25.1.1 Noninvasive Blood Pressure Measurement

In intensive care, blood pressure is generally monitored automatically and noninvasively using the oscillometric technique. In seriously ill patients and small children, conventional measurement techniques using the Riva–Rocci method or auscultation of the Korotkoff sounds often lead to false low values.

The devices available for oscillometric measurement automatically measure the blood pressure at freely selected intervals and, in addition to systolic and diastolic pressure, also indicate the mean pressure and pulse rate.

To take the measurement, the blood pressure cuff is inflated to a suprasystolic level and then vented gradually. As long as the cuff pressure is above the systolic blood pressure, no blood flows in a distal direction under the cuff. As soon as there is blood flow under the cuff, pressure oscillations are conducted and registered. The cuff pressure is then equivalent to the systolic blood pressure. When the oscillations reach their maximum, the cuff pressure has reached the mean pressure. However, the diastolic pressure is difficult to determine using the oscillatory method, so it is calculated from the systolic pressure and the mean pressure in most devices.

Oscillometric blood pressure monitoring is sufficient for hemodynamically relatively stable patients and correlates well with arterial blood pressure measurements.

25.1.2 Arterial (Direct) Pressure Measurement

In invasive measurement of arterial blood pressure, a cannula is inserted into an artery and the blood pressure in the artery is determined using a pressure transducer. Invasive arterial blood pressure monitoring is indicated when the following measures are necessary:

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  Continuous blood pressure monitoring (e.g., cardiac decompensation, shock, management of catecholamine treatment)

  
  
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  Frequent checks of blood gases (e.g., in a difficult ventilation situation)

Another advantage of direct blood pressure measurement is that the pressure curve provides information on the volume status. The mean pressure is also indicated in addition to the systolic and diastolic pressure.

The following vessels are theoretically suitable for arterial blood pressure measurement:

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  Radial artery (site of first choice)

  
  
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  Femoral artery

  
  
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  Brachial artery

  
  
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  Axillary artery

  
  
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  Dorsalis pedis artery

  
  
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  Umbilical artery in neonates

Allen test

Before cannulation of the radial artery, the Allen test is recommended. To check for adequate collateral circulation, first of all the radial and the ulnar arteries are both compressed manually until the hand becomes pale. Then the compression of the ulnar artery is released while the radial artery remains compressed. If there is sufficient collateral circulation, the hand will become pink again within a few seconds, although the radial artery is still compressed. The radial artery may be cannulated only if the Allen test is positive.

Arterial puncture

The arterial puncture is made under sterile conditions. It is easiest using the Seldinger technique. To prevent thrombi, the arterial access is continuously flushed, sometimes with a heparin solution.

Measurement

Before starting the measurement, a zero adjustment and calibration are needed. The pressure transducer must be at heart (atrial) level. This is for patients who are lying down, around the middle of the thorax. The tubing should be as short as possible and rigid.

A typical arterial pressure curve is shown in Fig. 25.1. The incisure is termed a dicrotic notch. The dicrotic wave arises at the transition from the systole to the diastole as a result of the Windkessel function after the valve closes. The area under the curve is proportional to the stroke volume (Fig. 25.1 a).

25.1.3 Central Venous Pressure

The central venous pressure (CVP) is the blood pressure in the vena cava at the transition to the right atrium. It is used to assess intravascular volume and right ventricular function.

The CVP is measured using a central venous catheter (CVC), through which it is also possible to apply medication, hyperosmolar infusions, and rapid volume substitution. The CVC should be located with the tip at the connection of the venae cavae to the right atrium. The CVC can be advanced via either the inferior or the superior vena cava. Typical access sites are:

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  Superior vena cava: internal jugular vein, subclavian vein, external jugular vein (as flow-directed catheter), basilic vein (as flow-directed catheter)

  
  
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  Inferior vena cava: femoral vein, umbilical vein (neonates)

The CVC is transferred via a tubing system to a pressure transducer that allows electronic measurement. Alternatively, a riser tube can also be used to measure CVP. Before measuring, a zero adjustment must be made. The pressure transducer or the zero point of the riser tube must be at the level of the heart (atrium).

Contraindications

Contraindications for the insertion of a CVC are coagulation disorders and inflammatory skin changes at the puncture site.

Complications

Typical complications from a CVC are infections, thromboses along the course of the catheter, and puncture-related problems such as compression of the artery, air embolism, vascular or nerve damage, pneumothorax, hematothorax, pericardial tamponade, AV fistulas, injury to the cervical sympathetic trunk in jugular vein punctures, or arrhythmia when the catheter is advanced into the right atrium or ventricle.

CVP Curve

The CVP curve has pressure fluctuations that are synchronous with heart action. There are three maximum pressures and two minimum pressures (Fig. 25.3):

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  A wave: contraction of the right atrium (ventricular diastole)

  
  
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  C wave: protrusion of the tricuspid valve into the right atrium during contraction of the right ventricle

  
  
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  X wave (atrial diastole): relaxation of the atrium and downward movement of the tricuspid valve

  
  
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  V wave: filling of the right atrium while the tricuspid valve is still closed (ventricular systole)

  
  
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  Y wave: opening of the tricuspid valve

The A wave and V wave are the most important in clinical routine.

Lack of coordination between the A wave and the V wave occurs in rhythm disorders in which there is no orderly sequence of atrial and ventricular contractions (so-called lack of atrioventricular synchonicity, e.g., 3rd degree AV block, junctional ectopic tachycardia).

Excessively high A waves suggest increased resistance during atrial emptying (tricuspid stenosis, pulmonary hypertension, reduced right ventricular compliance). A high V wave occurs in relevant tricuspid valve insufficiency.

The normal CVP value is around 5 mmHg (1–10 mmHg). The CVP is considered to be a parameter for right heart preload. The changes over time are more important than the absolute value. It should also be noted that if compliance of the right ventricle is reduced, there is no longer a linear relationship between the intravascular volume and the CVP, so volume therapy can no longer be reliably managed based on the CVP (e.g., after correction of a tetralogy of Fallot).

The CVP is elevated in the following situations:

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  Hypervolemia

  
  
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  Right heart failure, global heart failure

  
  
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  Reduced right ventricular compliance

  
  
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  Pulmonary hypertension

  
  
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  Low cardiac output

  
  
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  Pericardial effusion/tamponade

  
  
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  Tension pneumothorax

  
  
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  High PEEP (> 15 mmHg)

Causes of low CVP:

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  Hypovolemia

  
  
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  Shock

  
  
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  High cardiac output

25.1.4 Pulmonary Artery Catheter

The pressure in the pulmonary artery, CVP, pulmonary capillary wedge pressure, and mixed venous oxygen saturation can be measured directly using a pulmonary artery catheter. In addition, the cardiac output can be calculated using the thermodilution method.

The pulmonary artery catheter (also called the Swan–Ganz catheter after its developers) consists of a distal and a proximal port and a balloon tip and a thermistor (Fig. 25.4).

Like a CVC, the pulmonary artery catheter is inserted using the Seldinger technique into a large vein (right internal jugular vein, subclavian vein) and advanced so the tip with the distal port is located in the vicinity of the main pulmonary artery. Then the balloon tip is inflated so the balloon is carried by the bloodstream. During this flow-directed placement of the catheter, a continuous pressure curve is recorded that has a characteristic profile depending on the location of the catheter (Fig. 25.5).

The correct location of the proximal port is in the right atrium, so the CVP is measured via this port. There is also a temperature sensor (thermistor) at the tip of the catheter that is connected by a wire to the monitor. The thermistor is used to measure cardiac output using the thermodilution method (see Cardiac Output below).

Most pulmonary artery catheters also have another port that ends in the region of the proximal segment and can be used for applying medication.

A pulmonary artery catheter can be indicated in the following situations:

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  Severe left heart failure

  
  
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  Cardiomyopathy

  
  
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  Pulmonary hypertension

  
  
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  Shock

  
  
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  Sepsis

  
  
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  Acute lung failure

  
  
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  Acute pulmonary embolism

Generally speaking, the indication for a pulmonary artery catheter has become increasingly more restrictive as other methods are available, especially for measuring cardiac output (e.g., PiCCO system).

Contraindications:

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  Tricuspid or pulmonary stenosis

  
  
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  Tumor or thrombus in the right atrium or ventricle

  
  
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  Severe coagulation disorders

  
  
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  Severe arrhythmias

  
  
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  Newly placed pacemaker electrodes (risk of dislocation)

Cardiac output

Cardiac output is determined using the thermodilution method: an ice-cold solution of NaCl 0.9% is injected as quickly as possible into the right atrium and carried by the bloodstream to the pulmonary artery, where the change in temperature (result of dilution and warming by the blood) is measured by the thermistor. The cardiac output is then calculated using the area under the temperature–time curve (Stewart–Hamilton equation).

Central venous pressure

The proximal port of the pulmonary artery catheter is located in the right atrium, so the CVP can be measured from this port.

Pulmonary capillary wedge pressure

The pulmonary capillary wedge pressure (PCWP) is determined by inflating the balloon at the tip of the catheter and letting it be carried by the bloodstream until it is wedged into a branch of the pulmonary artery, occluding it completely. According to the principle of the static water column, the pressure in the occluded pulmonary artery capillary is equivalent to the pressure in the left atrium and thus also to the left ventricular end-diastolic pressure (LVEDP). The condition for these measurements is, of course, that there are no stenoses between the wedged pulmonary artery capillary, the left atrium, and the left ventricle. The left atrial pressure and the LVEDP are measures for left ventricular preload. The normal wedge pressure is 9 ± 4 mmHg.

After measuring, the balloon is deflated and the catheter tip is pulled back into the main pulmonary artery so that there is no risk of occluding the pulmonary artery capillary bed, which may lead to a pulmonary embolism.

Pulmonary artery pressure

The pulmonary artery pressure is measured at the distal tip of the catheter. The normal pressure is less than 22 mmHg. The diastolic pulmonary artery pressure is normally approximately equivalent to the wedge pressure. The pulmonary artery pressure is elevated in patients with pulmonary hypertension or a pulmonary embolism, for example.

Calculated values

The values described above can be used to calculate the following parameters:

Normal value: 2.5 to 4 l/min/m² body surface area (BSA).

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