Cardiac monitoring devices are invaluable in providing vital information, but their overuse can interfere with the patient’s haemodynamics. Although non-invasive monitors get round this problem, readings are not always reliable
Jan Poelaert MD, PhD
Department of anesthesiology and
Email: [email protected]
Monitoring of the critically-ill patient comprises a whole set of observations by medical care providers to analyse, follow up and control the consequences of disease and therapy. Obtained or stored electronic data recordings are compared with the
patient’s physiological state.
The simplest way to monitor haemodynamics is to estimate blood pressure in a noninvasive way. Since the introduction of the pulmonary artery catheter nearly 40 years ago, tremendous progress has been made in haemodynamic monitoring, with respect to both monitoring tools and the approach.
The pulmonary artery catheter provides important information on different issues such as cardiac output, right- and left-sided filling pressures, pulmonary artery pressures and oxygen delivery. Although the risks and shortcomings of this catheter have been stated unequivocally, the pulmonary artery catheter still has evident indications. Nevertheless, the whole discussion made medical care providers aware of potential non-invasive monitoring facilities or tools that offer similar information. Alongside the technological advances, this
awareness boosted the introduction of various adequate and less satisfactory haemodynamic monitoring devices. Direct and indirect information, on haemodynamics in general and on
perfusion and filling in particular, became available at the bedside.
Monitoring of cardiovascular function
The heart is the circulatory pump, pushing the oxygenated blood from the lungs towards the tissues and forcing venous blood back to the lungs. Non-invasive monitoring of cardiac function may be performed by different means, as listed in Table 1 (overleaf).
Presence of pulse gives the most basic information on oxygen saturation. This technique also allows monitoring of perfusion.
Hyoperfusion will dampen the signal, although little differentiation can be made between global hypoperfusion and local – such as vasoconstriction. Systolic respiratory variation is another important signal. Oxygen saturation with forehead
oximetry permits more rapid and stable SaO2 assessment in cases of shock. Nowadays, several systems to estimate blood and perfusion pressure are marketed: from classic automatic
oscillometric devices to more sophisticated wrist or finger monitors. Care should be taken to assess blood pressure at the level of the right atrium. When measurements are taken at other locations, refer to the cardiac position. In acute care and operating theatres, blood pressure monitoring is not meant to elucidate chronic arterial hypertension, but rather provides a follow-up of the evolution of blood pressure throughout a critical period in the emergency ward, during an operative procedure or intensive care unit (ICU) admission. In the late 1980s and through the 1990s, the Finapres method demonstrated that monitoring of a finger can provide blood pressure and heart rate. The Finapres method uses technology in which the volume of the digit is assessed by infra-red plethysmography. Despite altering blood pressure, it is kept constant by changing cuff pressure and is finally recorded as an arterial pressure trace. With more recent devices, perfusion pressure, stroke volume, cardiac output and data on ventilation-induced stroke volume variation are also available.
Oesophageal Doppler uses the technique of pulsed wave Doppler, in which one crystal is sent and the signal of a reflected wave is awaited. This technology allows assessment of stroke volume – and hence cardiac output – and all dynamic approaches of assessment of fluid responsiveness (see below). The shortcomings of this technique are based upon the limited possibilities of achieving the correct Doppler signal – including a low intercept angle between the Doppler beam and the aortic blood flow. As with each Doppler signal, the areas under the curve demonstrate the stroke volume; a decreased area depicts a reduced stroke volume either as a consequence of increased opposed forces (afterload) or hypovolaemia. The upstroke of the Doppler signal suggests the ejection force of the left ventricle, as a sign of contractility.
Echocardiographic evaluation of cardiac function encompasses different aspects. Although the transthoracic approach is sometimes difficult to obtain because of interfering factors – such as air, high-pressure ventilation mode, PEEP, chronic obstructive pulmonary disease (COPD) with hyperinflation – it is the most complete non-invasive monitoring tool that can be used at the bedside. Familiarity with the technology, and a background in physiology and medical knowledge are essential to get the most from this technology. Table 1 lists essential information that can be gained with this technique with respect to cardiovascular function.
Monitoring of preloading conditions and fluid responsiveness
Fluid loading is the most frequent therapeutic handling performed in anaesthetised or critically ill patients. Accurate blood volume determination, however, is cumbersome. First, it has to be determined whether a patient is indeed hypovolaemic with concurrent signs – such as low perfusion pressure, low diuresis, bleeding or known losses of third-space fluid and increasing lactate. In many situations, classical static variables
of preload are insufficient, urging the use of more dynamic descriptors of fluid status (Table 2). Optimal filling status could be determined by testing with a small loading trial. However, as
there is a risk of overfilling and subsequent pulmonary oedema, it is advisable to use the ‘passive leg raising’ or Trendlenburg position to test hypovolaemia  reversibly and evaluate the effect on perfusion pressure.
The passive leg raising test is preferable to Trendlenburg, as the first not only transfers blood acutely towards the thorax but allows squeezing of the intra-abdominal vessels, boosting the
increased preload. Several devices on the market rely on an invasive arterial pressure trace. However, the variation of stroke volume assessed by oesophageal Doppler or across the aortic valve with echo-Doppler techniques in mechanically ventilated patients is a reliable variable of fluid responsiveness.[6,7] When a stroke volume variation of >10-12% is present, fluid filling could be beneficial. Short-term increase of preload by the passive leg raising test would suffice to determine this need and circumvents the need of a device to assess fluid responsiveness.
Nevertheless, it would be advisable to use a non-invasive manner to demonstrate, clearly and safely, the presence of fluid responsiveness. Finapres monitoring of both blood pressure
and arterial oxygen saturation also provides information on the beat-to-beat variation of stroke volume in ventilated patients. Indeed, mechanical ventilation induces cyclic variations
of intra-thoracic pressures and therefore of venous return. In analogy with ventilationinduced systolic variation of arterial pressure, this technique offers the same information on a
non-invasive basis with respect to assessment of fluid responsiveness.
Monitoring of the microcirculation
For a number of years, devices have been available to assess the microcirculation directly. Both the orthogonal polarisation spectral (OPS) device and the Sidestream Dark Field (SDF) imaging device provide high-quality imaging of the microvasculature. They are both based on the principle that green light is absorbed by red blood cell haemoglobin in the superficial vessels – capillaries and small veins. Minor alterations of cardiac output and lactate provoke major changes of the microcirculation.
Improved technology resulting in improved image quality, in conjunction with a physiological and quantitative analysis, should lead to a comprehensive estimation of microcirculatory changes.
Pros and cons
Critical appraisal of intensive care medicine raised questions on benefits and risks as far back as the early 1980s.9 These ideas were enforced by the rather semantic discussion around the risk and benefits of the pulmonary artery catheter in the 1990s. Meanwhile, growing consensus and awareness lessens the push for high-level monitoring devices because of risks of over-interpretation and, thus, potentially harmful therapeutic
actions in fragile high-risk patients. One of the strengths of non-invasive haemodynamic monitoring is in relieving all potential risks of invasive and insidious monitoring
devices. However, there are equally grave risks arising from overestimating the value and power of the non-invasive tools and of deriving too much information out of them. Problems can arise from unsatisfactory equipment and from inadequate
and insufficient knowledge about its use. The only way to tackle this is by training to improve knowledge of the technology. This should ensure proper choice of monitoring tool on admission
to the ICU and ensure that as disease evolves it remains tailored on the needs of the patient.
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