IMPAIRED BLOOD PRESSURE COMPENSATION FOLLOWING HEMORRHAGE IN CONSCIOUS OBESE ZUCKER RATS

By: Lusha Xiang, John S. Clemmer, Silu Lu, and Peter N. Mittwede. Life Sci. 2013 August 14; 93(0): 214–219.

Hemorrhage shock has been linked to a higher risk for mortality and morbidity in obese patients. Blood loss can lead to things like organ failure and increased inflammatory response. The most  important factor is that because of this blood loss, volume is decreased. When volume is decreased, blood pressure (BP) follows suit. When hypovolemia occurs, sympathetic nerve activity (SNA) is increased, as well as the release of vasoconstrictors like angiotensin II and antidiuretic hormone (ADH). The purpose of these actions is to increase total peripheral resistance (TPR), in order to bring BP back up. The study focuses on obese patients exclusively since no other studies have been done relating obesity and severe hemorrhage.

This study focused on how well (or poorly) the obese rats are able to recover BP following a severe hemorrhage. Their previous studies showed that BP recovery is significantly impaired following a hemorrhage under anesthesia. This study will focus on conscious obese zucker rats (OZ), compared to lean Zucker rats (LZ). They hypothesized that BP compensation following a severe hemorrhage is impaired in conscious OZ.

The mean body weight for LZ was 306g. The mean body weight for OZ was 452g. Rats were anesthetized and catheters were inserted to record BP and heart rate (HR) before and after hemorrhage. Baroreflex sensitivity and basal sympathetic tone were also measured. Hemorrhage was induced by withdrawing 35% of total blood volume from the femoral catheter (0.5 mL/min, 6-8 ml). BP and heart rate was measured during and 1 hour following the hemorrhage.

The following results were observed:
Baroreflex sensitivity and basal sympathetic tone

1. Basal BP and heart rate did not differ between LZ and OZ
2. HR was blunted in OZ in response to dose of sodium nitroprusside (SNP)
3. OZ showed a greater decrease in basal BP when prazosin was injected

Responses to hemorrhage

1. Basal BP was not different between LZ and OZ. 
2. Hemorrhage dropped BP in both groups and was not different between LZ and OZ immediately after hemorrhage. 
3. At 30 and 60 min of recovery, OZ had significantly lower BP. 
4. Basel HR was higher in OZ. 
5. LZ HR decreased immediately after hemorrhage, but increased and stayed up thereafter.
6. OZ had decreased HR throughout recovery period and was lowered than LZ.
7. Oxygen consumption dropped in both groups following hemorrhage, OZ significantly lower.
8. Cardiac output dropped in similar levels in both groups
9. TPR, plasma renin activity (PRA), and ADH increased in both groups, but the increases were significantly lower in OZ

The results of the experiment have told us that OZ exhibited a blunted baroreflex sensitivity and elevated basal sympathetic tone compared to LZ. Following hemorrhage, OZ had an impaired increase of TPR, PRA, and ADH, as well as blunted recovery of BP. It is pretty clear that the ability to recover from a severe hemorrhage is more difficult in the obese group. The blunted baroreflex sensitivity explains why in OZ, they could not recover as well. The baroreflex is very important in controlling BP on a beat to beat basis. The mechanism to why this is happening is unknown.

The reason why I chose this paper was instead of studying obesity, we want to study sedentary conditions. The same type of study could be done in our lab, except we would be comparing seds to runners!

-Tsetse Fly