 L
Time it took to go past = 0.5 min
ie flow rate = 2.5 L /0.5 min = 5 L/min
time of passage (t) = 0.5 min
average conc (X) = 2 mg/L
mass of dye (Q g)
Flow rate =
average dye conc (X g/L) x time of passage (t min)
downslope until baseline
area under the curve is inversely proportional to the flow rate in the pulmonary artery which equals the cardiac output in absence of intracardiac shunt
Sources of error
Baseline temp fluctuation
Inaccurate in low flow- low output states (overestimation upto 35%)
Empirical correction factor to account for catheter warming may be inadequate
OVERSIMPLIFICATION- pulsatile flow in dynamic and diverse vascular beds
goal of the oxygen saturation run is to measure differences in oxygen saturation in various chambers of the heart
(difficulty advancing the catheter into the pulmonary artery)
( Mattaet al. found that rapid aspiration increased oxygen saturations)
End-hole catheter (e.g., Swan-G) or with side holes close to tip (e.g., GL)
EFFECTIVE BLOOD FLOW- fraction of the mixed venous return received by the lungs without contamination by the shunt flow
( PvO2 - MV O2)
1. SIZE OF L R SHUNT (NO R L SHUNT)
= PBF – SBF
2 . SIZE OF L R SHUNT (R L SHUNT ⁺)
= PBF – EBF
3. SIZE OF R L SHUNT
= SBF - EBF
( SVC - PA)
in the presence of net left-to-right shunting
pulmonary artery pressure less than two thirds systemic levels
PVR less than two thirds SVR
when responsive to either pulmonary vd therapy or
test occlusion of the defect
( Lock JE, Einzig S, Bass JL, Moller JH. The pulmonary vascular response to supplemental oxygen and its influence on operative results in children with ventricular septal defect. PediatrCardiol. 1982;3:41–6.
Balzer DT, Kort HW, Day RW, et al. Inhaled nitric oxide as a preoperative test (INOP Test I): The INOP Test Study Group. Circulation 2002;1060)
(National Consensus Meeting on “Management of Congenital Heart Diseases in India” held on 26thAugust 2007)
Operability-determination of the severity of PAH and the degree of vasoreactivity of the pulmonary circulation.
- balloon test occlusion
1)a low likelihood to benefit from permanent repair 2)higher perioperative risk
On O2 adm- if diastolic pressures fall significantly (10mm Hg) & mean pressure by about 5mm Hg- reversible PAP and operability.
Antegrade flow across MV & TV occur in diastole, AV & PV occur in systole
DFP begins at MV opening(PCW-LV Crossover) & until end-diastole(peak of R in ECG)SEP begins with AV opening & until dicrotic notch/other evidence of AV closure
CO- ml/min DFP/SEP- sec/beat HR- beats/min C- empirical constant (0.85-MVA,1.0- AVA)P- mm Hg ( C- empirical constant - calculated valve area (by Gorlin) -actual valve area (at surgery) Mitral Valve = constant 0.7 (later changed 0.85, Aortic valve: assumed to be 1 )
Peak – peak gradient
Distortion of pulse -FA
Peripheral pulse amplification
Catheter in LVOT
Central aorta - pressure recovery
Calculation of cardiac output :
Fick’s method - presence of MR - overestimate MVA
Thermodilution method - not possible in presence of TR
Confounding factors like anemia, fever AF & thyrotoxicosis
PCWP tracing - delay in transmission
over estimate LA pressure by 2 – 3 mmHg
difficult to obtain wedge position
confirm by noting mean Pr & oxymetry
Based on observation- HR × SEP/DFP × C ≈ 1
Hakki et al
Differs from Gorlins by 18 ± 13% in HR
Mean Ao V Gradient/CO per sec of syst flow
(h = pressure gradientG = gravitational constant (980 cm/sec2) for conversion cmH2 to units pressure. Cv- coefficient velocity for correcting energy loss (pressure energy- kinetic energy)
Valve area = cardiac output ÷ (HR X SEP)
44.3 X C X √ pressure gradient
( C- empirical constant calculated valve area (by Gorlin) actual valve area (at surgery)
Mitral Valve = constant 0.7 (later changed 0.85, Aortic valve: assumed to be 1 )
cardiac output (L/ min)
Heart rate: 60- 100/ min
Pull back hemodynamics :
Peak – peak gradient
Distortion of pulse - femoral artery
peripheral pulse amplification
catheter in LVOT
central aorta - pressure recovery
The current definition of PAH relies on the presence of a mean pulmonary arterial pressure exceeding 25 mmHg at rest or 30 mmHg during exercise, a left atrial pressure below 15 mmHg, and a normal resting cardiac output, suggesting a resting pulmonary vascular resistance above 3 Wood units.