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Hypovolaemia
CORE e TRAUMA
Hypovolaemia
Learning objectives
Aidan Marsh After reading this article, you should be able to: C C
Abstract Fluid therapy has, traditionally, been guided by static markers of cardiac preload such as central venous pressure and pulmonary artery wedge pressure. Fluid responsiveness, or an increase in stroke volume in response to fluid, is poorly predicted by these variables. This has led to increased interest in variables such as the fluctuation in blood pressure and stroke volume in response to mechanical ventilation. These changes are caused by the reduction in venous return associated with positivepressure ventilation, which, in turn, leads to a reduction in left ventricular stroke volume and arterial pressure. These cyclical changes, termed systolic pressure variation, pulse pressure variation and stroke volume variation, predict fluid responsiveness more accurately than static markers in positive-pressure ventilated patients with a stable cardiac rhythm. For patients with spontaneous respiratory efforts or an irregular cardiac rhythm, the response in stroke volume to a passive leg raise seems to show promise in its ability to predict fluid responsiveness.
C
define fluid responsiveness appreciate the limitations of central venous pressure and pulmonary artery wedge pressure when assessing fluid responsiveness understand the principles of dynamic indices of fluid responsiveness, and their limitations.
many years. However, it has become apparent that these variables perform poorly in their ability to predict fluid responsiveness. Neither their initial value nor their response to fluid appears to give an indication that a patient is fluid responsive.1,2 This has led to interest in other techniques, including more dynamic tests such as ventilation-induced pressure and volume variations.
Systolic pressure variation
Keywords central venous pressure; fluid responsiveness; preload;
The haemodynamic changes in response to positive-pressure ventilation have long been recognized. The term ‘systolic pressure variation’ is defined as the maximum and minimum values of systolic pressure following a single positive-pressure breath (see Figure 1). The raised intrathoracic pressure associated with mechanical ventilation leads to a reduction in right ventricular preload due to the reduction in venous return. This, in turn, leads to a decrease in left ventricular preload, and, as the arterial pulse pressure is proportional to left ventricular stroke volume, a decrease in systolic pressure. The magnitude of this reduction will be greater when operating on the steeper part of the FrankeStarling curve, and will be exacerbated by hypovolaemia and diminished by fluid administration (see Figure 2). Thus a patient’s position on the FrankeStarling curve can be identified by noting the change in systolic pressure associated with mechanical ventilation, and this will give a good indication of whether increasing the preload by giving fluid will lead to an increase in stroke volume (position A) or not (position B). Studies on intensive care patients in a regular cardiac rhythm who were positive-pressure ventilated with a tidal volume of at least 8 ml/kg show that, if the systolic pressure variation is 8e10 mmHg, then it is highly likely that the patient will be fluid responsive.1e3 No such threshold can be determined for either CVP or PAWP.
pulmonary artery wedge pressure; pulse pressure variation; stroke volume variation; systolic pressure variation
Fluid responsiveness Optimizing cardiac output, and hence oxygen delivery, is of fundamental importance in critically ill patients with haemodynamic insufficiency. The dangers of insufficient fluid resuscitation have been appreciated for many years. However, there is now increasing recognition of the problems associated with excessive fluid administration and its detrimental effects on organ systems caused by interstitial oedema. A key question is to determine whether the patient is fluid responsive; that is, will fluid administration increase stroke volume and cardiac output? If the patient is fluid responsive and unstable, then fluid will be the most commonly applied therapy. If the patient is not fluid responsive, then other therapies such as vasopressors and inotropes may be applied, thus avoiding the detrimental effects of excess fluid with no therapeutic benefit. Therefore, accurate predictors of fluid responsiveness are extremely important in haemodynamically unstable patients.
Pulse pressure variation (PPV) and stroke volume variation (SVV) Central venous pressure (CVP) and pulmonary artery wedge pressure (PAWP)
PPV is defined as the maximum pulse pressure in a respiratory cycle minus the minimum pulse pressure, divided by the mean of these two values, and is usually expressed as a percentage:
Traditional assessment of cardiac preload using static pressure indicators such as CVP and PAWP has been recommended for
PPVð%Þ ¼ 100 Aidan Marsh MRCP FRCA DICM is a Consultant in Anaesthesia and Intensive Care Medicine at Frenchay Hospital, Bristol, UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:2
ðpulse pressuremax pulse pressuremin Þ ððpulse pressuremax þ pulse pressuremin Þ=2Þ
PPV reflects the variation in left ventricular stroke volume during the respiratory cycle, and it has been suggested that it is
56
Ó 2009 Elsevier Ltd. All rights reserved.
CORE e TRAUMA
Frank Starling curve showing a fluid responsive patient (A) and a fluid unresponsive patient (B)
Arterial trace showing the systolic pressure variation (SPV) A
B
Stroke volume
SPV
A
Figure 1 Reproduced from Intensive Care Medicine 1997; 23: 651e7. Ventricular preload
a superior marker of fluid responsiveness than SPV. This is because systolic pressures may vary, not only owing to changes in transmural pressures due to an increase in stroke volume, but also owing to changes in pleural pressures associated with mechanical ventilation. PPV may be superior because pleural pressures will affect both systolic and diastolic pressures, thus PPV will reflect changes in stroke volume alone. In several studies, it performs better as an indicator of fluid responsiveness than CVP and PAWP, with thresholds ranging from 10 to 13%.1e3 Again, patients need to be in a stable cardiac rhythm and ventilated with tidal volumes of 8 ml/kg. It also seems to be superior to SPV in a few studies.2 New cardiac output monitoring technology, such as pulse contour analysis, has permitted the estimation of the actual variation in stroke volume rather than measuring surrogates such as SPV and PPV. SVV is defined as the percentage change between the maximum and minimum values divided by the mean of these two values: SVVð%Þ ¼ 100
Reproduced from Critical Care 2000; 4: 282–9.
Figure 2
Summary Fluid administration is a key component of resuscitating unstable patients. SPV, PPV and SVV are superior to static markers such as CVP and PAWP at predicting fluid responsiveness in appropriate patients. It should always be noted that identifying a patient as being fluid responsive does not necessarily mean that the patient needs fluid. This is a different question altogether because there will be many patients who are fluid responsive but are clinically stable and do not need resuscitation. Future studies should examine whether dynamic markers could be used to improve fluid administration strategies and lead to improved outcomes for patients. A
ðstroke volumemax stroke volumemin Þ ððstroke volumemax þ stroke volumemin Þ=2Þ REFERENCES 1 Michard F, Teboul J-L. Predicting fluid responsiveness in ICU patients. Chest 2002; 121: 2000e8. 2 Michard F, Boussat S, Chemla D, et al. Relation between respiratory changes in arterial pulse pressure and fluid responsiveness in septic patients with acute circulatory failure. Am J Respir Crit Care Med 2000; 162: 134e8. 3 Michard F. Changes in arterial pressure during mechanical ventilation. Anesthesiology 2005; 103: 419e28. 4 Monnet X, Rienzo M, Osman D, et al. Passive leg raising predicts fluid responsiveness in the critically ill. Crit Care Med 2006; 34: 1402e7.
SVV also reliably predicts fluid responsiveness in ventilated ITU patients in a regular cardiac rhythm, with threshold values ranging from 9 to 12%.3
Spontaneously breathing patients A limitation of SPV, PPV and SVV is that they cannot be used in patients that are making spontaneous efforts. Some studies have shown that measuring the change in stroke volume in response to a passive leg raise (a ‘reversible fluid challenge’) reliably predicts the response to a subsequent fluid challenge.4
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:2
57
Ó 2009 Elsevier Ltd. All rights reserved.
Hypovolaemia
Learning objectives
Aidan Marsh After reading this article, you should be able to: C C
Abstract Fluid therapy has, traditionally, been guided by static markers of cardiac preload such as central venous pressure and pulmonary artery wedge pressure. Fluid responsiveness, or an increase in stroke volume in response to fluid, is poorly predicted by these variables. This has led to increased interest in variables such as the fluctuation in blood pressure and stroke volume in response to mechanical ventilation. These changes are caused by the reduction in venous return associated with positivepressure ventilation, which, in turn, leads to a reduction in left ventricular stroke volume and arterial pressure. These cyclical changes, termed systolic pressure variation, pulse pressure variation and stroke volume variation, predict fluid responsiveness more accurately than static markers in positive-pressure ventilated patients with a stable cardiac rhythm. For patients with spontaneous respiratory efforts or an irregular cardiac rhythm, the response in stroke volume to a passive leg raise seems to show promise in its ability to predict fluid responsiveness.
C
define fluid responsiveness appreciate the limitations of central venous pressure and pulmonary artery wedge pressure when assessing fluid responsiveness understand the principles of dynamic indices of fluid responsiveness, and their limitations.
many years. However, it has become apparent that these variables perform poorly in their ability to predict fluid responsiveness. Neither their initial value nor their response to fluid appears to give an indication that a patient is fluid responsive.1,2 This has led to interest in other techniques, including more dynamic tests such as ventilation-induced pressure and volume variations.
Systolic pressure variation
Keywords central venous pressure; fluid responsiveness; preload;
The haemodynamic changes in response to positive-pressure ventilation have long been recognized. The term ‘systolic pressure variation’ is defined as the maximum and minimum values of systolic pressure following a single positive-pressure breath (see Figure 1). The raised intrathoracic pressure associated with mechanical ventilation leads to a reduction in right ventricular preload due to the reduction in venous return. This, in turn, leads to a decrease in left ventricular preload, and, as the arterial pulse pressure is proportional to left ventricular stroke volume, a decrease in systolic pressure. The magnitude of this reduction will be greater when operating on the steeper part of the FrankeStarling curve, and will be exacerbated by hypovolaemia and diminished by fluid administration (see Figure 2). Thus a patient’s position on the FrankeStarling curve can be identified by noting the change in systolic pressure associated with mechanical ventilation, and this will give a good indication of whether increasing the preload by giving fluid will lead to an increase in stroke volume (position A) or not (position B). Studies on intensive care patients in a regular cardiac rhythm who were positive-pressure ventilated with a tidal volume of at least 8 ml/kg show that, if the systolic pressure variation is 8e10 mmHg, then it is highly likely that the patient will be fluid responsive.1e3 No such threshold can be determined for either CVP or PAWP.
pulmonary artery wedge pressure; pulse pressure variation; stroke volume variation; systolic pressure variation
Fluid responsiveness Optimizing cardiac output, and hence oxygen delivery, is of fundamental importance in critically ill patients with haemodynamic insufficiency. The dangers of insufficient fluid resuscitation have been appreciated for many years. However, there is now increasing recognition of the problems associated with excessive fluid administration and its detrimental effects on organ systems caused by interstitial oedema. A key question is to determine whether the patient is fluid responsive; that is, will fluid administration increase stroke volume and cardiac output? If the patient is fluid responsive and unstable, then fluid will be the most commonly applied therapy. If the patient is not fluid responsive, then other therapies such as vasopressors and inotropes may be applied, thus avoiding the detrimental effects of excess fluid with no therapeutic benefit. Therefore, accurate predictors of fluid responsiveness are extremely important in haemodynamically unstable patients.
Pulse pressure variation (PPV) and stroke volume variation (SVV) Central venous pressure (CVP) and pulmonary artery wedge pressure (PAWP)
PPV is defined as the maximum pulse pressure in a respiratory cycle minus the minimum pulse pressure, divided by the mean of these two values, and is usually expressed as a percentage:
Traditional assessment of cardiac preload using static pressure indicators such as CVP and PAWP has been recommended for
PPVð%Þ ¼ 100 Aidan Marsh MRCP FRCA DICM is a Consultant in Anaesthesia and Intensive Care Medicine at Frenchay Hospital, Bristol, UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:2
ðpulse pressuremax pulse pressuremin Þ ððpulse pressuremax þ pulse pressuremin Þ=2Þ
PPV reflects the variation in left ventricular stroke volume during the respiratory cycle, and it has been suggested that it is
56
Ó 2009 Elsevier Ltd. All rights reserved.
CORE e TRAUMA
Frank Starling curve showing a fluid responsive patient (A) and a fluid unresponsive patient (B)
Arterial trace showing the systolic pressure variation (SPV) A
B
Stroke volume
SPV
A
Figure 1 Reproduced from Intensive Care Medicine 1997; 23: 651e7. Ventricular preload
a superior marker of fluid responsiveness than SPV. This is because systolic pressures may vary, not only owing to changes in transmural pressures due to an increase in stroke volume, but also owing to changes in pleural pressures associated with mechanical ventilation. PPV may be superior because pleural pressures will affect both systolic and diastolic pressures, thus PPV will reflect changes in stroke volume alone. In several studies, it performs better as an indicator of fluid responsiveness than CVP and PAWP, with thresholds ranging from 10 to 13%.1e3 Again, patients need to be in a stable cardiac rhythm and ventilated with tidal volumes of 8 ml/kg. It also seems to be superior to SPV in a few studies.2 New cardiac output monitoring technology, such as pulse contour analysis, has permitted the estimation of the actual variation in stroke volume rather than measuring surrogates such as SPV and PPV. SVV is defined as the percentage change between the maximum and minimum values divided by the mean of these two values: SVVð%Þ ¼ 100
Reproduced from Critical Care 2000; 4: 282–9.
Figure 2
Summary Fluid administration is a key component of resuscitating unstable patients. SPV, PPV and SVV are superior to static markers such as CVP and PAWP at predicting fluid responsiveness in appropriate patients. It should always be noted that identifying a patient as being fluid responsive does not necessarily mean that the patient needs fluid. This is a different question altogether because there will be many patients who are fluid responsive but are clinically stable and do not need resuscitation. Future studies should examine whether dynamic markers could be used to improve fluid administration strategies and lead to improved outcomes for patients. A
ðstroke volumemax stroke volumemin Þ ððstroke volumemax þ stroke volumemin Þ=2Þ REFERENCES 1 Michard F, Teboul J-L. Predicting fluid responsiveness in ICU patients. Chest 2002; 121: 2000e8. 2 Michard F, Boussat S, Chemla D, et al. Relation between respiratory changes in arterial pulse pressure and fluid responsiveness in septic patients with acute circulatory failure. Am J Respir Crit Care Med 2000; 162: 134e8. 3 Michard F. Changes in arterial pressure during mechanical ventilation. Anesthesiology 2005; 103: 419e28. 4 Monnet X, Rienzo M, Osman D, et al. Passive leg raising predicts fluid responsiveness in the critically ill. Crit Care Med 2006; 34: 1402e7.
SVV also reliably predicts fluid responsiveness in ventilated ITU patients in a regular cardiac rhythm, with threshold values ranging from 9 to 12%.3
Spontaneously breathing patients A limitation of SPV, PPV and SVV is that they cannot be used in patients that are making spontaneous efforts. Some studies have shown that measuring the change in stroke volume in response to a passive leg raise (a ‘reversible fluid challenge’) reliably predicts the response to a subsequent fluid challenge.4
ANAESTHESIA AND INTENSIVE CARE MEDICINE 11:2
57
Ó 2009 Elsevier Ltd. All rights reserved.