Cardiac Output Monitoring Flashcards
What is fluid responsiveness?
Fluid responsiveness is a term applied to a situation in which a patient’s cardiovascular
system (CVS) is challenged with a bolus of IV fluid and the response
assessed. Patients fit into one of two groups:
1 Non-responder
– No response to a fluid challenge
– This may mean that the patient is intravascularly replete and that another
cause of shock should be sought (e.g. obstructive, cardiogenic) or they are
so hypovolaemic that not enough fluid has been given to result in improvement
(e.g. active haemorrhage)
2 Responder
– Patient responds to a fluid challenge with improvement in clinical and/or
haemodynamic parameters (response may be sustained or short-lived)
– This may imply that the patient is volume deplete and requires further fluid
What methods can we use to determine whether someone
might be fluid responsive?
There are several indications that a patient may benefit from a fluid challenge:
1 Clinical signs
– Tachypnoea
– Tachycardia +/– hypotension
– Peripheral-core temperature difference
– Evidence of end organ dysfunction (oliguria, confusion/low GCS, high
lactate)
– Observing a swing in the arterial line or SpO2 trace
2 Administering a fluid challenge (i.e. a straight-leg raise or administering
250 ml of fluid over 5 minutes) and reviewing the following parameters:
– HR
– BP
– CVP
3 Cardiac output monitoring
4 Echocardiography
– LV kissing walls
– RV volume status
– IVC collapsibility/distensibility index (in spontaneously ventilating and
mechanically ventilated patients respectively)
Classify types of cardiac output monitoring and give
examples of each
Non-invasive
- Transthoracic impedence - Bioimpedence (changes in electrical
current produced by fluid shifts in
thorax)
- Transthoracic echo- LV and RV volume status, IVC
collapsibility index
Minimally
invasive
-PiCCO - Pulse contour analysis, calibrated using
transpulmonary thermodilution,
needs CVC
-LiDCO - Pulse power analysis, calibrated with transpulmonary lithium dilution, CVC
or venflon
-FloTrac - Pulse contour analysis, non-calibrated
-Oesophageal Doppler - Doppler principle, aortic cross-sectional
area probe
-TOE - LV and RV volume status
Invasive
-Pulmonary artery catheter Area under thermodilution curve (CO calculated using Stewart-Hamilton
equation)
Describe the pulmonary artery catheter
The pulmonary artery catheter (PAC, also known as the Swan-Ganz catheter) is
usually 8 French in calibre and 110 cm in length. It has the following components:
1 Distal lumen that is used to measure the PCWP and for sampling mixed venous
blood
2 Proximal lumen that opens about 30 cm from the tip that is used to monitor
CVP and may be used to administer cold injectate to permit measurement of
CO by thermodilution
3 There may be another proximal lumen 26 cm proximal to the tip that can be
used for fluids or drugs
4 Thermistor located 3.7 cm proximal to the tip of the PAC, which is transduced
by the cardiac output monitor
5 10 cm long thermal filament, enabling calculation of CO using thermodilution
without the need for a cold saline bolus
6 A further lumen with a built-in clamp is connected to a balloon that is located
at the catheter tip: this is used to inject up to 1.5 ml of air into the balloon to
facilitate floatation.
Describe the normal pressures and waveforms that you
encounter as the PAC is advanced
The balloon is inflated once the PAC is within the RA, allowing the catheter to
float through the heart into the PA. Right atrial pressure (RAP) is similar to CVP
(usually 3–8 mmHg in a non-ventilated patient). Once the PAC advances into
the RV the pressure trace develops a systolic component (~25 mmHg) and a low diastolic component (0–10 mmHg). As the catheter floats into the pulmonary
artery (PA), the diastolic pressure increases (10–20 mmHg, due to pulmonary
vascular resistance) and a dicrotic notch can be seen (due to closure of the
pulmonary valve). The balloon then carries the catheter into a branch of the
PA into which it wedges. This trace is similar to the CVP waveform (PCWP,
4–12 mmHg) and reflects left atrial pressure (LAP).
It is important that the PAC tip sits in West zone 3, so that there is a continuous
column of blood between the PAC and the LA.
If the PAC is inserted via the right internal jugular vein (IJV), the RA is located at
approximately 15–20 cm, the RV is at 25–30 cm and the PAis a further 10 cm distally.
It is important to deflate the balloon once the PCWP has been taken, otherwise
the lung supplied by the artery in which the PAC is wedged will become ischaemic
and risk infarction
What information is measured and what is derived?
Measured
CO
CVP
RAP
RVP
PAP
PCWP
SvO2
Core temp
Derived
Cardiac index (CO/BSA, l/min/m2)
SV (CO/HR, ml/beat)
SVI (SV/BSA, ml/beat/m2)
SVR (dynes.sec/m5)
SVRI (dynes.sec/m5)
PVR (dynes.sec/m5)
PVRI (dynes.sec/m5)
Tell me about SvO2. How does it compare to central venous
saturation (ScvO2)?
Mixed venous oxygen saturation (SvO2) is the oxygen saturation of venous
blood in the pulmonary arterial tree, after mixing with anatomical and physiological
shunt. The SvO2 exceeds ScvO2 in healthy patients because it samples
blood from the superior vena cava (SVC) and the brain has a high oxygen
extraction ratio (OER).
In several situations, the ScvO2 may exceed the SvO2:
1 Anaesthesia (the cerebral blood flow (CBF) increases but cerebral metabolic
rate decreases so there is reduced OER)
2 Traumatic brain injury (TBI)
3 Shock (blood is diverted from the splanchnic circulation, there is increased
oxygen extraction and therefore a lower oxygen saturation (SO2) in
the IVC)
What are the complications associated with use of pulmonary artery catheters?
Complications associated with obtaining central venous access:
1 Bleeding / haematoma
2 Air embolism
3 Vascular injury
4 Arterial puncture
5 Pneumothorax
6 Tamponade
Complications associated with floating the catheter:
1 Arrhythmias
2 Tamponade
3 Valvular trauma
4 Misplacement (incorrect West zone, unable to pass through the heart)
5 Knotting of catheter
Complications associated with the PAC being in situ:
1 Venous thromboembolism
2 Pulmonary infarction
3 Pulmonary arterial rupture
Do you know of any evidence for or against the use of PAC?
The PAC-man trial (2005) was a pragmatic RCT that showed no difference in inhospital,
ICU or 28-day mortality, or ICU or hospital LOS. There was a 10%
complication rate associated with PAC use. The authors concluded that whilst
there was no clear evidence of benefit or of harm, the PAC is potentially useful in
undifferentiated shock, right ventricular failure (RVF) and pulmonary hypertension
(although the study was underpowered for this).
Tell me about the oesophageal doppler as a CO monitor
The Doppler effect states that when a sound wave is reflected off a moving
object, the frequency shift is proportional to the velocity of the object. This
principle is utilised in the oesophageal Doppler probe, which emits ultrasound
waves that are reflected off red blood cells (RBCs) travelling in the descending
aorta, producing a velocity-time curve of blood flow. The stroke distance (the
distance travelled by the blood in one heartbeat) is then calculated. The aortic
cross-sectional area (CSA) is determined from a nomogram based on the patient’s height and weight, and multiplied by the stroke distance to give stroke volume.
The probe is 90 cm long, with markers at 35, 40 and 45 cm to aid placement.
The descending aortic Doppler trace is normally obtained between 35–40 cm
when the probe is placed orally. The probe must be directed posteriorly, pointing
towards the descending aorta. Audible and visual signals are used to aid focussing, but the oesophageal Doppler monitor (ODM) is significantly operator
dependent.
What assumptions are made when using the Oesophageal Doppler monitor
1 That the angle of the probe to the direction of blood flow is constant
2 That the aortic CSA is constant throughout the cardiac cycle
3 That there is laminar flow within the aorta
4 That 70% of the cardiac output enters the descending aorta
Draw a typical trace and tell me what each part means
- Stroke distance (SD) is the distance in cm that a column of blood moves along the aorta with each contraction of the LV. It is the area under the velocity-time curve. This is converted into stroke volume by multiplying by aortic CSA.
- Mean acceleration (MA) and peak velocity (PV) may be used as markers of LV contractility. The peak velocity decreases with age (90–120 cm/s for a 20-yearold;
70–100 cm/s for a 50-year-old; and 50–80 cm/s for a 70-year-old). - Flow time corrected (FTc) is the time in milliseconds that the heart spends in
systole, corrected for heart rate. Typical values in a healthy adult are 330–360 ms.
A low value indicates high afterload (often due to hypovolaemia) and a high
value indicates low afterload, e.g. vasoplegia in sepsis.
Are there any guidelines for the use of ODM? What are they based on?
The concept of goal-directed therapy came from the introduction of protocolised
care for patients with sepsis by Rivers in his trial in 2001, which produced
compelling results. However, the recent ARISE, ProCESS and ProMISe trials all
concluded that protocolised care is no better than usual care. These trials are
discussed in the ‘Sepsis’ topic
There are several studies that have demonstrated reduced complication
rates and shorter LOS in hospital when the ODM is used perioperatively to
guide fluid therapy. NICE advises that the oesophageal Doppler should be considered for use in patients undergoing high risk or major surgery
(MTG3).
