Patient Monitoring

Term used for the automatic visual display of measurements such as blood pressure, respiration, pulse and temperature. (Churchill Livingstone, 1989)

On the whole it is the ITU nurse who monitors the patients 24/7

They have to observe, communicate analyse and interpret data in order to care effectively for the patient (Field 1997) and it has never been easier with all of the equipment now available to us.

The introduction of the Swan Ganz  catheter in the 70’s has helped improve our knowledge of cardiovascular function (Boldt et al, 1995) we can now monitor cardiac output continuously using heated electrodes, so results in therapy and can be observed in real time and adjustments made.

Can the use of Continuous Cardiac Output (CCO) be justified from a clinical point of view? A multi centre trial failed to show increased survival in disease matched ITU patients receiving a Pulmonary Artery (PA) Catheter versus those who did not (Gresham Bayne, 1997) So What are the alternatives?

Pulmonary Artery Catheters are still the “Gold Standard” in the assessment of cardiac output (Hoeft, 1995). However the interpretation of Pulmonary Artery Occlusion Pressure (PAOP) or Wedge pressure can be difficult due to many artifacts in measurement. (Paul et al, 1987)

Factors like Intrathoracic pressure, myocardial contractility may affect the results of PAOP estimation.

Lichtwarck-Aschoff et al (1992) were able to show that a much better indicator of circulating blood volume and cardiac pre load is to  measure the intrathoracic blood volume (ITBV) or the intrathoracic blood volume index (ITBVI) and extra vascular lung water (EVLW) content of the lungs.

These measurements can be obtained through arterial pulse contour cardiac output and trans pulmonary thermodilution (Sakka et al, 1999)

This method uses a standard central line with a temperature sensor on the distal lumen and a thermo dilution sensor arterial catheter, placed in either the femoral or brachial artery. This is a much less invasive method and reduces the risk associated with the insertion of a PA catheter. e.g., ventricular dysrhthmias, endocardidtis, valvular damage,  cardiac rupture, cardiac tamponade, pulmonary artery rupture, thrombosis, embolism, hemorrhage and infarction. (Thelan et al 1998).

Although the thermodilution curve is longer and flatter than in traditional PA thermodilution it is not influenced by respiratory cycle. The algorithm used computes left ventricular stroke volume by measuring the area under the systolic part of the waveform from the end of diastole to the end of the ejection phase and dividing the area by the aortic impedance (Godje at al 1999). This calculation provides a measure of stroke volume.

Changes in ITBV have been shown to correlate well with changes in cardiac output and may be a more appropriate monitoring parameter for cardiac preload (Sakka et al 1999)

Another interesting aspect of PiCCO is the ability to measure extra vascular lung water (EVLW) a measurement Paul et al (1997) suggested may be an alternative approach determining pulmonary function and a guide to fluid resuscitation in shock.  Godenheim et al (1985) suggests that the restricting of excessive intravasclular fluid volume expansion in patients with ARDS the patients out come may be improved. Robin et al (1985) note that resolution of pulmonary odema may be more rapid when EVLW is used as a therapeutic guide especially if the cause is due to cardiac failure.

New methods of haemodynamic monitoring that are less invasive can only be a positive step forward. Utilizing all available information will help us deliver the best possible care for our patients.

WHAT IS PiCCO?

PiCCO technology is based on a haemodynamic monitoring method, which is a combination of transpulmonary thermodilution and arterial pulse contour analysis. By “transpulmonary”, this means the injection of cold fluid which traverses the lungs after being injected through a CVP line, and that the thermodilution curve is being measured in a systemic artery.  Generally, we use the CVP lumen of a quad line and ideally a femoral artery and gaining readings of cardiac output, cardiac index, stroke volume, cardiac function index, extravascular lung water and intrathoracic blood volume, totally eliminates the need for a right heart (pulmonary artery) catheter.

• PERFORMING A PiCCO THERMODILUTION MEASUREMENT

The PiCCO  Plus monitor requires calibration and re-zeroing every 12 hours, so doing this at the beginning of each shift will familiarise you with the monitor, and the patient’s cardiovascular and fluid status.

A calibration simply means performing a set of thermodilution measurements by injecting cold saline past the inline sensor on the CVP line.

You may perform as many intermittent measurements as necessary to check the patient’s cardiovascular and volume status, and also after changes to, or initiation of, new treatments.

PLEASE REFER TO STAGE 5 ON THE QUICK SET-UP GUIDE ON THE FOLLOWING PAGE to perform a measurement.

• SETTING UP OF PiCCO AND INPUT SCREEN

• RE-ZEROING THE PiCCO

The PiCCO transducer needs to be re-zeroed before attaching to the patient and approximately every 12 hours thereafter.

Open transducer to air. Press enter key, then AP key then ‘O’key. When the numbers go to zero, close to air and press enter.  AP correction should be left at 0 cm if transducer is at usual mid-heart level.  Press enter again, then select waveform page to monitor blood pressure.

• PATIENT INPUT SCREEN

When PiCCO is first inserted, you need to key in certain patient details:

Height in cm

Weight in kg

Injectate volume 20 ml (should be preset by PiCCO)      

Injectate temperature < 8^oC (should be preset by PiCCO)

CVP  (This is acceptable for changes up to 5, but it is advisable to re-input CVP if it changes, and check when receiving patient or performing measurements)

PiCCO Plus will display “invalid/faulty catheter” under ‘catheter type’, ignore this as PiCCO will automatically read the catheter type and input it when the cable is attached to the patient.

(Our catheter types are usually PV2015L20 for a 20 cm line).

• INTRA-THORACIC BLOOD VOLUME

ITBVi = 850 – 1000 ml/m²

<850 = underfilled   > 1000 = adequate – overfilled

ITBV consists of the GEDV (global end diastolic volume), the volume of blood within the heart plus the pulmonary blood volume.  Three volumes are found within the thorax, the intrathoracic blood volume, the intrathoracic gas volume and the extravascular lung water.  Due to limited expansion of the thorax, the three volumes interact and change proportionally to each other.  ITBV is a volumetric measurement of cardiac preload (ventricular end-diastolic pressure), and in mechanically ventilated patients is a sensitive indicator of the circulatory blood volume.

• EXTRAVASCULAR LUNG WATER

EVLWi = 3.0 – 7.0 ml/kg

>7.0 ml/kg indicates pulmonary oedema

The water content in the lungs increases in left heart failure, pneumonia, sepsis, burns etc. Measurement of EVLW relates specifically to pulmonary oedema eg . in ARDS,   Prognosis based on EVLW has indicated a higher risk of mortality with EVLW of greater than 14 ml/kg.

• CARDIAC FUNCTION INDEX

CFI = 4.5 – 6.5 l/min

< 4.5 indicates poor myocardial contractility

CFI reflects myocardial contractile function.  It is measured independently of cardiac preload and reflects the inotropic state of the heart.  Positive inotropic stimulation increases the curve, therefore the reading.

• STROKE VOLUME VARIATION

< 10%  = well-filled/overloaded

10 – 15%  =  normal

> 15%  = dehydrated/underfilled

Stroke volume is the amount of blood ejected from the ventricle with each contraction.

The SVV is measured as the mean difference between the highest and lowest stroke volume over the last 30 seconds.  Stroke volume will change on inspiration (increases with venous return) and on expiration (against venous return).  Positive pressure ventilation impedes venous return, therefore there may be a larger variation in stroke volume in mechanically ventilated patients who are dehydrated, as the heart cannot compensate for the differences in stroke volume on inspiration and expiration if the blood volume is not available ie. the patient is dry, and the blood is not available for refill into the ventricles on preload (ventricular end diastole).

This is only accurately measured in ventilated patients.

If the patient is tachyarrhythmic eg. atrial fibrillation, SVV will be inaccurate as ventricular refill will be irregular, therefore stroke volume is not measured properly.

• CONTINUOUSLY MONITORED AND DISPLAYED 

Arterial Blood Pressure

Heart Rate

Blood/Body Temperature - BT

Continuous Pulse Contour Cardiac Output/Index – CO/CI

Stroke Volume - SV

Stroke Volume Variation - SVV

Systemic Vascular Resistance - SVR

• OBTAINED BY THERMODILUTION MEASUREMENT 

Transpulmonary Cardiac Output  - CO

Intrathoracic Blood Volume - ITBV

Extravascular Lung Water - EVLW

Cardiac Function Index - CFI

(Global End Diastolic Volume *) - GEDV

(Global Ejection Fraction *) - GEF