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Pressure Parameters

Pressure Parameters

The EDM+ uses the proven Doppler technology to control both its Flow Monitoring Mode of use and the calibration of the chosen Pulse Pressure Waveform Analysis (PPWA) algorithm for its Pressure Monitoring Mode of cardiac output (CO). Flow based parameters are also available on the EDM+ monitor.

The direct flow Doppler EDM, is preferred for guidance of intervention with fluid and drugs. EDM can do this effectively in the hemodynamically challenging environment of the Operating Theatre, where anesthesia and surgery result in rapid and frequent changes in compliance. Pressure based technologies are useful in stable postoperative patients but are limited in their ability to guide interventional treatment.  A combination of both technologies, EDM+ is the best of both worlds.

Pressure Based Parameters

Stroke Volume (SV)

Stroke Volume is the amount of blood in millilitres pumped from the heart during every heart beat. It is the volume ejected from the left ventricle due to the contraction of the heart muscle. Stroke Volume can be calculated as a Doppler Flow Based Parameter by the EDM+ (when in flow-monitoring mode). In addition the EDM+ also calculates SV from the arterial pressure wave using the Liljestrand and Zander algorithm (when calibrated and in pressure-monitoring mode).

The EDM+ first calibrates the arterial pressure wave signal against the Doppler Flow Based calculation of Stroke Volume. During calibration the mean Stroke Volume of a minimum of 10 Doppler flow waveforms is established, alongside simultaneous measurements of mean systolic and diastolic pressures. The EDM+ then applies the Liljestrand and Zander formula (with the constant (k) generated during the calibration period), to calculate beat-by-beat Stroke Volume (SV) from the arterial pressure waveform via the following equation:

SV = k (Ps – Pd) (Ps + Pd)

Stroke Volume Index (SVI)

Stroke Volume Index is the amount of blood in millilitres pumped from the human heart every heart beat indexed for body surface area. Stroke Volume Index can be calculated as a Doppler Flow Based Parameter by EDM and EDM+ and after calibration of the PPWA algorithm EDM+ can calculate this parameter as a Pressure Based Parameter Stroke Volume Index relates the Stroke Volume to body surface area (BSA),  relating heart performance to the size of the individual. The unit of measurement is millilitres per square metre (ml/m2).

Stroke Volume Index = Stroke Volume/Body Surface Area

The typical value for Stroke Volume Index is 50 ml/ m2 with arrange of between 35 ml/m2 to 65ml/ m2).

Heart Rate (HR)

Heart Rate as displayed on the EDM and EDM+ from Doppler based measures and the EDM+ can also calculate Heart Rate from the ABP waveform. From the Doppler the heart beats per minute is calculated from the analysed waveform. The number of beats used to make the calculation can be set by the user from ’every beat’ to a maximum of 20. If set to ‘every beat’ the time in milliseconds of the complete cardiac cycle is measured and then this is divided into 60,000 (milliseconds in a minute) to give the number of beats per minute. If the monitor is set to 20 cycles for calculation then the time in milliseconds for 20 full cycles would be used as the basis of the parameter calculations.

The monitor updates the heart rate display after each calculation period depending on the number of cycles set. The EDM+ can also calculate the Heart Rate from the ABP waveform. The system uses the same method as described for the Doppler as it measures the length of the cardiac cycle in milliseconds and divides this into one minute to give the number of beats per minute.

Cardiac Output (CO)

Cardiac Output is the volume of blood being pumped by the heart, in particular by the left ventricle in the time interval of one minute. The units of Cardiac Output are litres per minute (L/min).

The EDM and EDM+ calculate Cardiac Output in Doppler flow mode. Additionally, after the calibration of the Stroke Volume, the EDM+ can simultaneously calculate the Cardiac Output from the arterial pressure waveform.

The EDM+ averages the Cardiac Output based on the setting of ‘cycles for calculation’, ranging from beat-by-beat estimation of cardiac output, to Cardiac Output based on Stroke Volume averaged over 20 cycles.

Cardiac Output = Stroke Volume x Heart Rate

Cardiac Index (CI)

The EDM and EDM+ can calculate Cardiac Index in Doppler flow mode and EDM+ can additionally calculate this from the Pulse Pressure Waveform Analysis algorithm.

Cardiac Index relates the Cardiac Output to body surface area (BSA), thus relating heart performance to the size of the individual. The unit of measurement is litres per minute per square metre (l/min/m2).

Cardiac Index = Cardiac Output/Body Surface Area

Typical values for Cardiac Index are between 3.5-4.5 L/min/m2

“Low cardiac output syndrome” is typically associated with CI values < 2.5 L/min/m2.

Stroke Volume Variation (SVV)

The EDM and EDM+ can calculate Stroke Volume Variation in Doppler flow mode and EDM+ can additionally calculate this from the Pulse Pressure Waveform Analysis algorithm.

Stroke Volume Variation is widely considered as a useful indicator of fluid responsiveness. The mechanism of generation of this parameter relates to the observation of variations in left ventricular ejection volumes (Stroke Volume).  It has been shown that return blood flow through the thorax is affected by the positive pressure of the ventilator.  As the ventilator cycles it creates varying periods of higher and lower flow. These fluctuations traverse the lung and are manifest as variations in stroke volume of the heart.

These variations can be detected as variations in flow and pressure. The EDM uses these variations in flow to calculate the percentage variation between the maximum stroke volume and the minimum.

The limitations of this parameter is that the patient must meet the following criteria: Fully mechanically ventilated, sinus rhythm, tidal volume ≥ 7-8 mL/kg and higher tidal volumes elicit higher variations. Increasing PEEP will result in higher variations. HR: Respiratory rate ratio ≥4. Changes in lung or chest compliance, or patient position and right ventricular dysfunction or  abdominal insufflation may affect readings.

Caution is advised and clinicians need to be aware of the particular ‘cut off’ or ‘grey zone’ threshold values for the technology being used and the limitations described in the literature.

Systemic Vascular Resistance (SVR)

Systemic Vascular Resistance can be calculated as a Doppler Flow Based Parameter by EDM and EDM+ and after calibration of the PPWA algorithm EDM+ can calculate this parameter as a Pressure Based Parameter

Systemic Vascular Resistance is the resistance to blood flow due to the peripheral vascular system. The formula used in the EDM is as follows:

SVR = 80 (MAP-CVP)
CO

Where MAP is the Mean Arterial Pressure, CVP is the Central Venous Pressure and CO the Cardiac Output.

The Cardiac Output is automatically provided from the flow readings calculated from Stroke Volume and Heart Rate. The user is required to input the MAP and CVP from other sources. The monitor then provides the measurement continuously as the CO is derived.

Systemic Vascular Resistance Index (SVRI)

Systemic Vascular Resistance Index can be calculated as a Doppler Flow Based Parameter by EDM and EDM+ and after calibration of the PPWA algorithm EDM+ can calculate this parameter as a Pressure Based Parameter

Systemic Vascular Resistance Index is the resistance to blood flow due to the peripheral vascular system indexed for patient body size. The formula uses the Body Surface Area as calculated from the input weight and height is as follows:

SVRI = SVR x BSA*

*Where BSA is the Body Surface Area.

The SVRI is automatically updated as SVR changes with CO calculated from the flow readings calculated from Stroke Volume and Heart Rate.

Delivered Oxygen (DO2)

Delivered Oxygen can be calculated as a Doppler Flow Based Parameter by EDM and EDM+ and after calibration of the PPWA algorithm EDM+ can calculate this parameter as a Pressure Based Parameter

Delivered Oxygen is the amount of oxygen in the blood delivered to the body’s tissues. The EDM and EDM+ can calculate this parameter but require the user to input measurements of haemoglobin concentration and the saturated oxygen concentration. The Cardiac Output as calculated by the monitor is automatically updated as DO2 changes with CO calculated from the flow readings, the formula used is as follows:

DO2 = 1.34 x Hb x SaO2 x CO

Where Hb is the concentration of haemoglobin, SaO2 is the saturation of hemoglobin and the amount of dissolved oxygen, all multiplied by the Cardiac Output (CO).

Delivered Oxygen Index (DO2I)

Delivered Oxygen Index can be calculated as a Doppler Flow Based Parameter by EDM and EDM+ and after calibration of the PPWA algorithm EDM+ can calculate this parameter as a Pressure Based Parameter

Delivered Oxygen Index is the amount of oxygen in the blood delivered to the bodies tissues indexed for patient body size. The EDM and EDM+ can calculate this parameter but require the user to input measurements of hemoglobin concentration (Hb) and the saturated oxygen concentration (SaO2). The Body Surface Area (BSA) is calculated by the monitor from the input patient weight and height and this used to automatically updated as DO2 changes with CO calculated from the flow readings, the formula used is as follows:

DO2I = DO2
BSA

Pulse Pressure Variation (PPV)

Pulse Pressure Variation is available only on the EDM+ as a pressure based parameter only. PPV has been reported to be a useful predictor of fluid responsiveness.

Intermittent Fluid Loading

As with Stroke Volume Variation the mechanism of generation of this parameter relates to the observation of variations in pulse pressure due to variations in ventricular ejection volumes. It has been shown that return blood flow through the thorax is affected by the positive pressure of the ventilator.

These fluctuations traverse the lung and are manifest as variations in pulse pressure and stroke volume of the heart.

The EDM+ uses the measured variations in pulse pressure to calculate the percentage variation between the maximum and minimum pulse pressures. The effect of the thoracic pressure changes due to ventilation (PAW) creates a respiratory swing in the magnitude of the pulse pressure wave (PA). The resulting PPMax and PPMin measurements are the basis of calculating the PPV.

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Publications have shown that the sampling plan used for collecting the data and the formula have a significant bearing on the sensitivity of the PPV methodology.

The EDM+ uses a state of the art method where PPMax values are calculated separately for each of three successive breaths. The resulting %PPV has improved accuracy as a result. The formula used is as follows:

PPV = (2(PPmax1 – PPmin1)) + 2(PPmax2 – PPmin2) + 2(PPmax3 – PPmin3) ÷ 3 x 100%
(PPmax1 + PPmin1) (PPmax2 + PPmin2) (PPmax3 + PPmin3)

The limitations of this parameter is that the patient must meet the following criteria: Fully mechanically ventilated, sinus rhythm, tidal volume ≥ 7-8 mL/kg and higher tidal volumes elicit higher variations. Increasing PEEP will result in higher variations. HR: Respiratory rate ratio ≥4. Changes in lung or chest compliance, or patient position and right ventricular dysfunction or  abdominal insufflation may affect readings.

Caution is advised and clinicians need to be aware of the particular ‘cut off’ or ‘grey zone’ threshold values for the technology being used and the limitations described in the literature.

Mean Arterial Pressure (MAP)

The Mean Arterial Pressure (MAP) is the average blood pressure of an individual and is measured in mmHg. It is the average arterial blood pressure during a single cardiac cycle. In ‘Pressure Monitoring Mode’ the EDM+ calculates the MAP from the systolic and diastolic pressures of each heart beat.

MAP is considered to be the perfusion pressure required to perfuse the cells with oxygen.

The following formula can be used:

MAP = (2 X diastolic pressure) + systolic pressure
3

Curve of the arterial pressure during one cardiac cycle (sourced from Wikipedia).

Blood Pressure (BP)

Blood pressure (BP) is the pressure exerted by the circulating blood upon the walls of the blood vessels and is created from the pumping action of the heart. Blood pressure decreases as the circulating blood moves away from the heart through the vascular system.

BP is measured in mmHg and consists of systolic pressure (SP) and diastolic pressure (DP) displayed as SP/DP. The systolic pressure is the pressure created when the heart contracts and diastolic pressure is the resting pressure when the heart relaxes.

Various factors influence BP and will include blood volume, resistance and viscosity.

Increased circulating blood volume allows more blood to return to the heart ready to be pumped to the organs and cells, which therefore influences cardiac output and the pressure required to achieve this.

Resistance is related to vessel radius, vessel length and it’s smoothness and also to blood viscosity. The larger the radius, the lower the resistance and the longer the vessel area, the higher the resistance. Vasoconstrictors can reduce the radius of a vessel thereby increasing BP, while vasodilators can increase the radius causing the BP to fall.

Viscosity is the thickness of the fluid and refers to the red cell concentration. If viscosity increases, resistance will increase.

Cardiac Power Output (CPO)

Cardiac Power Output requires flow and pressure to be measured simultaneously and describes the pumping ability of the heart. This be easily achieved with the EDM+ since flow is measured by the esophageal Doppler probe at the same time as arterial pressure from the arterial waveform. The formula for Cardiac Power Output is as follows:

CPO = MAP x CO
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CPO has been found to be the strongest independent hemodynamic correlate of in-hospital mortality in patients with cardiogenic shock and chronic heart failure, following the review of the SHOCK trial results (2000). A cut off value of 0.53 watts had a predictive value for in hospital mortality. Patients with a value below 0.53 watts had a 71% probability of in hospital mortality, whereas those with a value above 0.53 watts had a 58% probability of mortality before discharge. Increasing age and female gender are independently associated with a lower CPO.

Cardiac Power Index (CPI)

Cardiac Power Index as with CPO require flow and pressure to be measured simultaneously. This be easily achieved with the EDM+ since flow is measured by the esophageal Doppler probe at the same time as arterial pressure from the arterial waveform.

CPI = MAP x CI
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CPI is similar to CPO where cardiac output has been substituted for cardiac index. Women had a lower CPI than men and there was an inverse correlation between CPI and age.