9: Intensive care

TOPIC 9 Intensive care



Topic Contents


Cardiac output monitoring 165
Test: Pulmonary artery catheter 165

Test: Oesophageal Doppler monitor (ODM) 167

Test: Lithium dilution cardiac output (LiDCO plus) 168

Test: Pulse-induced contour cardiac output (PiCCO) 170

Test: Noninvasive cardiac output (NICO) monitor 170

Test: Impedance cardiography 171

Test: Echocardiography 171

Perioperative optimization 171
Test: Central venous pressure (CVP) 172

Test: Invasive arterial pressure monitoring 173

Monitors of organ perfusion 174
Indication 174

Test: Serum lactate 174

Test: Base deficit 175

Test: Anion gap 175

Test: Mixed venous oxygen saturation (SvO2) 176

Test: Gastric tonometry and the CO2 gap 177

Other investigations performed in intensive care 177
Test: Intra-abdominal pressure (IAP) 177

Test: Ultrasound in intensive care 178


Cardiac output monitoring



Indications



1. To aid management of patients with haemodynamic instability. This may be in the setting of isolated cardiac dysfunction, e.g. myocardial infarction or acute left ventricular failure; more often in critically ill patients there is combined pathology – hypovolaemia and distributive shock as well as associated cardiac dysfunction. Rationalizing appropriate vasoconstrictor, inodilator and fluid therapy is enhanced with knowledge of cardiovascular parameters.

2. To aid management of surgical patients at high risk due to intrinsic cardiac disease or other comorbidities. Cardiac output (CO) monitoring instituted in the perioperative period with goal-directed therapy has a growing body of clinical evidence of outcome benefit.


Method


New technology has resulted in a number of techniques, which range vary in invasiveness (Table 9.1).


Table 9.1 Invasiveness of monitoring methods











Invasive Moderately invasive Noninvasive
Pulmonary artery catheter
ODM

LIDCO

PICCO

NICO

TOE
Impedance cardiography (ICG)

Transthoracic echocardiography


Test: Pulmonary artery catheter



• Considered by many to be the gold standard of cardiac output monitoring.

• Position less assured with advent of newer techniques and uncertainty of outcome benefit in addition to suggestion of possible harm.

• Balloon-tipped catheter is floated through right heart and ‘wedged’ in medium-sized pulmonary artery.

• Thermodilution principle is used to measure CO using preterminal thermistor.

• Two methods:
– Cold injectate – 10 mL normal saline or 5% dextrose solution (cold or room temperature) injected quickly through proximal port produces a temperature change in blood detected by thermistor. Area under curve of resultant temperature/time curve (Fig. 9.1) is inversely proportional to CO. Usually averaged over three measurements at end-expiration.

– Continuous cardiac output – uses same principle, but uses pulses of heat from a filament to produce the temperature/time curve.

image

Fig. 9.1 Cold injectate temperature change. Area under curve inversely proportional to Cardiac Output.


Provides information regarding pulmonary as well as systemic vasculature.


• Measured variables (normal values in brackets):
– Right atrial pressure (5–8 mmHg)

– Pulmonary artery pressure (systolic 15–25 mmHg, diastolic 8–15 mmHg)

– Pulmonary artery occlusion pressure (PAOP) (6–15 mmHg)

– Cardiac output – derived via Stewart Hamilton method (4–7 L/minute)

– Mixed venous oxygen saturation (SvO2) drawn from distal port (70–75%) – see below.

• Calculated variables:
– Stroke volume (60–100 mL/beat, varies with age)

– Systemic vascular resistance (SVR) (1000–1500 dyne s/cm5)

– Pulmonary vascular resistance (PVR) (<250 dyne s/cm5).


Interpretation



• PAOP low in hypovolaemia, raised due to left ventricular failure or mitral valve disease.

• Low SVR represents vasodilated state, high suggests vasoconstriction.

• CO low may reflect hypovolaemia or impaired cardiac function, CO is raised in hyperdynamic states (e.g. sepsis, thyrotoxicosis, inotrope therapy).


Limitations



– Invasive

– Requires technical skills

– Arrhythmias result in unreliable data

– Risk of:
• catheter-related infection (limit placement to maximum 72 hours)

• Pulmonary infarction if continuously wedged

• Potential damage to cardiac and pulmonary structures (NB always deflate balloon prior to withdrawing catheter).


Test: Oesophageal Doppler monitor (ODM)



• Flexible ultrasound probe placed in distal oesophagus under anaesthesia/sedation via mouth or nose and focused to produce a Doppler waveform trace corresponding to flow of blood in descending aorta (Fig. 9.2).

• Softer flexible nasal probes available for awake patients.

• Doppler ultrasound is used to quantify velocity of blood in descending aorta during systole.

• Combining this with estimated aortic cross-sectional area gives the stroke volume.

• Stroke volume × heart rate = cardiac output.

• Flow time (FT) represents systolic time and is corrected (FTc) to a heart rate of 60 bpm with a further correction factor for the unmeasured upper extremity blood flow (i.e. carotid/subclavian flow).

• Measured variables: Mean acceleration.

• Calculated variables: Stroke volume, peak velocity (PV), cardiac output, flow time, systemic vascular resistance.

image

Fig. 9.2 OCM waveform.



Interpretation


Based on pattern recognition using all the indices. Using one single parameter should not be relied upon for decision making (normal values in brackets):


• FTc (330–360 ms) – reduced in hypovolaemia and vasoconstriction. High values may represent cardiac failure or vasodilatation in a volume replete patient.

• PV (varies with age, lower limit of normal approximately 120 − age) – marker of contractility.

• MA (10–12 m/s2) – affected primarily by contractility.

• SV – low values occur in hypovolaemia, cardiac failure, increased afterload.


Contraindications and limitations



• Pharyngo-oesophageal pathology.

• Presence of intra-aortic balloon pump (IABP) makes data unreliable.

• No pulmonary data obtained.

• Difficult to interpret with arrhythmias.

• User variability in focusing probe.


Suprasternal Doppler



• Similar principle to ODM, however ultrasound signal probe placed in suprasternal notch to measure flow in aortic arch.

• Direction of probe manipulated to achieve best audible signal and trace.

• Not widely used partly due to concerns of reproducibility of measured results.


Test: Lithium dilution cardiac output (LiDCO plus)



• Lithium chloride injected as a bolus via central (ideally) or peripheral line.

• Blood is drawn from an arterial line, past a lithium sensor producing a lithium concentration/time curve.

• Mathematical application to the area under this curve provides an estimate of cardiac output.

• Pulse power cardiac output software (Pulse CO) uses this information to calibrate the arterial waveform such that each pulse is a quantified estimate of the patient’s stroke volume (Fig. 9.3).

• Hence provides ‘beat to beat’ (time averaged) cardiac output monitoring.

• Also has the facility to quantify the pulse pressure variation (PPV), systolic pressure variation (SPV) and stroke volume variation (SVV) – each providing preload status information.

• Calculates oxygen delivery (DO2).

image

Fig. 9.3 Pulse power cardiac output software (Pulse CO) display.




Limitations



• Contraindicated in lithium therapy.

• Stroke volume variation interpretation difficult with arrhythmias.

• Pregnancy (first trimester).

• Regular recalibration recommended.

• Muscle paralysis affects calibration.

• Requires arterial +/− central access.

• Aortic valve regurgitation will cause inaccuracies.

• Excessive positive end-expiratory pressure (PEEP) will overestimate SVV.

• Intra-aortic balloon pump affects data.


Interpretation



• CO/SV/SVR as above.

• SVV, PPV, SPV values above 10% suggest hypovolaemia, predicting fluid responsiveness in the fully mechanically ventilated patient.

• DO2 (850–1200 mL/minute): low values suggest hypovolaemia, low cardiac output, hypoxaemia or anaemia. Raised with hyperdynamic state.


Test: Pulse-induced contour cardiac output (PiCCO)



• Some similarities to cold injectate thermodilution pulmonary artery catheter (PAC) and LIDCO method.

• Bolus injectate given through central venous catheter.

• Thermistor at tip of centrally placed arterial catheter – femoral (usual), brachial or axillary.

• Transpulmonary thermodilution curve produced and modified Stewart-Hamilton method applied to calculate cardiac output.

• Pulse contour analysis (Table 9.2) applies a calibrated algorithm to calculate the stroke volume from each pulse wave.

• Further mathematical algorithms are applied to obtain other volumetric parameters (listed).

Table 9.2 Variables in monitoring contour cardiac output











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May 31, 2016 | Posted by in ANESTHESIA | Comments Off on 9: Intensive care

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Thermodilution variables Pulse contour variables
Cardiac output Continuous cardiac output
Global end-diastolic volume index (GEDI)