Context: Survival outcome following cardiac arrest in children is poor, and recent evidence suggests that the quality of Cardiopulmonary Resuscitation(CPR) is critically important. Venous blood return to the thorax to refill the heart is essential for good quality CPR and critical organ perfusion. Adult cardiac arrest studies suggest that incomplete chest wall decompression (i.e. "leaning" on the sternum of the chest) during CPR affects intrathoracic pressure and impedes venous return. The consequence of "leaning" on the chest during CPR is increased intrathoracic pressure, which creates a "back-pressure" preventing optimal return of blood to the heart. The critical importance of manipulating positive and negative intrathoracic pressures during Cardiopulmonary Resuscitation (CPR) has been recently demonstrated in both animal and human studies. Food and Drug Administration (FDA) approved defibrillators with a force and depth sensor can monitor the depth, rate and complete release of sternal pressure during CPR. These FDA approved defibrillators have been introduced and implemented in the Pediatric Intensive Care Unit (PICU) at the Children's Hospital of Philadelphia (CHOP). These defibrillators can provide feedback on the force and amount of "leaning", but there is no data on the minimal amount of sternal pressure (or "leaning pressure") that affects intrathoracic or intravascular pressures or venous return to the heart. In addition, there is no data on how much force on the sternal pressure sensor (e.g. leaning on the sensor) begins to affect intrathoracic pressure in children. We propose to observe, measure and report the effect of incremental gentle sternal pressure increases on intrathoracic pressure, and other surrogates of hemodynamic function, in stable but critically ill and mechanically ventilated children. This study will provide preliminary data to inform the resuscitation research community and assist development of evidence-based pediatric resuscitation guidelines in the future.

Objectives: 1) To characterize the effect of gentle, incremental increases in sternal chest pressure on intrathoracic pressure in mechanically ventilated children. 2) To characterize the effect of gentle, incremental increases in sternal pressure on regional perfusion pressures, when existing catheters (arterial, central venous, intracranial) are present.

Study Design/Setting/Participants: This protocol is a prospective, pilot, observational study in the Pediatric Intensive Care Unit (PICU), Progressive Care Unit (PCU) and the Operating Room (OR)settings at the Children's Hospital of Philadelphia. The participants are a convenience sample of stable mechanically ventilated children from 6 months to \< 8 years of age. A total of 20 patients will be enrolled, including a minimum of 10 with vascular catheters.

Study Measures: The primary outcome variable is the change in intrathoracic pressure with incremental increase in gentle sternal pressure, measured by the peak airway pressure detected at the proximal end of the tracheal tube during end inspiration. Secondary outcomes include additional measures of intrathoracic pressure (end inspiratory pressure, mean pressure, area under the curve over 15 seconds, plateau pressure). For patients with indwelling central venous, arterial or intracranial pressure monitors, perfusion pressure changes will also be analyzed.