Beat-to-beat changes in stroke volume precede the general circulatory effects of mechanical ventilation: a case report

Abstract

The haemodynamic as well as the ventilatory consequences of mechanical ventilation can be harmful in critically ill neonates. Newly developed ventilatory lung protective strategies are not always available immediately and in an acute situation the haemodynamic changes caused by mechanical ventilation can affect the oxygen delivery considerably. We report the case of a male neonate who was treated with conventional pressure-controlled mechanical ventilation because of respiratory distress and progressive respiratory acidosis resulting from meconium aspiration. Because of poor arterial oxygenation despite 100% inspired oxygen and increased ventilator settings, echocardiography was performed to exclude central haemodynamic reasons for low oxygen delivery. Doppler echocardiography was used for the measurement of stroke volume and cardiac output. Pulse oximetry and aortic blood pressure were monitored continuously. Echocardiography revealed no cardiac malformations or signs of persistent fetal circulation. When inspiratory pressures and duration were increased, beat-to-beat variation in stroke volume preceded decay in cardiac output. Stroke volume variations and oxygen saturation values guided ventilator settings until extracorporal membrane oxygenation could be arranged for. After recovery and discharge 4 weeks later the boy is progressing normally. Because oxygen delivery is dependent on both blood flow and arterial oxygen content, measurement of cardiac output as well as left heart oxygen saturation is a useful guide to optimizing oxygen delivery. This case report demonstrates how Doppler echocardiographic monitoring of beat-to-beat changes in stroke volume can be used to detect early negative haemodynamic effects of increased mechanical ventilation settings before cardiac output is affected.

Background:

The haemodynamic as well as the ventilatory consequences of mechanical ventilation can be harmful in critically ill neonates. Newly developed ventilatory lung protective strategies are not always available immediately and in an acute situation the haemodynamic changes caused by mechanical ventilation can affect the oxygen delivery considerably. We report the case of a male neonate who was treated with conventional pressure-controlled mechanical ventilation because of respiratory distress and progressive respiratory acidosis resulting from meconium aspiration. Because of poor arterial oxygenation despite 100% inspired oxygen and increased ventilator settings, echocardiography was performed to exclude central haemodynamic reasons for low oxygen delivery.

Method:

Doppler echocardiography was used for the measurement of stroke volume and cardiac output. Pulse oximetry and aortic blood pressure were monitored continuously.

Results:

Echocardiography revealed no cardiac malformations or signs of persistent fetal circulation. When inspiratory pressures and duration were increased, beat-to-beat variation in stroke volume preceded decay in cardiac output. Stroke volume variations and oxygen saturation values guided ventilator settings until extracorporal membrane oxygenation could be arranged for. After recovery and discharge 4 weeks later the boy is progressing normally.

Conclusion:

Because oxygen delivery is dependent on both blood flow and arterial oxygen content, measurement of cardiac output as well as left heart oxygen saturation is a useful guide to optimizing oxygen delivery. This case report demonstrates how Doppler echocardiographic monitoring of beat-to-beat changes in stroke volume can be used to detect early negative haemodynamic effects of increased mechanical ventilation settings before cardiac output is affected.

Introduction

Meconium aspiration syndrome (MAS) is an acute illness affecting full-term babies immediately after birth. Prenatal warning signs, if present, are non-specific and late. Transportation to special neonatal intensive care units is not always possible. Newly developed ventilatory lung protective strategies such as high-frequency ventilation and/or nitric oxide might not be immediately available. For patients with MAS, conventional pressure-controlled mechanical ventilation might therefore be the only option available while waiting for other measures to be taken.

].

]. Variability in stroke volume with preserved CO during mechanical ventilation has, to our knowledge, not previously been reported. We here present the case of a patient with MAS in which early beat-to-beat-changes in stroke volume, as determined on-line by Doppler echocardiography, guided the ventilator settings to levels not affecting overall central blood flow.

Clinical case

Signs of fetal asphyxia prompted acute caesarean section at 41 weeks of gestation in a previously healthy primipara. The patient's birth weight was 3610 g and the Apgar score was 4-8-8. Heavily meconium-stained amniotic fluid was aspirated from the trachea and larynx. A pneumothorax was successfully drained. Progressive acidosis and respiratory difficulties necessitated mechanical ventilation with 100% oxygen through a pressure-limited time-cycled continuous-flow ventilator (Babylog 2; Dräger, Lübeck, Germany). A suitable high-frequency ventilator was not available at the time. In spite of an increasing pressure setting, oxygen saturation deteriorated progressively. Echocardiography showed no signs of organic heart disease or persistent fetal circulation. Dimensional values for both right and left ventricles were in the lower normal range.

Methods

Blood pressure was monitored with a TmSet1 (30 ml flush; Codan Triplus AB, Kirchseeon, Germany) attached to an indwelling umbilical arterial argyle catheter (French 5). Oxygen saturation was measured on the right hand by pulse oximetry (OxiNellcor Sensor II N25, Pleasanton, California, USA) and bipolar electrocardiography leads were applied.

]. The transducer beam was kept as close to parallel to the blood flow as possible; no angle correction was made because the angle was judged to be less than 20°. The flow area, which was assumed to be circular, was calculated from the mean of three two-dimensional diameter measurements of the LV outflow tract in parasternal long-axis view. Beat-to-beat changes in stroke volume (systolic velocity-time integral multiplied by flow area) were calculated and oxygen saturation was measured while ventilator adjustments were made. Changes from baseline values as well as changes within the respiratory cycle were calculated. CO was calculated as the product of heart rate and mean stroke volume.

Recordings at baseline

]) and the mean blood pressure from the monitors remained unchanged (within ± 3 mmHg). Oxygen saturation remained low (46%) and no pressure plateau was visible on the ventilator airway pressure display at this baseline setting.

Central circulatory effects of peak inspiratory pressure

O), a small decrease in LV CO was recorded. The maximal stroke volume increased slightly but we found a 30% beat-to-beat variation in stroke volume. This effect was further exaggerated when the inspiratory time was increased and also when the ventilatory rate was reduced. A further increase in inspiratory pressure resulted in an overall reduction in Doppler measurements and consequently a decrease in CO.

Central circulatory effects of inspiratory time and inspiratory:expiratory ratio

Measurements of maximal blood flow velocity and stroke volume were stable during inspiration and expiration when inspiratory : expiratory ratio increased from 1:2 to 1:1 if inspiratory time was kept constant (ie an increase in ventilator rate). An increase in inspiratory time to 0.5 s caused an inspiratory plateau but also an immediate cyclic increase and a decrease in LV stroke volume, initially keeping the CO constant. When PIP was increased, a stroke volume variation remained but the CO decreased. When inspiratory time was reduced to 0.4 s, respiratory variation in stroke volume was again very small and the CO returned to the baseline value.

Heart rate, blood pressure and oxygen saturation

The heart rate was 103-110 beats/min. The systolic and diastolic aortic blood pressures were 42-46/28-29 mmHg (mean 32-37 mmHg) according to the monitor throughout the procedure. A detailed analysis of beat-to-beat changes in blood pressure, pulse pressure or heart rate within these limits was not performed. Arterial oxygen saturation rose from 46% to a maximum of 87% with a slight increased inspiratory time and frequency without seriously affecting stroke volume or CO. Oxygen saturation values above 80% were otherwise recorded independently of the central circulatory effects of ventilation.

Further course

Neither high-frequency ventilation nor nitric oxide was available for the patient at the time of the study. The baby was not stable enough for transportation to another unit and a national team considered that the baby fulfilled the criteria to be accepted for extracorporal membrane oxygenation (ECMO), which took place 10 hours later. In the meantime the arterial blood pressure and heart rate were stable and echocardiography together with oxygen saturation guided the ventilator settings. ECMO was continued for 3 days, mechanical ventilation for a further 2 days and supplementary oxygen for a further week. The baby was initially tube fed and eventually allowed home at the age of 4 weeks. At discharge, computed tomography of the brain was normal, as was his neurological status; the boy is progressing normally.

Discussion

]. We show here that in a neonate a decrease in CO induced by increased inspiratory pressure or time is preceded by beat-to-beat variation in stroke volume. Doppler echocardiography can detect those early changes and in this regard is a useful bedside diagnostic tool in the neonatal intensive care unit.

].

]. These acute effects on left heart output were monitored in our patient by Doppler echocardiography.

].

Our case report confirms that mean blood pressure, heart rate and arterial oxygenation in the clinically available setting might not provide enough information to ensure that the oxygen supply to the tissues is adequate in the management of critically ill babies on mechanical ventilation. Because oxygen delivered to the tissues is dependent on both blood flow and arterial oxygen content, a non-invasive stroke volume measurement provides useful information. An increased inspiratory pressure and duration can initially cause an increased beat-to-beat variation in stroke volume while CO remains unaffected. Not until LV volume loading and compliance are severely disturbed will the CO be affected. As well as pulmonary effects themselves, Doppler echocardiographic monitoring of beat-to-beat variation in stroke volume, preceding the decay in CO, might therefore be an important indicator in the guidance of the ventilator settings to prevent further tissue hypoxia.

Acknowledgement

BJ-S was supported by the Swedish Medical Foundation (grant no. 99P-12313). Support was also obtained from the Swedish Heart Lung Foundation.

Figures and Tables

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Ventilatory and haemodynamic effects (lower panel) caused by changes in ventilator settings (upper panel)

saturation and CO.

Keywords

• cardiac output
• Doppler echocardiography
• haemodynamics
• mechanical ventilation
• newborn infant