Initial Assessment and Management

The patient is a 35-year-old man involved in a motor vehicle crash with significant mechanisms of injury, including being hit on the driver's side. His symptoms include severe pain on the left side of the chest and belly, tachycardia (heart rate of 120 bpm), hypotension (blood pressure 100/60 mmHg), tachypnea (breathing rate of 26 bpm), and decreased oxygen saturation (90% on room air). The initial step is to follow the basic trauma management principles, which include assessing the airway, breathing, circulation, disability, and exposure (ABCDE approach).

Fluid Resuscitation and Initial Orders

Given the patient's hypotension and tachycardia, fluid resuscitation should be initiated promptly to ensure adequate circulation. An order for 2 liters of crystalloid fluid (e.g., lactated Ringer's solution) should be given to help stabilize the patient's blood pressure. Additionally, oxygen supplementation should be provided to improve oxygen saturation, and the patient should be placed on a cardiac monitor.

Laboratory and Imaging Workup

The laboratory workup should include:

• Complete Blood Count (CBC) to assess for signs of bleeding or infection.
• Basic Metabolic Panel (BMP) to evaluate electrolyte balance and renal function.
• Liver Function Tests (LFTs) and Amylase/Lipase to assess for liver or pancreatic injury.
• Type and Screen in case blood transfusion is needed.
• Urinalysis to check for hematuria or signs of urinary tract injury.

Imaging studies should include:

• Chest X-ray to evaluate for thoracic injuries such as pneumothorax or hemothorax.
• Focused Assessment with Sonography for Trauma (FAST) to quickly assess for free intraperitoneal fluid, which could indicate internal bleeding.
• CT scans of the chest, abdomen, and pelvis with contrast to further evaluate the extent of injuries, if the patient's condition allows.

Differential Diagnosis

Single Most Likely Diagnosis

• Splenic injury: Given the mechanism of injury (side impact) and the patient's symptoms (left-sided chest and abdominal pain, hypotension), a splenic injury is highly likely. The spleen is the most commonly injured organ in blunt abdominal trauma.

Other Likely Diagnoses

• Rib fractures: The chest pain could be indicative of rib fractures, especially given the mechanism of injury.
• Pneumothorax or Hemothorax: These conditions could explain the chest pain and decreased oxygen saturation.
• Liver injury: Although less common than splenic injury in this context, liver lacerations could also present with abdominal pain and hypotension.

Do Not Miss Diagnoses

• Aortic injury: Although less likely, aortic injuries can be catastrophic if missed. The mechanism of injury (high-speed deceleration) increases the risk.
• Diaphragmatic rupture: This condition can present with chest or abdominal pain and is often associated with other injuries.
• Pancreatic injury: Pancreatic injuries can be subtle initially but have significant long-term consequences if not recognized.

Rare Diagnoses

• Gastric or intestinal perforation: These injuries might not be immediately apparent but could become evident with time, especially if there's a delay in seeking medical attention or if the initial assessment does not include imaging that would reveal such injuries.
• Adrenal gland injury: While rare, adrenal injuries can occur in the setting of severe trauma and might not be immediately suspected.

Each of these diagnoses should be considered in the context of the patient's overall clinical presentation, mechanism of injury, and the results of the laboratory and imaging studies.