Bronchoconstriction during alveolar hypocapnia and systemic hypercapnia in dogs with a cardiopulmonary bypass

Acute sodium bicarbonate administration improves ventilatory efficiency in experimental respiratory acidosis: clinical implications

Administering sodium bicarbonate (NaHCO3) to patients with respiratory acidosis breathing spontaneously is contraindicated because it increases carbon dioxide load and depresses pulmonary ventilation. Nonetheless, several studies have reported salutary effects of NaHCO3 in patients with respiratory acidosis but the underlying mechanism remains uncertain. Considering that such reports have been ignored, we examined the ventilatory response of unanesthetized dogs with respiratory acidosis to hypertonic NaHCO3 infusion (1 N, 5 mmol/kg) and compared it with that of animals with normal acid-base status or one of the remaining acid-base disorders. Ventilatory response to NaHCO3 infusion was evaluated by examining the ensuing change in PaCO2 and the linear regression of the PaCO2 vs. pH relationship. Strikingly, PaCO2 failed to increase and the ΔPaCO2 vs. ΔpH slope was negative in respiratory acidosis, whereas PaCO2 increased consistently and the ΔPaCO2 vs. ΔpH slope was positive in the remaining study groups. These results cannot be explained by differences in buffering-induced decomposition of infused bicarbonate or baseline levels of blood pH, PaCO2, and pulmonary ventilation. We propose that NaHCO3 infusion improved the ventilatory efficiency of animals with respiratory acidosis, i.e., it decreased their ratio of total pulmonary ventilation to carbon dioxide excretion (VE/VCO2). Such exclusive effect of NaHCO3 infusion in animals with respiratory acidosis might emanate from baseline increased VD/VT (dead space/tidal volume) caused by bronchoconstriction and likely reduced pulmonary blood flow, defects that are reversed by alkali infusion. Our observations might explain the beneficial effects of NaHCO3 reported in patients with acute respiratory acidosis.

Changes in lung mechanics and ventilation-perfusion match: comparison of pulmonary air- and thromboembolism in rats

BACKGROUND

Pulmonary air embolism (AE) and thromboembolism lead to severe ventilation-perfusion defects. The spatial distribution of pulmonary perfusion dysfunctions differs substantially in the two pulmonary embolism pathologies, and the effects on respiratory mechanics, gas exchange, and ventilation-perfusion match have not been compared within a study. Therefore, we compared changes in indices reflecting airway and respiratory tissue mechanics, gas exchange, and capnography when pulmonary embolism was induced by venous injection of air as a model of gas embolism or by clamping the main pulmonary artery to mimic severe thromboembolism.

METHODS

Anesthetized and mechanically ventilated rats (n = 9) were measured under baseline conditions after inducing pulmonary AE by injecting 0.1 mL air into the femoral vein and after occluding the left pulmonary artery (LPAO). Changes in mechanical parameters were assessed by forced oscillations to measure airway resistance, lung tissue damping, and elastance. The arterial partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2) were determined by blood gas analyses. Gas exchange indices were also assessed by measuring end-tidal CO2 concentration (ETCO2), shape factors, and dead space parameters by volumetric capnography.

RESULTS

In the presence of a uniform decrease in ETCO2 in the two embolism models, marked elevations in the bronchial tone and compromised lung tissue mechanics were noted after LPAO, whereas AE did not affect lung mechanics. Conversely, only AE deteriorated PaO2, and PaCO2, while LPAO did not affect these outcomes. Neither AE nor LPAO caused changes in the anatomical or physiological dead space, while both embolism models resulted in elevated alveolar dead space indices incorporating intrapulmonary shunting.

CONCLUSIONS

Our findings indicate that severe focal hypocapnia following LPAO triggers bronchoconstriction redirecting airflow to well-perfused lung areas, thereby maintaining normal oxygenation, and the CO2 elimination ability of the lungs. However, hypocapnia in diffuse pulmonary perfusion after AE may not reach the threshold level to induce lung mechanical changes; thus, the compensatory mechanisms to match ventilation to perfusion are activated less effectively.

Pathophysiology and Management of Pulmonary Embolism

Pulmonary embolism (PE) is one of the most common etiologies of cardiovascular mortality. It could be linked to several risk factors including advanced age. The pathogenesis of PE is dictated by the Virchow's triad that includes venous stasis, endothelial injury, and a hypercoagulable state. The diagnosis of PE is difficult and is often missed due to the nonspecific symptomatology. Hypoxia is common in the setting of PE, and the degree of respiratory compromise is multifactorial and influenced by underlying cardiac function, clot location, and ability to compensate with respiratory mechanics. Right ventricular dysfunction/failure is the more profound cardiovascular impact of acute PE and occurs due to sudden increase in afterload. This is also the primary cause of death in PE. High clinical suspicion is required in those with risk factors and presenting signs or symptoms of venous thromboembolic disease, with validated clinical risk scores such as the Wells, Geneva, and pulmonary embolism rule out criteria in estimating the likelihood for PE. Advancement in capture time and wider availability of computed tomographic pulmonary angiography and D-dimer testing have further facilitated the rapid evaluation and diagnosis of suspected PE. Treatment is dependent on clinical presentation and initially involves providing adequate oxygenation and stabilizing hemodynamics. Anticoagulant therapy is indicated for the treatment of PE. Treatment is guided by presence or absence of shock and ranges from therapeutic anticoagulation to pharmacologic versus mechanical thrombectomy. The prognosis of patients can vary considerably depending on the cardiac and pulmonary status of patient and the size of the embolus.

Intraoperative end-tidal carbon dioxide and postoperative mortality in major abdominal surgery: a historical cohort study

PURPOSE

There is a paucity of data on the effect of intraoperative end-tidal carbon dioxide (EtCO₂) levels on postoperative mortality. The purpose of this study was to investigate the relationship between intraoperative EtCO₂ and 90-day mortality in patients undergoing major abdominal surgery under general anesthesia.

METHODS

We conducted a historical cohort study of patients undergoing major abdominal surgery under general anesthesia at Kyoto University Hospital. We measured the intraoperative EtCO₂, and patients with a mean EtCO₂ value < 35 mm Hg were classified as low EtCO₂. The time effect was determined based on minutes below an EtCO₂ of 35 mm Hg, and cumulative effects were evaluated by measuring the area under the threshold of 35 mm Hg for each patient.

RESULTS

Of 4,710 patients, 1,374 (29%) had low EtCO₂ and 55 (1.2%) died within 90 days of surgery. Multivariable Cox regression analysis-adjusted for age, American Society of Anesthesiologists Physical Status classification, sex, laparoscopic surgery, emergency surgery, blood loss, mean arterial pressure, duration of surgery, type of surgery, and chronic obstructive pulmonary disease-revealed an association between low EtCO₂ and 90-day mortality (adjusted hazard ratio, 2.2; 95% confidence interval [CI], 1.2 to 3.8; P = 0.006). In addition, severity of low EtCO₂ was associated with an increased 90-day mortality (area under the threshold; adjusted hazard ratio; 2.9, 95% CI, 1.2 to 7.4; P =0.02); for long-term exposure to an EtCO₂ < 35 mm Hg (≥ 226 min), the adjusted hazard ratio for increased 90-day mortality was 2.3 (95% CI, 0.9 to 6.0; P = 0.08).

CONCLUSION

A mean intraoperative EtCO₂ < 35 mm Hg was associated with increased postoperative 90-day mortality.

Hypoxic pulmonary vasoconstriction as a regulator of alveolar-capillary oxygen flux: A computational model of ventilation-perfusion matching

The relationship between regional variabilities in airflow (ventilation) and blood flow (perfusion) is a critical determinant of gas exchange efficiency in the lungs. Hypoxic pulmonary vasoconstriction is understood to be the primary active regulator of ventilation-perfusion matching, where upstream arterioles constrict to direct blood flow away from areas that have low oxygen supply. However, it is not understood how the integrated action of hypoxic pulmonary vasoconstriction affects oxygen transport at the system level. In this study we develop, and make functional predictions with a multi-scale multi-physics model of ventilation-perfusion matching governed by the mechanism of hypoxic pulmonary vasoconstriction. Our model consists of (a) morphometrically realistic 2D pulmonary vascular networks to the level of large arterioles and venules; (b) a tileable lumped-parameter model of vascular fluid and wall mechanics that accounts for the influence of alveolar pressure; (c) oxygen transport accounting for oxygen bound to hemoglobin and dissolved in plasma; and (d) a novel empirical model of hypoxic pulmonary vasoconstriction. Our model simulations predict that under the artificial test condition of a uniform ventilation distribution (1) hypoxic pulmonary vasoconstriction matches perfusion to ventilation; (2) hypoxic pulmonary vasoconstriction homogenizes regional alveolar-capillary oxygen flux; and (3) hypoxic pulmonary vasoconstriction increases whole-lobe oxygen uptake by improving ventilation-perfusion matching.

The relationship between regional ventilation (airflow) and perfusion (blood flow) is a major determinant of gas exchange efficiency. Atelactasis and pulmonary vascular occlusive diseases, such as acute pulmonary embolism, are characterized by ventilation-perfusion mismatching and decreased oxygen in the bloodstream. Despite the physiological and medical importance of ventilation-perfusion matching, there are gaps in our knowledge of the regulatory mechanisms that maintain adequate gas exchange under pathological and normal conditions. Hypoxic pulmonary vasoconstriction is understood to be the primary regulator of ventilation-perfusion matching, where upstream arterioles constrict to direct blood flow away from areas that have low oxygen supply, yet it is not understood how this mechanism affects oxygen transport at the system level. In this study we present a computational model of the ventilation-perfusion matching and hypoxic pulmonary vasoconstriction to better understand how physiological regulation at the regional level scales to affect oxygen transport at the system level. Our model simulations predict that this regulatory mechanism improves the spatial overlap of airflow and blood flow, which serves to increase the uptake of oxygen into the bloodstream. This improved understanding of ventilation-perfusion matching may offer insights into the etiology of, and therapeutic interventions for diseases characterized by ventilation-perfusion mismatching.

Sodium bicarbonate therapy for acute respiratory acidosis

PURPOSE OF REVIEW

Respiratory acidosis is commonly present in patients with respiratory failure. The usual treatment of hypercapnia is to increase ventilation. During the recent surge of COVID-19, respiratory acidosis unresponsive to increased mechanical ventilatory support was common. Increasing mechanical ventilation comes at the expense of barotrauma and hemodynamic compromise from increasing positive end-expiratory pressures or minute ventilation. Treating acute respiratory acidemia with sodium bicarbonate remains controversial.

RECENT FINDINGS

There are no randomized controlled trials of administration of sodium bicarbonate for respiratory acidemia. A recent review concluded that alkali therapy for mixed respiratory and metabolic acidosis might be useful but was based on the conflicting and not conclusive literature regarding metabolic acidosis. This strategy should not be extrapolated to treatment of respiratory acidemia. Low tidal volume ventilation in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) has beneficial effects associated with permissive hypercapnia. Whether the putative benefits will be negated by administration of alkali is not known. Hypercapnic acidosis is well tolerated, with few adverse effects as long as tissue perfusion and oxygenation are maintained.

SUMMARY

There is a lack of clinical evidence that administration of sodium bicarbonate for respiratory acidosis has a net benefit; in fact, there are potential risks associated with it.

The role of hypercapnia in acute respiratory failure

The biological effects and physiological consequences of hypercapnia are increasingly understood. The literature on hypercapnia is confusing, and at times contradictory. On the one hand, it may have protective effects through attenuation of pulmonary inflammation and oxidative stress. On the other hand, it may also have deleterious effects through inhibition of alveolar wound repair, reabsorption of alveolar fluid, and alveolar cell proliferation. Besides, hypercapnia has meaningful effects on lung physiology such as airway resistance, lung oxygenation, diaphragm function, and pulmonary vascular tree.

In acute respiratory distress syndrome, lung-protective ventilation strategies using low tidal volume and low airway pressure are strongly advocated as these have strong potential to improve outcome. These strategies may come at a price of hypercapnia and hypercapnic acidosis. One approach is to accept it (permissive hypercapnia); another approach is to treat it through extracorporeal means. At present, it remains uncertain what the best approach is.

An observational study of end-tidal carbon dioxide trends in general anesthesia

PURPOSE

Despite growing evidence supporting the potential benefits of higher end-tidal carbon dioxide (ETCO₂) levels in surgical patients, there is still insufficient data to formulate guidelines for ideal intraoperative ETCO₂ targets. As it is unclear which intraoperative ETCO₂ levels are currently used and whether these levels have changed over time, we investigated the practice pattern using the Multicenter Perioperative Outcomes Group database.

METHODS

This retrospective, observational, multicentre study included 317,445 adult patients who received general anesthesia for non-cardiothoracic procedures between January 2008 and September 2016. The primary outcome was a time-weighted average area-under-the-curve (TWA-AUC) for four ETCO₂ thresholds (< 28, < 35, < 45, and > 45 mmHg). Additionally, a median ETCO₂ was studied. A Kruskal-Wallis test was used to analyse differences between years. Random-effect multivariable logistic regression models were constructed to study variability.

RESULTS

Both TWA-AUC and median ETCO₂ showed a minimal increase in ETCO₂ over time, with a median [interquartile range] ETCO₂ of 33 [31.0-35.0] mmHg in 2008 and 35 [33.0-38.0] mmHg in 2016 (P <0.001). A large inter-hospital and inter-provider variability in ETCO₂ were observed after adjustment for patient characteristics, ventilation parameters, and intraoperative blood pressure (intraclass correlation coefficient 0.36; 95% confidence interval, 0.18 to 0.58).

CONCLUSIONS

Between 2008 and 2016, intraoperative ETCO₂ values did not change in a clinically important manner. Interestingly, we found a large inter-hospital and inter-provider variability in ETCO₂ throughout the study period, possibly indicating a broad range of tolerance for ETCO₂, or a lack of evidence to support a specific targeted range. Clinical outcomes were not assessed in this study and they should be the focus of future research.

Pulmonary embolism

Pulmonary embolism (PE) is caused by emboli, which have originated from venous thrombi, travelling to and occluding the arteries of the lung. PE is the most dangerous form of venous thromboembolism, and undiagnosed or untreated PE can be fatal. Acute PE is associated with right ventricular dysfunction, which can lead to arrhythmia, haemodynamic collapse and shock. Furthermore, individuals who survive PE can develop post-PE syndrome, which is characterized by chronic thrombotic remains in the pulmonary arteries, persistent right ventricular dysfunction, decreased quality of life and/or chronic functional limitations. Several important improvements have been made in the diagnostic and therapeutic management of acute PE in recent years, such as the introduction of a simplified diagnostic algorithm for suspected PE as well as phase III trials demonstrating the value of direct oral anticoagulants in acute and extended treatment of venous thromboembolism. Future research should aim to address novel treatment options (for example, fibrinolysis enhancers) and improved methods for predicting long-term complications and defining optimal anticoagulant therapy parameters in individual patients, and to gain a greater understanding of post-PE syndrome.

Importance of carbon dioxide in the critical patient: Implications at the cellular and clinical levels

Important recent insights have emerged regarding the cellular and molecular role of carbon dioxide (CO₂) and the effects of hypercapnia. The latter may have beneficial effects in patients with acute lung injury, affording reductions in pulmonary inflammation, lessened oxidative alveolar damage, and the regulation of innate immunity and host defenses by inhibiting the expression of inflammatory cytokines. However, other studies suggest that CO₂ can have deleterious effects upon the lung, reducing alveolar wound repair in lung injury, decreasing the rate of reabsorption of alveolar fluid, and inhibiting alveolar cell proliferation. Clearly, hypercapnia has both beneficial and harmful consequences, and it is important to determine the net effect under specific conditions. The purpose of this review is to describe the immunological and physiological effects of carbon dioxide, considering their potential consequences in patients with acute respiratory failure.

Role of bronchodilation and pattern of breathing in increasing tidal expiratory flow with progressive induced hypercapnia in chronic obstructive pulmonary disease

Hypercapnia (HC) in vitro relaxes airway smooth muscle; in vivo, it increases respiratory effort, tidal expiratory flows (V̇_exp), and, by decreasing inspiratory duration (Ti), increases elastic recoil pressure (Pel) via lung viscoelasticity; however, its effect on airway resistance is uncertain. We examined the contributions of bronchodilation, Ti, and expiratory effort to increasing V̇_exp with progressive HC in 10 subjects with chronic obstructive pulmonary disease (COPD): mean forced expiratory volume in 1 s (FEV₁) 53% predicted. Lung volumes (Vl), V̇_exp, esophageal pressure (Pes), Ti, and end-tidal Pco₂ ([Formula: see text]) were measured during six tidal breaths followed by an inspiratory capacity (IC), breathing air, and at three levels of HC. V̇_exp and V̇ with submaximal forced vital capacities breathing air (V̇_sFVC) were compared. Pulmonary resistance ( Rl) was measured from the Pes-V̇ relationship. V̇_exp and Pes at end-expiratory lung volume (EELV) + 0.3 tidal volume [V̇_(0.3Vt) and Pes_(0.3Vt), respectively], Ti, and Rl correlated with [Formula: see text] ( P < 0.001 for all) and were independent of tiotropium. [Formula: see text], Ti, and Pes_(0.3Vt) predicted the increasing V̇_(0.3Vt)/V̇_sFVC(0.3Vt) [multiple regression analysis (MRA): P = 0.001, 0.004, and 0.025, respectively]. At [Formula: see text] ≥ 50 Torr, V̇_(0.3Vt)/V̇_sFVC(0.3Vt) exceeded unity in 30 of 36 measurements and was predicted by [Formula: see text] and Pes_(0.3Vt) (MRA: P = 0.02 and 0.025, respectively). Rl decreased at [Formula: see text] 45 Torr ( P < 0.05) and did not change with further HC. IC and Vl_(0.3Vt) did not change with HC. We conclude that in COPD HC increases V̇_exp due to bronchodilation, increased Pel secondary to decreasing Ti, and increased expiratory effort, all promoting lung emptying and a stable EELV. NEW & NOTEWORTHY The response of airways to intrapulmonary hypercapnia (HC) is uncertain. In chronic obstructive pulmonary disease (COPD), progressive HC increases tidal expiratory flows by inducing bronchodilation and via an increased rate of inspiration and lung viscoelasticity, a probable increase in lung elastic recoil pressure, both changes increasing expiratory flows, promoting lung emptying and a stable end-expiratory volume. Bronchodilation with HC occurred despite optimal standard bronchodilator therapy, suggesting that in COPD further bronchodilation is possible.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.