Urine output

When any patient becomes oliguric, perfusion should be assessed. Oliguria can be a sign that the patient is deteriorating.

As vital signs such as blood pressure and pulse can be unhelpful, multiple clinical variables must be assessed in order to gain an overall picture when deciding on volume status. No one variable should be taken in isolation. If there is any uncertainty about volume status, a fluid challenge is the safest way to assess this further. The fluid challenge is a fundamental manoeuvre - the aim is to produce a small but rapid increase in plasma volume and then to assess the response.

The physiological relationship between cardiac filling and cardiac output is described in Starling's curve (see Figure 6.1, p 106). This curve describes the functional consequences of alterations in preload on cardiac output and is the rationale behind improving cardiac output with volume administration. Up to a certain point, volume expansion leads to an increased cardiac output owing to increased end-diastolic ventricular filling. Beyond this, increased end-diastolic volume will lead to a reduced cardiac output. Measurement of cardiac responses in response to a fluid challenge can therefore be used to define optimal filling pressures (Figure 5.5).

A suitable fluid challenge is 250 ml colloid over 10 minutes. This may need to be repeated.

Colloids are effective volume expanders for resuscitation, especially in situations where changes in capillary permeability have occurred. Different colloids have different properties. Crystalloids are isotonic and rapidly distribute throughout the extracellular space causing oedema. Crystalloids also have to be given in large volumes to achieve significant volume expansion: 1000 ml of 0 9% sodium chloride would increase the plasma volume by 200 ml and be short-lived.

It is also important to understand that simply speeding up maintenance fluid is not an effective way to treat volume depletion, as shown in Figure 5.6. The increased maintenance fluid could take hours to make a patient euvolaemic. During this time hypoperfusion could cause irreversible renal damage and metabolic acidosis - problems that become more difficult to correct with time.

A fluid challenge should be given through a large bore cannula (14-16G) and wide diameter giving set to give an adequate flow rate (Table 5.4).

A triple lumen central venous catheter has different sized ports - the largest is designed for fluid and the others for measurement and infusions. The relatively small diameter and long length of a central line compared with a 14G cannula makes it less suitable for resuscitation when fluid needs to be given quickly.

Assessment of the fluid challenge can be either pressure-based (for example, blood pressure and CVP) or flow-based (for example, cardiac output measurements). When pressure based, a common approach is to continue giving boluses of fluid until a sustained rise in central venous pressure (CVP) occurs, rather

< 3 underfilled

Blood volume

Filled

Filled

Blood volume

Figure 5.5 The response of CVP/PAOP and stroke volume to small volume fluid challenges. In a hypovolaemic patient, an increase in stroke volume with no significant rise in CVP/PAOP would be expected. In the optimally filled patient, a rise in CVP/PAOP with no significant rise in stroke volume would be expected. CVP, central venous pressure; PAOP, pulmonary artery occlusion pressure

Fluid challenge(s) followed by maintenance

\

^^---—-—- Euvolaemia

/ /

s tu tat

m lu ol

>

^ ^ Maintenance fluid speeded up y

/

Time (h)

Figure 5.6 The effect of fluid challenge(s) versus maintenance fluids

Figure 5.6 The effect of fluid challenge(s) versus maintenance fluids

Table 5.4 Flow rates and diameters of cannulae

Size of cannula Colour code Flow rate (ml min 1)

Table 5.4 Flow rates and diameters of cannulae

Size of cannula Colour code Flow rate (ml min 1)

24G

Yellow

18

22G

Blue

36

20G

Pink

61

18G

Green

90

16G

Grey

200

14G

Brown

300

than be limited by a predetermined value. A normal heart rate, blood pressure, and CVP do not exclude volume depletion and a single CVP reading is unreliable in assessing volume status. The "rule of threes" is a more useful tool. If the CVP increases by 3 mmHg and then falls back to the original value after 5 minutes, a further fluid challenge should be administered. If the CVP increases by 3 mmHg and remains elevated, no further fluid is indicated. If the CVP increases by more than 3 mmHg and is above 10 mmHg, the patient could be overfilled. However there is no "normal" CVP and initial measurements greater than 10 mmHg are common and can be due to reasons other than volume status (see Chapter 6).

Some patients continue to show signs of inadequate perfusion (cold extremities, increased respiratory rate, metabolic acidosis, and oliguria) despite an apparently optimal intravascular volume. Early referral to the ICU is required as more advanced monitoring and treatment with vasoactive drugs may be necessary. Pulmonary artery catheters, oesophageal Doppler, and other flow-based monitors permit measurement of cardiac output and other derived measurements, such as systemic vascular resistance, so that predefined oxygen delivery goals may be achieved.

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