The evaluation and management of hypoxemia in the chronic critically ill patient

Management of Respiratory Failure

The management of acute respiratory failure varies according to the etiology. A clear understanding of physiology of respiration and pathophysiological mechanisms of respiratory failure is mandatory for managing these patients. The extent of abnormality in arterial blood gas values is a result of the balance between the severity of disease and the degree of compensation by cardiopulmonary system. Normal blood gases do not mean that there is an absence of disease because the homeostatic system can compensate. However, an abnormal arterial blood gas value reflects uncompensated disease that might be life threatening.

Conventional mechanical ventilation in acute lung injury and acute respiratory distress syndrome

Acute lung injury and acute respiratory distress syndrome are inflammatory conditions involving a broad spectrum of lung injury from mild respiratory abnormality to severe respiratory derangement. Regardless of cause (direct or indirect lung injury), pulmonary physiology and mechanics are altered, leading to hypoxemic respiratory failure. the use of positive pressure ventilation itself may cause lung injury (ventilator-induced lung injury, or VILI). VILI may amplify preexisting injury, delay lung recovery, and result in adverse outcomes. This article examines the evidence supporting lung-protective ventilation strategies and addresses the methods, outcomes, and potential obstacles to implementation of such approaches.

Febrile-range hyperthermia augments pulmonary neutrophil recruitment and amplifies pulmonary oxygen toxicity

Febrile-range hyperthermia (FRH) improves survival in experimental infections by accelerating pathogen clearance, but may also increase collateral tissue injury. We hypothesized that FRH would worsen the outcome of inflammation stimulated by a non-replicating agonist and tested this hypothesis in a murine model of pulmonary oxygen toxicity. Using a conscious, temperature-controlled mouse model, we showed that maintaining a core temperature at FRH (39 degrees C to 40 degrees C) rather than at euthermic levels (36.5 degrees C to 37 degrees C) during hyperoxia exposure accelerated lethal pulmonary vascular endothelial injury, reduced the inspired oxygen threshold for lethality, induced expression of granulocyte-colony stimulating factor, and expanded the circulating neutrophil pool. In these same mice, FRH augmented pulmonary expression of the ELR(+) CXC chemokines, KC and LPS-induced CXC chemokine, enhanced recruitment of neutrophils, and changed the histological pattern of lung injury to a neutrophilic interstitial pneumonitis. Immunoblockade of CXC receptor-2 abrogated neutrophil recruitment, reduced pulmonary vascular injury, and delayed death. These combined data demonstrate that FRH may enlist distinct mediators and effector cells to profoundly shift the host response to a defined injurious stimulus, in part by augmenting delivery of neutrophils to sites of inflammation, such as may occur in infections. In certain conditions, such as in the hyperoxic lung, this process may be deleterious.

A prospective, randomized comparison of the Volume Diffusive Respirator vs conventional ventilation for ventilation of burned children. 2001 ABA paper

The Volume Diffusive Respirator (VDR) is a high-frequency time cycled pressure ventilator that can ventilate, oxygenate, and promote secretion removal. The VDR provides ventilation at lower airway pressures than those required for conventional ventilation in the pressure control mode (PCV). A prospective, randomized, institutional review board-approved study was conducted comparing the VDR to PCV in burned children with respiratory failure from all causes.

METHODS

Pediatric burn patients requiring ventilation were stratified by presence of inhalation injury and ventilated by VDR or PCV to achieve predefined arterial blood gases.

RESULTS

Sixty-four patients were prospectively assigned ventilator type; 32 to VDR, 32 to PCV. Data are reported as mean + SEM. Patient age was 7.4 + 0.7 years, TBSA was 56 + 3%, and number of patients with inhalation injuries was 55 (86%). Maximum peak inspiratory pressure with the VDR was significantly less than with PCV (30.9 + 0.8 cm H2O vs 39.5 + 1.8 cm H2O,P < 0.05) and the best PaO2 /FIO2 ratio was significantly higher with the VDR compared with PCV (563 + 15 vs 507 + 13, P < 0.05). No patient in the VDR group had evidence of barotrauma compared with two in the PCV group. Five patients in the PCV group died compared with two in the VDR group.

CONCLUSION

Patients ventilated with the VDR required significantly lower peak inspiratory pressure and achieved a significantly higher PaO2 /FIO2 ratio compared with PCV. This demonstrates the VDR is a safe and effective method of ventilation for pediatric burn patients and it offers advantages when compared with conventional ventilation.

High-Frequency Percussive Ventilation as a Salvage Modality in Adult Respiratory Distress Syndrome: A Preliminary Study

Despite multiple advances in critical care patients with severe adult respiratory distress syndrome (ARDS) can exhaust the capability of conventional ventilation; this results in respiratory failure and death. High-frequency percussive ventilation (HFPV), which was initially utilized for salvage of burn patients with smoke inhalation injury refractory to conventional ventilation, has evolved as a standard of burn care. Based on our experience with HFPV in burn patients the burn team was consulted to provide salvage ventilation for non-burn surgical intensive care unit patients with refractory respiratory failure. Over a 14-month period ten patients with refractory ARDS from multiple causes were treated. Retrospective chart review was performed. Respiratory parameters were assessed before and 24 hours after initiation of HFPV. Mean values of fraction of inspired oxygen (Fio2), pH, partial pressure of O2 in arterial blood (Pao2), partial pressure of CO2 in arterial blood (Paco2), HCO3, oxygen saturation in arterial blood (Sao2), Pao2/Fio2, and peak inspiratory pressure were compared. Significant improvement in oxygenation was reflected by increases in Sao2, Pao2, and the Pao2/Fio2 ratio in the first 24 hours of HFPV. No significant increase in peak inspiratory pressure was documented by conversion from conventional ventilation to HFPV. No hemodynamic changes directly associated with HFPV were noted. Seven of ten patients failing conventional ventilation survived to hospital discharge after salvage therapy with HFPV. We advocate further studies of HFPV in non-burn patients with ARDS both as salvage therapy and as replacement for conventional ventilation for the initial treatment for ARDS.