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PiCCO - All you need to know !

The heart's ability to function as a pump can be evaluated through the use of a Swan Ganz catheter. During end diastole, under most conditions, ventricular pre load is indirectly reflected by their respective atria. Left ventricular pre load can be evaluated by observing the PADP (pulmonary artery diastolic pressure), or more accurately the PAOP, as while the catheter is wedged, left heart pressures are reflected.

As left ventricular function deteriorates, end diastolic pressure (pre load) increases. This increase is reflected back to the atria where for the left heart the PAOP will also be recorded higher. Cardiac output will decline as a result, and clinically the patient will exhibit signs of poor organ perfusion.

It was thought that by obtaining the CVP the left heart function could be assessed. At that time the only readily available monitoring means was the CVP catheter. Since the utilization of the PA catheter this concept has been dispelled.

By using the RA port on the PA catheter the RV preload can be assessed. Increased RV preload as a result of severe lung disease or RV dysfunction will be reflected in an elevated RA pressure. There are certain pulmonary and right-sided heart diseases where only the right-sided pressures will be abnormal. In these conditions a CVP catheter is inadequate for assessing left ventricular function.

Both the right and left ventricular function can be assessed by using a PA catheter. The information obtained from the PAOP reflects the left heart function while the RA lumen reflects right heart function. Under conditions of severe left heart failure and resultant right heart failure both values will be elevated.

In the 1950’s Fegler first described measuring cardiac output by the Thermodilution method. It was not until the early that Drs. Swan and Ganz demonstrated reliability and reproducibility of this method with a special temperature sensing pulmonary artery catheter. Since that time, the Thermodilution method of obtaining cardiac output has become a standard for clinical practice.

The Thermodilution method applies indicator dilution principles, using temperature change as the indicator. A known amount of solution at a known temperature is injected rapidly into the right atrial lumen of the catheter. This cooler solution mixes with and cools the surrounding blood, and the temperature is measured downstream in the pulmonary artery by a thermistor embedded in the catheter. The resultant change in temperature is then plotted on a time-temperature curve.

A normal curve characteristically shows a sharp upstroke from rapid injection of the injectate. This is followed by a smooth curve and slightly prolonged downslope back to the baseline. Since this curve is representing a change from warmer temperature to cooler and then back to warmer temperature, the actual curve is in a negative direction. For continuity for most graphs the curve is produced in an upright fashion. The area under the curve is inversely proportional to the cardiac output.

When cardiac output is low, more time is required for the temperature to return to baseline, producing a larger area under the curve. With high cardiac out put the cooler injectate is carried faster through the heart, and the temperature returns to baseline faster. This produces a smaller area under the curve.

A modified Stuart- Hamilton equation is used to calculate cardiac output taking into consideration the change in temperature as the indicator, modification include the measured temperature of the injectate and the patients blood temperature along with the specific gravity of the solution injected.

CO = V x (TB – TI) x (SI x CI) x 60 x CT x K
         A            (SB X CB)           1

Where:

CO = Cardiac Output

V = Volume of injectate in mls.

A = area of Thermodilution curve in square mm divided by paper speed (mm/sec)

K = calibration constant in mm/°C

TB. TI = temperature of blood and injectate

SB, SI = specific gravity of blood and injectate

CB, CI = specific heat of blood and injectate

(SI x CI) = 1.08 when 5% dextrose is used
(SB x CB)

60 = 60 sec/min

CT = correction factor for injectate warming

Don’t worry you do not need to know or even understand this equation as the computer in the monitor calculates it for you (thank God for technology!)

The thermistor port of the catheter is attached to the monitor. Calculations are performed internally with the results displayed on the screen. You can see from the equation why 5% dextrose must be used, however it is possible to use another solution as long as the computer has been told which fluid you are using in order to adjust the value for the specific gravity. For ease and to reduce errors custom and practice dictates that 5% dextrose is used.

Most monitors display the actual cardiac output time - temperature curve. By observing the Thermodilution curve assessment of injection technique and artificial influences can be noted.

The temperature of the injectate is room temp. The computer is registering a change (signal) in temperature from the patient’s base line (noise). In some conditions, a variation in temperature can occur with respirations, this decreases the "signal-to-noise" ratio and produce an abnormally low cardiac output. Other conditions where an increased signal to noise ratio may be beneficial is in the febrile patient, low cardiac output states, and patients with wide respiratory variations.

We use either the closed system or the continuous cardiac output monitor Conditions where Thermodilution may produce unreliable results are those that have a backward flow of blood on the right side; tricuspid or pulmonic valve regurgitation, and ventricular or atrial septal defects.

The Swan-Ganz Thermodilution catheter is a powerful tool for the clinician in assessment and management of the critically ill patient. Use of the catheter by itself is not an intervention. Instead it is a diagnostic adjunct that if utilized and the data interpreted properly leads to appropriate therapeutic interventions.

The determinants of cardiac performance are heart rate (HR), preload, afterload and contractility, but by indirect calculations other performance factors can be assessed.

· Heart rate One of the more easily obtainable values for assessing haemodynamic status is the heart rate. As a component of cardiac output, the heart rate plays an integral part as to the diastolic filling time and therefore end diastolic volume. It can be palpated or obtained via an ECG.

· Systolic and Diastolic Blood Pressures. Blood pressure is the measured tension within the blood vessels during ventricular systole and resting diastole. This measurement can be obtained indirectly with a sphygmomanometer or more accurately with an arterial catheter.

· Pulmonary Artery Pressure. With the use of the Swan-Ganz catheter P.A. systolic and diastolic can be obtained, as can wedge pressure values.

· Right Atrial Pressure. Right ventricular filling pressures can be obtained using the R.A. port can provide information about right ventricular function.

· Waveform Analysis. Measurement of the "a" and "v" wave of the RA and PAW tracings can provide valuable information as to filling pressures and disease states.

· Cardiac Output. Through the use of Thermodilution an accurate determination of cardiac performance can be made.

From the direct measurements obtained, derived parameters can be calculated to further assess cardiac performance and normalize values obtained for body size (indexing)

· Mean Arterial Pressure. This is the average pressure throughout the vascular system during systole and diastole. The maintenance of a minimal pressure is necessary for coronary artery and tissue perfusion. This value can be measured using the following formula:

 

MAP = SBP + (DBP X 2)
3

Normal MAP = 70 –105 mmHg

· Cardiac Indexing. The normal range for cardiac output is wide 4-8 litres per min. Since the value assesses the function of the ventricle, normalizing the value to body size can offer more precise information. To index a haemodynamic value, the patient’s body surface area (BSA) is obtained from a nonogram using the patient’s height and weight. Any value that is to be indexed can then be divided by the BSA.
 

CI = CO
        BSA

Normal cardiac index = 2.5 to 4 l/min/m²
 

· Stroke Volume. This is the amount of blood pumped out of the ventricle with one contraction. Since stoke volume is part of the cardiac output equation the value can be derived mathematically,

SV =CO X 1000 ml/L
 HR

Normal Stroke volume is 60 to 100 ml/beat
 

· Stroke volume index. As with cardiac output Stroke Volume can also be indexed by dividing CI by HR.

Normal SVI = 33 to 47 ml/beat/m²

 

By calculating the SV or SVI, some indication of the state of contractility can be evaluated.

· Vascular Resistance. Another variable of ventricular function is vascular resistance. Resistance is the relationship of pressure to flow. As blood flows through the vascular system there is resistance. This value is the clinical representation of afterload; the amount of resistance the ventricle must overcome to eject blood volume.

· Systemic Vascular Resistance. (SVR) measures the afterload or resistance for the left ventricle. Since resistance relates to pressure to flow, pressure is arrived at by measuring the gradient between the beginning of the circuit (MAP) and the end (CVP). This value is then divided by the flow or cardiac output. A rounded conversion factor of 80 is used to adjust the value into units of force; Dyne-sec-cm5

 

SVR = (MAP – CVP) X 80
CO

Normal SVR = 800 to 1200 dynes/sec/cm5
 

· Pulmonary Vascular Resistance. The Right ventricular afterload is clinically measured by calculating pulmonary vascular resistance (PVR). Again the gradient of the circuit is measured MPAP minus the end PAOP then divided by the flow or cardiac output. The conversion factor of 80 is again used to convert to a unit of force.
 

PVR = (MPAP – PAOP) X 80
C0

Normal PVR = < 250 dynes/sec/cm5

· Stroke Work. Another way to evaluate ventricular function is by measuring the external work for the ventricle in one contraction. This value can be calculated from obtaining the average pressure generated by a ventricle during one heart beat and multiply that by the amount of blood ejected in one beat and multiplying it by a conversion factor to convert it into a measure of "work", 0.0136 is utilized. We also index this value for BSA and sometimes evaluate both ventricles’ stroke work index.

SW = (MAP – LVEDP) X SV X 0.0136

LVSWI = SVI (MAP – PAOP) X 0.0136

Normal Values 45 – 75 mg-m/m²/Beat
 

Obtaining haemodynamic parameters can assist the clinician with not only assessing the status of the ventricular function, but also provide important information that assists in the differentiating disease when the clinical presentation makes diagnosis unclear.

The nurse should be aware of normal A & P of the heart and lungs and the flow of blood through these organs.

The nurse shall demonstrate knowledge of PAOP, PA & CVP interpretations on ventilated and spontaneously breathing patients.

The nurse should understand the significance of the haemodynamic profile and be able to titrate inotropes according to the results as directed by the doctors.

The nurse should be familiar with the set up of the monitor and be able to input relevant data i.e. height and weight of the patient

 

· Check the PA trace to make sure the Swan is in correct position.

· Check that nothing else is running on the CVP lumen

· Preferably no fast IV’s should be running as this can affect the baseline temp

· Need exactly 10 ml of 5% Dextrose (the calculation takes into account the specific gravity of this solution)

· Inject as rapidly as possible, timed with end expiration

· Check no leaks while injecting

· Watch the shape of the curve

· Repeat injections (usually 3 or 4) when the monitor is "ready"

· Awareness of errors – Hypothermic patient, incorrect volume injected (leaks), rapid infusions of other fluids, shunts or valvular heart disease, very low cardiac outputs.

 

 

------------------------------------------- has demonstrated proficiency in this skill.

Signed ----------------------------------------------------------------------------------------

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To ensure CO studies are performed skillfully and appropriate interpretations are made resulting in optimization of CVS parameters in accordance with medical guidelines.

The directorate manager/ clinical director & senior nurse manager have jointly agreed that registered nurses who have the appropriate skills, knowledge and experience (> 1 year ITU experience) in the care of patients with pulmonary artery catheters and are IV drug administration competent may perform this task following appropriate training and assessment.

The role of the nurse is to augment the proactive care of the patient, if the nursing dependency/priority of care deem so

The nurse makes regular assessment and recording of CVS parameters in patient with pulmonary artery catheters insitu - vasoactive drug titration is performed by competent nurses under medical guidelines – regular CO measurement will enable monitoring of therapeutic intervention more closely.

Nurses perform this task in many ITU’s many nurses feel it is a fundamental requirement of a skilled ITU nurse.

Performance of these parameters will be shared with medical staff

The role will only be undertaken by nurses who have been assessed as competent and if the patient dependency allows – otherwise medical assistance will be sought.

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