Impact of Non-invasive Positive Pressure Ventilation on Cardiac Function and Echocardiographic Parameters

Background

Mechanical ventilation (MV) is commonly used in intensive care units (ICU) to improve gas exchange and to reduce breathing effort. The need for MV is the single most prominent reason for admission to an intensive care unit, either for acute cardiac or respiratory failure, postoperative ventilation or other reasons. Its usefulness is not limited to critically ill patients but has also gained recognition in treating sleep-related breathing disorders. Soon after the introduction of MV to clinical practice its adverse hemodynamic effects were noted and have been studied ever since. It is now well accepted that MV, by applying external positive pressure in order to achieve tidal breathing, affects lung volume and intrathoracic pressure which in turn influence cardiac loading and function.

Echocardiography is increasingly used in intensive care to evaluate hemodynamics and especially Doppler assessments of mitral inflow and annular tissue velocity to describe left ventricular (LV) diastolic function. There is a growing body of evidence stating that diastolic dysfunction is a key factor for weaning failure from MV, a clinical issue affecting up to one third of patients. Worsened diastolic LV filling under MV may decisively lower cardiac performance and cause hemodynamic instability especially in case of pre-existing diastolic dysfunction in patients with arterial hypertension, a disease with a high prevalence of 30-45% in the general population. So far, despite the extensive use of echocardiography in ventilated patients, the causative pathophysiological mechanisms underlying ventilation-induced changes of echocardiographic parameters used to determine diastolic LV function have never been thoroughly evaluated. Assessing diastolic function during MV is complex since Doppler derived surrogate parameters all depend on cardiac loading conditions and loading itself is inevitably coupled to MV. The question therefore arises whether echocardiography allows for detection of deteriorating LV diastolic function in terms of impaired intrinsic ventricular relaxation or LV stiffness or whether it primarily mirrors MV-induced loading alterations. It is important to differentiate if echocardiographic signs of diastolic dysfunction in the patient under MV are caused by changes in loading or a LV filling restraint since in case of hemodynamic instability the two scenarios would ask for different therapeutic measures. In case of a loading problem, pre- and afterload can be targeted using drugs or fluid resuscitation. If a MV-induced filling restraint is the predominant problem one may consider special ventilatory modes or even deep sedation and the use of muscle relaxants to minimize Pit.

The basic concepts of heart-lung interactions under positive pressure ventilation make it conceivable that extrinsic pressure applied to the thorax and therefore, as explained later in more detail, to the heart may mimic diastolic dysfunction even in a totally healthy heart if assessed by echocardiography. Echocardiographic parameters to assess diastolic function have been evaluated against the gold standard - invasive LV pressure and volume measurements - in spontaneously breathing patients. Such an evaluation is lacking for the patient with ventilatory support and therefore the value of echocardiography for assessment of diastolic function under MV needs further clarification. The investigators intend to comparatively study cardiac and particularly left ventricular diastolic function in ventilated patients using both invasive and ultrasound methods.

Objective

The presented study will elucidate the influence of positive pressure ventilation on cardiac function in humans. Furthermore, the investigators aim to describe the changes of echocardiographic parameters under different levels of positive pressure ventilation and link these changes to the underlying pathophysiological alterations in hemodynamics induced by positive pressure ventilation.

Methods

Prospective single-centre study at the University Hospital Berne. 30 patients scheduled for an elective coronary angiogram will be included. During spontaneous breathing and two different levels of non-invasive PPV transthoracic echocardiography is performed while recording LV-pressure-volume loops using an impedance catheter. Simultaneously the pulmonary artery occlusion pressure, the right atrial pressure and the intrathoracic pressure are recorded using a pulmonary artery catheter and an oesophageal balloon respectively.