Left ventricular preload and function during graded haemorrhage and retranfusion in pigs: analysis of arterial pressure waveform and correlation with echocardiography

S. Preisman*,1, E. DiSegni2, Z. Vered2 and A. Perel1

1Department of Anesthesiology and Intensive Care and the 2Institute of Cardiology, Sheba Medical Center, Sackler School of Medicine, Tel-Aviv University, Tel Hashomer, Israel*Corresponding author

Accepted for publication: November 22, 2001


    Abstract
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Background. Arterial pressure waveform analysis is a new method for assessment of cardiac preload. Despite the close correlation of parameters derived by its use with the degree of blood loss, their relationship with more precise estimates of cardiac preload remains controversial.

Methods. Systolic pressure variation (SPV), delta up (dUp), and delta down (dDown), which are the changes in the arterial blood pressure (BP) during mechanical ventilation, were measured during graded haemorrhage and retransfusion in seven pigs under light halothane anaesthesia, and compared with changes in cardiac filling pressures and left ventricular end-diastolic volume (LVEDV), measured by echocardiography.

Results. Significant changes in preload parameters and stroke volume (SV) but not in BP and heart rate occurred. SPV, dDown, and cardiac filling pressures correlated significantly with LVEDV. Following retransfusion, LVEDV returned to baseline values but the SV and left ventricular ejection fraction were significantly low. This deterioration in myocardial performance was associated with elevated dUp.

Conclusions. During mechanical ventilation, dDown and the SPV may serve as minimally invasive indicators of preload. The retransfusion stage that follows significant blood loss may be associated with deterioration in LV function.

Br J Anaesth 2002; 88: 716–18

Keywords: arterial pressure, measurement; blood, loss; heart, echocardiography; pig


    Introduction
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
One of the suggested methods for assessing cardiac preload in mechanically ventilated patients is arterial pressure waveform analysis.1 The increase in intrathoracic pressure during mechanical ventilation normally causes a biphasic change in left ventricular (LV) output, which is reflected in a similar way in arterial blood pressure (BP).13 An early elevation in BP, termed delta up (dUp), reflects augmentation of LV stroke volume (SV), and is normally followed by a later decrease in BP, which is termed delta down (dDown). The difference between the maximal and minimal values of the systolic BP during one mechanical breath is termed the systolic pressure variation (SPV). The SPV and dDown were found to correlate with the degree of blood removal during experimental haemorrhage2 and during surgery,4 and with echocardiographic estimates of LV filling in postoperative patients.5 Moreover, SPV and dDown were found to predict the haemodynamic response to volume loading better than the pulmonary artery occlusion pressure or LV end-diastolic area in mechanically ventilated septic patients.6 dUp was, in turn, found to become more prominent during hypervolaemia2 and during experimental congestive heart failure.3 Recently, however, Dalibon and colleagues7 questioned the usefulness of dDown in assessing the degree of blood loss during experimental haemorrhage, and claimed that although it correlated significantly with the degree of blood loss, it was not a better measure than mean BP.

The aim of our study was to examine the correla tion between parameters of arterial pressure waveform analysis to echocardiographic estimates of cardiac preload and function during graded haemorrhage and retransfusion.


    Methods and results
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Seven German Landrace pigs (30.4 (6.8) kg) were anaesthetized with sodium pentothal 10 mg kg–1, the trachea intubated and the lungs ventilated with a tidal volume of 12 ml kg–1 (peak inspiratory pressure 15–18 cm H2O), 8–12 min–1 (PE'CO2 35–40 mm Hg), FIO2 0.5. Anaesthesia was maintained with halothane and pancuronium. A thermodilution pulmonary and a femoral arterial catheter were inserted. The animals were bled by increments of 5% of their blood volume (estimated to be 75 ml kg–1), up to a blood loss of 30%, following which the shed blood was retransfused over 4–5 min.

Haemodynamic measurements were made at baseline and 2–3 min after each change in blood volume. SPV was calculated as a mean over five consecutive mechanical breaths. dUp and dDown were measured as the difference between the value of systolic BP during 5 s of apnoea and its maximal and minimal values, respectively, during the respiratory cycle immediately preceding the apnoeic period. SPV, dUp, and dDown were expressed as percentages of systolic BP during apnoea.

Echocardiographic measurements were performed by a cardiologist who was blinded to the haemodynamic data. More details of the echocardiographic methods used in the study may be found elsewhere.8 Changes in the variables during the experiment were evaluated by analysis of variance for repeated measures. Coefficients of linear correlation between the LV end-diastolic volume (LVEDV) and the SPV, dDown, central venous pressure (CVP), and pulmonary artery occlusion pressure (PAOP) for each animal were derived by calculation of the mean r-values by reversed Fisher transformation.

Changes in the haemodynamic variables during the experiment are presented in Table 1. The gradual nature of the haemorrhage caused no significant changes in the arterial BP, and a significant decrease in the mean SV occurred only after 20% haemorrhage. CVP, PAOP, and LVEDV decreased, and %SPV and %dDown increased from the very early stages of haemorrhage. Following retransfusion, LVEDV returned to its baseline value. SV and left ventricular ejection fraction (LVEF) significantly decreased. dUp increased significantly, as did left ventricular end-systolic wall stress (ESWS). Changes of PAOP, CVP, and %SPV and %dDown correlated significantly with LVEDV (r values of 0.76, 0.77, –0.77, and –0.82, respectively).


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Table 1 Cardiovascular parameters following haemorrhage and retransfusion. Values are presented as mean (SD). *P<0.05, **P<0.01, ***P<0.005 compared with baseline values. PAP, systolic pulmonary artery pressure
 

    Comment
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
The early steps of graded haemorrhage create an occult hypovolaemia, which cannot be detected by conventional monitoring. In this set up SPV and dDown have been shown again, as in previous studies,2 4 5 to be sensitive indices of LVEDV, and to change significantly at the very early stage of blood loss. The results are contrary to those of Dalibon and colleagues who found dDown to be of less benefit than arterial BP itself in reflecting the degree of haemorrhage.7 However, in that study haemorrhage caused a severe reduction in arterial BP, and dDown was not expressed as per cent of BP as recommended.1

The retransfusion stage was associated with a global deterioration in LV function, which was reflected by the sharp decrease in LVEF. This phenomenon may have been caused by an elevation of afterload and/or by myocardial depression. Indeed, although no significant changes of systemic vascular resistance (SVR) occurred, ESWS, a more precise characteristic of myocardial afterload,9 was significantly increased after retransfusion. The decrease of myocardial performance was accompanied by a prominent increase of dUp, as was shown previously in a model of heart failure.3

The physiologic responses to blood loss and rapid volume restoration in our experiment may have been influenced by the anaesthetic agent and muscular relaxant used, as halothane was shown to reduce SVR and blunt the cardiovascular response to acute haemorrhage,10 whereas pancuronium bromide possesses vagolytic and sympathomimetic properties.11 However, a lack of changes in BP and elevation in heart rate in response to haemorrhage suggest a relatively superficial plane of anaesthesia in our experiment.

In conclusion, during graded haemorrhage and rapid blood volume restoration parameters of arterial pressure waveform analysis correlate with echocardiographic estimates of LV preload. The combination of a small dDown and a prominent dUp reflects the possible myocardial depression that can occur following haemorrhage and retransfusion. These results suggest that the changes in the arterial pressure waveform may be used as readily available monitoring tools in the haemodynamic assessment of mechanically ventilated patients.


    References
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
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8 Di Segni E, Preisman S, Ohad DG, et al. Echocardiographic left ventricular remodelling and pseudohypertrophy as markers of hypovolemia. An experimental study on bleeding and volume repletion. J Am Soc Echocardiogr 1997; 10: 926–36[ISI][Medline]

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