Head down position: hypoxemia

Respiratory Effects

a. Increases Atelectasis: leads to decreases in FRC, causing 1)cephalad displacemnent of diaphragm, 2)weight of abdominal contents on diaphragm, 3)increased central blood volume

b. Increased work of breathing secondary to increased impedance to stretching of chest wall and diaphragm

c. Movement of the ETT: cephalad movement of diaphragm can shift carina carina leading to endobronchial intubation. Give larger tidal volumes and add PEEP to increase O₂ delivery

Why is head down BAD in cases of hypotension?

When trendelenberg position is used with the hopes of raising a pt’s blood pressure, INITIALLY central blood volume increases by 1L in the adult patient and this will cause reflex barostimulation and then systemic vasodilation, which will then lead to decreased CO, SV and therefore DECREASED perfusion to BRAIN. Current literature has shown in many studies( Johansson, Sing) HEAD DOWN position (ie Trendelenburg) will cause increases MAP (mean arterial pressure), PCW (pulmonary capillary wedge pressure), SVR (systemic vascular resistance). Furthermore, there was NO CHANGE in OXYGEN DELIVERY, CI, O₂ consumption.

HEAD DOWN IS NO LONG RECOMMENDED AS THERAPY FOR HYPOVOLEMIC SHOCK

HOWEVER LAWN CHAIR IS BETTER for shock because 1. it elevates HEAD: minimizes cerebral congestion, IMPROVES cerebral oxygenation and 2. it elevates LEGS: increases VENOUS return, augments CO.

Physiologic Effects of Head-Down Positioning

• Cardiac: initial fluid bolus (1L), quickly offset by reflex barostimulation, vasodilation, and potentially decreased perfusion of the brain
• Respiratory: increased atelectasis, work of breathing, movement of ETT