The Non-pneumatic Antishock Garment (NASG) is a compression device that has shown significantly decreased blood loss in cases of obstetric hemorrhage. However, there are no physiologic studies of the NASG in postpartum women. This study used Doppler ultrasound to measure the resistive index (RI) in the internal iliac artery, thus approximating blood flow to the pelvis with and without the garment applied.
In this study, RI of the internal iliac artery was measured in a sample of 10 postpartum volunteers with and without the NASG applied. Median RI was calculated and compared between baseline and full application.
Internal iliac artery median RI was 0.83 (SD 0.11) at baseline and increased to 1.05 (SD 0.15) with full NASG application ( P = .02).
This study suggests a significant increase in internal iliac artery RI with NASG application and provides a physiological explanation of how the NASG might reduce postpartum hemorrhage.
Postpartum hemorrhage remains the leading cause of maternal death worldwide, particularly in developing countries where access to definitive treatment is often delayed and difficult to access. Thousands of women die of this preventable complication of childbirth each year. The Non-pneumatic Antishock Garment (NASG; ZOEX Corporation, Ashland, OR) is a neoprene and velcro device designed to reverse the effects of shock by shunting blood from the lower extremities and pelvis to the vital organs. The NASG is composed of lower extremity segments, a pelvic segment, and an abdominal segment, which includes an abdominal compression ball to provide increased pressure specifically to the lower abdomen and pelvis. This device has been shown in preliminary trials in Egypt and Nigeria to not only reverse shock and improve maternal outcomes, but also to significantly decrease blood loss by 33-78%. This finding suggests that in addition to shunting blood from the lower extremities and providing autotransfusion, the NASG may have a direct effect on the pelvic vasculature, inhibiting blood flow to the pelvic organs and decreasing blood loss.
The Pneumatic Antishock Garment (PASG) predated the NASG and although it initially showed promise for use in general trauma, several studies including a Cochrane Database Review raised concerns about the safety and efficacy of the PASG in these patients. Despite these findings in general trauma, the PASG gained recognition as a possible first-aid device for obstetric hemorrhage based on several case reports that documented favorable outcomes in cases of severe hemorrhage and shock. These findings are consistent with the thinking that the PASG has differential effectiveness depending on whether blood loss is from injuries below the waist vs above the waist. The favorable case reports in obstetrics were further supported by studies of the hemodynamic impact of the PASG showing a significant decrease in aortic blood flow below the level of the renal arteries, suggesting that the device would be useful for stemming blood loss from the uterus that is supplied by a branch of the internal iliac artery, a branch of the distal aorta below the renal arteries.
The presumed mechanisms of action underlying the utility of the antishock garment are based on 3 laws of physics. Poiseuille law states that the flow rate through a vessel is exponentially related to the radius of the vessel. Laplace law describes how tension is related to transmural pressure and the radius of a vessel. Bernoulli principle describes how the rate of bleeding depends on the size of the opening of the vessel and the transmural pressure. The circumferential pressure provided by the antishock garment compresses the radius of blood vessels and decreases the transmural pressure, which, according to these laws, should have important physiologic effects. First, the compression of the vessels should cause increased systemic vascular resistance, which decreases blood flow peripherally. Thus, if the blood cannot flow forward, it will back up and increase the blood flow through the noncompressed vessels, which should lead to increased preload and cardiac output, effectively providing autotransfusion. Second, it should lessen blood flow through the compressed vessels and if blood is being lost through these open vessels, this should result in a decrease or cessation of bleeding due to tamponade.
Like the PASG, the NASG applies circumferential counterpressure, but uses neoprene, a foam compression ball, and velcro rather than inflatable segments to do so. It is thought that the NASG is particularly well designed for treatment of obstetric hemorrhage because it can be rapidly applied, has a compression ball over the abdomen, and avoids the risk of overinflation that can lead to side effects. Although there is evidence from nonrandomized clinical trials that the NASG can decrease blood loss and reverse shock associated with obstetric hemorrhage, and a recent study has demonstrated decreased blood flow in the distal aorta with application of the NASG, there are no published studies of the impact of the NASG on pelvic blood flow in postpartum patients.
The objective of this study was to use a noninvasive method to estimate blood flow to the pelvis in nonhemorrhaging postpartum patients with the NASG applied. Delineating the physiologic mechanism of action of the NASG on pelvic blood flow is important for understanding how the NASG can impact blood loss due to obstetric hemorrhage.
Materials and Methods
Approval for this study was obtained from the University of California, San Francisco, Institutional Review Board and from the Program for Appropriate Technology in Health Institutional Review Board. Participation was voluntary and all study subjects gave written informed consent.
This study was conducted at San Francisco General Hospital on the labor and delivery ward from June 2008 through January 2009. Healthy female postpartum volunteers were recruited within 24 hours of a vaginal delivery. They were excluded if they had any underlying health problems, a cesarean section, an operative delivery, blood loss >500 mL postpartum, delivered >24 hours ago, a nonsingleton pregnancy, preeclampsia, an unstable hematocrit, pulmonary or cardiac problems, an unhealthy newborn, did not speak English or Spanish, or were <18 years of age. Demographic data were collected including patient age, gravidity, parity, and body mass index. The type of delivery, time since delivery, and uterotonic administration were noted. The objective of this study was to estimate the change in blood flow to the pelvis with application of the NASG. To use the least invasive form of evaluation possible, the blood flow was measured using transabdominal ultrasonography of the internal iliac artery. The internal iliac artery provides the blood supply to the pelvis and was able to be visualized using transabdominal ultrasound even with the NASG in place.
The resistive index (RI) of the internal iliac artery was measured as an approximation of pelvic blood flow. RI measurement is noninvasive and can be easily obtained using standard equipment available in the obstetrics department and provides information on the distal vessels of interest. The RI is defined as the peak of systole divided by the sum of systole and diastole [RI = S/(S + D)]. A higher RI is correlated with decreased blood flow to a given vessel. A value <1.0 indicates forward flow; whereas a value of ≥1.0 indicates absent or reverse flow ( Figure 1 ).