HEMORRHAGE PROGRESSIVELY DISTURBS INTERALVEOLAR PERFUSION IN THE LUNGS OF RATS

Vital sign patterns before shock-related cardiopulmonary arrest

OBJECTIVE

Traditional vital sign thresholds reflect an increased risk of mortality, which may occur hours, days, or weeks following illness/injury, limiting immediate clinical significance to guide rescue therapy to avoid arrest. Our objective is to explore vital sign patterns prior to arrest due to shock.

DESIGN

This retrospective observational analysis utilized physiological data from adult helicopter patients suffering provider-witnessed arrest. Pre-arrest values for systolic blood pressure (SBP), mean arterial pressure (MAP), heart rate (HR), shock index, and end-tidal carbon dioxide (EtCO2) were modeled against time using polynomial linear regression. The "terminal inflection point" beyond which arrest was imminent was identified where slope equals 1.0 (shock index) or -1.0 (SBP, MAP, HR, EtCO2) and was then compared to initial values.

SETTING

Air ambulance services.

PATIENTS

70 helicopter patients over age fourteen suffering cardiac arrest.

RESULTS

SBP and MAP demonstrated a gentle decline followed by acceleration beyond the inflection point (SBP 80.7 mmHg, MAP 61.9 mmHg). HR demonstrated an increase followed by a terminal drop, but inflection point values fell within normal range. Shock index increased gradually from a mean of 0.9 to the inflection point of 1.1. Initial EtCO2 values declined gradually from normal (34.4 mmHg) to the inflection point (24.7 mmHg), then dropped precipitously into arrest. All inflection points occurred 2-5 min prior to arrest.

CONCLUSIONS

Vital sign patterns were defined for SBP, MAP, HR, shock index, and EtCO2 with clear inflection points identified 2-5 min prior to arrest. These patterns may help guide therapy to reverse deterioration and prevent arrest.

Obstructive apnea causes microvascular perfusion maldistribution in the lungs of rats

Obstructive sleep apnea (OSA) is associated with significant cardiovascular consequences, including pulmonary hypertension, yet little is known about its effects on pulmonary microvascular perfusion. To investigate effects of OSA on pulmonary microvascular perfusion, we clamped the tracheal cannulas of anesthetized, spontaneously breathing rats to simulate obstructive apnea. The clamp remained in place for 10 breaths before it was released to allow the animals to again breathe spontaneously. We repeated this protocol every 20 s until the rat experienced a total of five apneic episodes of 10 breaths each. We then infused into a femoral vein 10⁸ fluorescent latex particles (4 µm diameter), which became trapped within the pulmonary microcirculation. We removed the lungs, allowed them to air-dry, and quantified the particle distributions in sections of the lungs using dispersion index (DI) analysis, a method we developed previously. The log of the DI (logDI) is a measure of perfusion maldistribution. Greater log(DI) values correspond to greater maldistribution. Apneic lungs had average logDI values of 1.28 (SD 0.24). Rats not subjected to apnea had average logDI values of 0.85 (SD 0.08) ( P ≤ 0.05). Rats that received latex particles 10 min or 24 h after apnea had average logDI values of 0.97 (SD 0.31) and 0.84  (SD 0.38), respectively (not significant). Our results demonstrate, for the first time, that a few apneic events produced significant, but temporary, perfusion maldistribution within the pulmonary microcirculation. Repeated nightly episodes of apnea over months and years may explain why human patients with OSA suffer from significantly greater cardiovascular disease than those without OSA.

Arterio-venous anastomoses in isolated, perfused rat lungs

Several studies have suggested that large-diameter (>25 μm) arterio-venous shunt pathways exist in the lungs of rats, dogs, and humans. We investigated the nature of these pathways by infusing specific-diameter fluorescent latex particles (4, 7, 15, 30, or 50 μm) into isolated, ventilated rat lungs perfused at constant pressure. All lungs received the same mass of latex (5 mg), which resulted in infused particle numbers that ranged from 1.7 × 10⁷ 4 μm particles to 7.5 × 10⁴ 50 μm particles. Particles were infused over 2 min. We used a flow cytometer to count particle appearances in venous effluent samples collected every 0.5 min for 12 min from the start of particle infusion. Cumulative percentages of infused particles that appeared in the samples averaged 3.17 ± 2.46% for 4 μm diameter particles, but ranged from 0.01% to 0.17% for larger particles. Appearances of 4 μm particles followed a rapid upslope beginning at 30 sec followed by a more gradual downslope that lasted for up to 12 min. All other particle diameters also began to appear at 30 sec, but followed highly irregular time courses. Infusion of 7 and 15 μm particles caused transient but significant perfusate flow reductions, while infusion of all other diameters caused insignificant reductions in flow. We conclude that small numbers of bypass vessels exist that can accommodate particle diameters of 7-to-50 μm. We further conclude that our 4 μm particle data are consistent with a well-developed network of serial and parallel perfusion pathways at the acinar level.

Inhaled thrombolytics reduce lung microclot and leukocyte infiltration after acute blood loss

We showed previously that a 30% blood loss in rats, without resuscitation, caused significant accumulation of microthrombi and leukocytes within the pulmonary circulation by 24 h. We hypothesized that the microthrombi formed spontaneously as a consequence of hemorrhage-induced stasis within the low-pressure pulmonary circuit and that the leukocytes were attracted to them. This suggested that elimination of the microthrombi, using an inhaled thrombolytic agent, could prevent the neutrophil sequestration after blood loss. To test this hypothesis, we removed 30% of the calculated blood volume from isoflurane-anesthetized, male Sprague-Dawley rats (350-500 g) over 5 min and allowed them to recover. Six hours later, we re-anesthetized the rats and nebulized tissue plasminogen activator (80 or 320 µg/kg), lactated Ringer's solution (LRS), or ipratropium bromide (i-bromide) into their lungs. We used i-bromide as a control after we discovered that nebulized LRS had thrombolytic properties. At 24 h, we removed and fixed the lungs and prepared sections for immunohistochemistry using antibodies against fibrinogen (microthrombi) and CD16 (leukocytes). Digital images of each section were obtained using a confocal microscope. Pixel counts of the images showed significantly less accumulation of microthrombi and leukocytes in lungs nebulized with tissue plasminogen activator or LRS than in non-nebulized lungs or in lungs nebulized with i-bromide (P ≤ 0.05). Lactated Ringer's solution becomes positively charged when nebulized (unlike i-bromide), suggesting that it eliminated microthrombi by fibrin depolymerization. We confirmed this using an in vitro assay. Our results demonstrate that lyses of microthrombi that accumulate in the lung after acute blood loss prevent subsequent leukocyte sequestration.

A method for quantifying blood flow distribution among the alveoli of the lung

This article describes a method for quantifying blood flow distribution among lung alveoli. Our method is based on analysis of trapping patterns of small diameter (4 μm) fluorescent latex particles infused into lung capillaries. Trapping patterns are imaged using confocal microscopy, and the images are analyzed statistically using SAS subroutines. The resulting plots provide a quantifiable way of assessing interalveolar perfusion distribution in a way that has not previously been possible. Methods for using this technique are described, and the SAS routines are included. This technique can be an important tool for learning how this critical vascular bed performs in health and disease.

Microthrombus formation may trigger lung injury after acute blood loss

We showed previously that acute blood loss, without resuscitation, caused marked maldistribution of interalveolar perfusion. Because hemorrhage is a known risk factor for the development of lung injury, the goal of our present studies was to determine if there was a correlation between perfusion maldistribution and the subsequent development of lung injury after blood loss. Specifically, we wanted to know if the perfusion maldistribution might be due to microthrombus formation and/or leukocyte sequestration within the pulmonary microcirculation. We bled rats (30% blood loss) and harvested their lungs 45 min or 24 h later. Lungs were prepared for perfusion distribution analysis, Western blot analysis to measure whole-lung fibrinogen concentrations, and for immunohistochemistry to measure fibrin deposition and leukocyte deposition (CD16 fluorescence). Perfusion was significantly maldistributed at 45 min and 24 h (P < 0.05). At 45 min, whole-lung fibrinogen concentrations were less than half that in controls (P < 0.05), whereas numbers of fibrin microthrombi were 2.5-fold greater than control by 45 min (not statistically significant) and were 4.5-fold greater by 24 h (P = 0.01). Leukocyte deposition was two-fold greater than control by 45 min (not statistically significant) and was 4-fold greater by 24 h (P = 0.02). Fibrin-to-leukocyte nearest-neighbor distances remained unchanged (18.1 [SD, 1.1] μm) even as the numbers of both increased with time after blood loss. Our results suggest that soluble fibrinogen polymerized to insoluble fibrin within minutes after acute blood loss, which caused perfusion maldistribution and attracted leukocytes. The development of lung injury after blood loss may be a consequence of leukocyte chemoattraction to fibrin microthrombi that seem to form within minutes after blood loss.